database/postgres
database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-03 01:21:48 +03:00
Code
postgres/doc/src/sgml/plpgsql.sgml
Tom Lane fc8d970cbc Replace functional-index facility with expressional indexes. Any column 
of an index can now be a computed expression instead of a simple variable.
Restrictions on expressions are the same as for predicates (only immutable
functions, no sub-selects).  This fixes problems recently introduced with
inlining SQL functions, because the inlining transformation is applied to
both expression trees so the planner can still match them up.  Along the
way, improve efficiency of handling index predicates (both predicates and
index expressions are now cached by the relcache) and fix 7.3 oversight
that didn't record dependencies of predicate expressions.
2003-05-28 16:04:02 +00:00

2877 lines
93 KiB
Plaintext
Raw Blame History

<!--
$Header: /cvsroot/pgsql/doc/src/sgml/plpgsql.sgml,v 1.19 2003/05/28 16:03:55 tgl Exp $
-->
<chapter id="plpgsql">
<title><application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language</title>
<indexterm zone="plpgsql">
<primary>PL/pgSQL</primary>
</indexterm>
<para>
<application>PL/pgSQL</application> is a loadable procedural
language for the <productname>PostgreSQL</productname> database
system. The design goals of <application>PL/pgSQL</> were to create
a loadable procedural language that
<itemizedlist>
<listitem>
<para>
can be used to create functions and trigger procedures,
</para>
</listitem>
<listitem>
<para>
adds control structures to the <acronym>SQL</acronym> language,
</para>
</listitem>
<listitem>
<para>
can perform complex computations,
</para>
</listitem>
<listitem>
<para>
inherits all user-defined types, functions, and operators,
</para>
</listitem>
<listitem>
<para>
can be defined to be trusted by the server,
</para>
</listitem>
<listitem>
<para>
is easy to use.
</para>
</listitem>
</itemizedlist>
</para>
<sect1 id="plpgsql-overview">
<title>Overview</title>
<para>
The <application>PL/pgSQL</> call handler parses the function's source text and
produces an internal binary instruction tree the first time the
function is called (within each session). The instruction tree
fully translates the
<application>PL/pgSQL</> statement structure, but individual
<acronym>SQL</acronym> expressions and <acronym>SQL</acronym> commands
used in the function are not translated immediately.
</para>
<para>
As each expression and <acronym>SQL</acronym> command is first used
in the function, the <application>PL/pgSQL</> interpreter creates
a prepared execution plan (using the <acronym>SPI</acronym>
manager's <function>SPI_prepare</function> and
<function>SPI_saveplan</function> functions). Subsequent visits
to that expression or command reuse the prepared plan. Thus, a
function with conditional code that contains many statements for
which execution plans might be required will only prepare and save
those plans that are really used during the lifetime of the
database connection. This can substantially reduce the total
amount of time required to parse, and generate execution plans for the
statements in a <application>PL/pgSQL</> function. A disadvantage is
that errors in a specific expression or command may not be detected
until that part of the function is reached in execution.
</para>
<para>
Once <application>PL/pgSQL</> has made an execution plan for a particular
command in a function, it will reuse that plan for the life of the
database connection. This is usually a win for performance, but it
can cause some problems if you dynamically
alter your database schema. For example:
<programlisting>
CREATE FUNCTION populate() RETURNS integer AS '
DECLARE
-- declarations
BEGIN
PERFORM my_function();
END;
' LANGUAGE plpgsql;
</programlisting>
If you execute the above function, it will reference the OID for
<function>my_function()</function> in the execution plan produced for
the <command>PERFORM</command> statement. Later, if you
drop and recreate <function>my_function()</function>, then
<function>populate()</function> will not be able to find
<function>my_function()</function> anymore. You would then have to
recreate <function>populate()</function>, or at least start a new
database session so that it will be compiled afresh. Another way
to avoid this problem is to use <command>CREATE OR REPLACE
FUNCTION</command> when updating the definition of
<function>my_function</function> (when a function is
<quote>replaced</quote>, its OID is not changed).
</para>
<para>
Because <application>PL/pgSQL</application> saves execution plans
in this way, SQL commands that appear directly in a
<application>PL/pgSQL</application> function must refer to the
same tables and columns on every execution; that is, you cannot use
a parameter as the name of a table or column in an SQL command. To get
around this restriction, you can construct dynamic commands using
the <application>PL/pgSQL</application> <command>EXECUTE</command>
statement --- at the price of constructing a new execution plan on
every execution.
</para>
<note>
<para>
The <application>PL/pgSQL</application>
<command>EXECUTE</command> statement is not related to the
<command>EXECUTE</command> statement supported by the
<productname>PostgreSQL</productname> server. The server's
<command>EXECUTE</command> statement cannot be used within
<application>PL/pgSQL</> functions (and is not needed).
</para>
</note>
<para>
Except for input/output conversion and calculation functions
for user-defined types, anything that can be defined in C language
functions can also be done with <application>PL/pgSQL</application>.
For example, it is possible to
create complex conditional computation functions and later use
them to define operators or use them in index expressions.
</para>
<sect2 id="plpgsql-advantages">
<title>Advantages of Using <application>PL/pgSQL</application></title>
<para>
<acronym>SQL</acronym> is the language <productname>PostgreSQL</>
(and most other relational databases) use as query language. It's
portable and easy to learn. But every <acronym>SQL</acronym>
statement must be executed individually by the database server.
</para>
<para>
That means that your client application must send each query to
the database server, wait for it to process it, receive the
results, do some computation, then send other queries to the
server. All this incurs interprocess communication and may also
incur network overhead if your client is on a different machine
than the database server.
</para>
<para>
With <application>PL/pgSQL</application> you can group a block of computation and a
series of queries <emphasis>inside</emphasis> the
database server, thus having the power of a procedural
language and the ease of use of SQL, but saving lots of
time because you don't have the whole client/server
communication overhead. This can make for a
considerable performance increase.
</para>
<para>
Also, with <application>PL/pgSQL</application> you can use all
the data types, operators and functions of SQL.
</para>
</sect2>
<sect2 id="plpgsql-overview-developing-in-plpgsql">
<title>Developing in <application>PL/pgSQL</application></title>
<para>
One good way to develop in
<application>PL/pgSQL</> is to simply use the text editor of your
choice to create your functions, and in another window, use
<command>psql</command> to load those functions. If you are doing it this way, it
is a good idea to write the function using <command>CREATE OR
REPLACE FUNCTION</>. That way you can reload the file to update
the function definition. For example:
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS '
....
end;
' LANGUAGE plpgsql;
</programlisting>
</para>
<para>
While running <command>psql</command>, you can load or reload such a
function definition file with
<programlisting>
\i filename.sql
</programlisting>
and then immediately issue SQL commands to test the function.
</para>
<para>
Another good way to develop in <application>PL/pgSQL</> is using a
GUI database access tool that facilitates development in a
procedural language. One example of such as a tool is
<application>PgAccess</>, although others exist. These tools often
provide convenient features such as escaping single quotes and
making it easier to recreate and debug functions.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-quote">
<title>Handling of Quotations Marks</title>
<para>
Since the code of any procedural language function is specified
<command>CREATE FUNCTION</command> as a string literal, single
quotes inside the function body must be escaped. This can lead to
rather complicated code at times, especially if you are writing a
function that generates other functions, as in the example in <xref
linkend="plpgsql-statements-executing-dyn">. The list below gives
you an overview over the needed levels of quotation marks in
various situations. Keep this chart handy.
</para>
<variablelist>
<varlistentry>
<term>1 quotation mark</term>
<listitem>
<para>
To begin/end function bodies, for example:
<programlisting>
CREATE FUNCTION foo() RETURNS integer AS '...'
LANGUAGE plpgsql;
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>2 quotation marks</term>
<listitem>
<para>
For string literals inside the function body, for example:
<programlisting>
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
</programlisting>
The second line is interpreted as
<programlisting>
SELECT * FROM users WHERE f_name='foobar';
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>4 quotation marks</term>
<listitem>
<para>
When you need a single quote in a string inside the function
body, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''
</programlisting>
The value of <literal>a_output</literal> would then be: <literal>
AND name LIKE 'foobar' AND xyz</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>6 quotation marks</term>
<listitem>
<para>
When a single quote in a string inside the function body is
adjacent to the end of that string constant, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar''''''
</programlisting>
The value of <literal>a_output</literal> would then be:
<literal> AND name LIKE 'foobar'</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>10 quotation marks</term>
<listitem>
<para>
When you want two single quotes in a string constant (which
accounts for 8 quotes) and this is adjacent to the end of that
string constant (2 more). You will probably only need that if
you are writing a function that generates other functions. For
example:
<programlisting>
a_output := a_output || '' if v_'' ||
referrer_keys.kind || '' like ''''''''''
|| referrer_keys.key_string || ''''''''''
then return '''''' || referrer_keys.referrer_type
|| ''''''; end if;'';
</programlisting>
The value of <literal>a_output</literal> would then be:
<programlisting>
if v_... like ''...'' then return ''...''; end if;
</programlisting>
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="plpgsql-structure">
<title>Structure of <application>PL/pgSQL</application></title>
<para>
<application>PL/pgSQL</application> is a block-structured language.
The complete text of a function definition must be a
<firstterm>block</>. A block is defined as:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
<replaceable>declarations</replaceable> </optional>
BEGIN
<replaceable>statements</replaceable>
END;
</synopsis>
</para>
<para>
Each declaration and each statement within a block is terminated
by a semicolon.
</para>
<para>
All key words and identifiers can be written in mixed upper and
lower case. Identifiers are implicitly converted to lower-case
unless double-quoted.
</para>
<para>
There are two types of comments in <application>PL/pgSQL</>. A double dash (<literal>--</literal>)
starts a comment that extends to the end of the line. A <literal>/*</literal>
starts a block comment that extends to the next occurrence of <literal>*/</literal>.
Block comments cannot be nested, but double dash comments can be
enclosed into a block comment and a double dash can hide
the block comment delimiters <literal>/*</literal> and <literal>*/</literal>.
</para>
<para>
Any statement in the statement section of a block
can be a <firstterm>subblock</>. Subblocks can be used for
logical grouping or to localize variables to a small group
of statements.
</para>
<para>
The variables declared in the declarations section preceding a
block are initialized to their default values every time the
block is entered, not only once per function call. For example:
<programlisting>
CREATE FUNCTION somefunc() RETURNS integer AS '
DECLARE
quantity integer := 30;
BEGIN
RAISE NOTICE ''Quantity here is %'', quantity; -- Quantity here is 30
quantity := 50;
--
-- Create a subblock
--
DECLARE
quantity integer := 80;
BEGIN
RAISE NOTICE ''Quantity here is %'', quantity; -- Quantity here is 80
END;
RAISE NOTICE ''Quantity here is %'', quantity; -- Quantity here is 50
RETURN quantity;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
<para>
It is important not to confuse the use of <command>BEGIN</>/<command>END</> for
grouping statements in <application>PL/pgSQL</> with the database commands for
transaction control. <application>PL/pgSQL</>'s <command>BEGIN</>/<command>END</> are only for grouping;
they do not start or end a transaction. Functions and trigger procedures
are always executed within a transaction established by an outer query
--- they cannot start or commit transactions, since
<productname>PostgreSQL</productname> does not have nested transactions.
</para>
</sect1>
<sect1 id="plpgsql-declarations">
<title>Declarations</title>
<para>
All variables used in a block must be declared in the
declarations section of the block.
(The only exception is that the loop variable of a <literal>FOR</> loop iterating
over a range of integer values is automatically declared as an integer
variable.)
</para>
<para>
<application>PL/pgSQL</> variables can have any SQL data type, such as
<type>integer</type>, <type>varchar</type>, and
<type>char</type>.
</para>
<para>
Here are some examples of variable declarations:
<programlisting>
user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;
</programlisting>
</para>
<para>
The general syntax of a variable declaration is:
<synopsis>
<replaceable>name</replaceable> <optional> CONSTANT </optional> <replaceable>type</replaceable> <optional> NOT NULL </optional> <optional> { DEFAULT | := } <replaceable>expression</replaceable> </optional>;
</synopsis>
The <literal>DEFAULT</> clause, if given, specifies the initial value assigned
to the variable when the block is entered. If the <literal>DEFAULT</> clause
is not given then the variable is initialized to the
<acronym>SQL</acronym> null value.
The <literal>CONSTANT</> option prevents the variable from being assigned to,
so that its value remains constant for the duration of the block.
If <literal>NOT NULL</>
is specified, an assignment of a null value results in a run-time
error. All variables declared as <literal>NOT NULL</>
must have a nonnull default value specified.
</para>
<para>
The default value is evaluated every time the block is entered. So,
for example, assigning <literal>'now'</literal> to a variable of type
<type>timestamp</type> causes the variable to have the
time of the current function call, not the time when the function was
precompiled.
</para>
<para>
Examples:
<programlisting>
quantity integer DEFAULT 32;
url varchar := ''http://mysite.com'';
user_id CONSTANT integer := 10;
</programlisting>
</para>
<sect2 id="plpgsql-declaration-aliases">
<title>Aliases for Function Parameters</title>
<synopsis>
<replaceable>name</replaceable> ALIAS FOR $<replaceable>n</replaceable>;
</synopsis>
<para>
Parameters passed to functions are named with the identifiers
<literal>$1</literal>, <literal>$2</literal>,
etc. Optionally, aliases can be declared for <literal>$<replaceable>n</replaceable></literal>
parameter names for increased readability. Either the alias or the
numeric identifier can then be used to refer to the parameter value.
Some examples:
<programlisting>
CREATE FUNCTION sales_tax(real) RETURNS real AS '
DECLARE
subtotal ALIAS FOR $1;
BEGIN
RETURN subtotal * 0.06;
END;
' LANGUAGE plpgsql;
CREATE FUNCTION instr(varchar, integer) RETURNS integer AS '
DECLARE
v_string ALIAS FOR $1;
index ALIAS FOR $2;
BEGIN
-- some computations here
END;
' LANGUAGE plpgsql;
CREATE FUNCTION use_many_fields(tablename) RETURNS text AS '
DECLARE
in_t ALIAS FOR $1;
BEGIN
RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-declaration-type">
<title>Copying Types</title>
<synopsis>
<replaceable>variable</replaceable>%TYPE
</synopsis>
<para>
<literal>%TYPE</literal> provides the data type of a variable or
table column. You can use this to declare variables that will hold
database values. For example, let's say you have a column named
<literal>user_id</literal> in your <literal>users</literal>
table. To declare a variable with the same data type as
<literal>users.user_id</> you write:
<programlisting>
user_id users.user_id%TYPE;
</programlisting>
</para>
<para>
By using <literal>%TYPE</literal> you don't need to know the data
type of the structure you are referencing, and most importantly,
if the data type of the referenced item changes in the future (for
instance: you change the type of <literal>user_id</>
from <type>integer</type> to <type>real</type>), you may not need
to change your function definition.
</para>
</sect2>
<sect2 id="plpgsql-declaration-rowtypes">
<title>Row Types</title>
<synopsis>
<replaceable>name</replaceable> <replaceable>table_name</replaceable><literal>%ROWTYPE</literal>;
<replaceable>name</replaceable> <replaceable>composite_type_name</replaceable>;
</synopsis>
<para>
A variable of a composite type is called a <firstterm>row</>
variable (or <firstterm>row-type</> variable). Such a variable
can hold a whole row of a <command>SELECT</> or <command>FOR</>
query result, so long as that query's column set matches the
declared type of the variable.
The individual fields of the row value
are accessed using the usual dot notation, for example
<literal>rowvar.field</literal>.
</para>
<para>
A row variable can be declared to have the same type as the rows of
an existing table or view, by using the
<replaceable>table_name</replaceable><literal>%ROWTYPE</literal>
notation; or it can be declared by giving a composite type's name.
(Since every table has an associated datatype of the same name,
it actually does not matter in <productname>PostgreSQL</> whether you
write <literal>%ROWTYPE</literal> or not. But the form with
<literal>%ROWTYPE</literal> is more portable.)
</para>
<para>
Parameters to a function can be
composite types (complete table rows). In that case, the
corresponding identifier <literal>$<replaceable>n</replaceable></> will be a row variable, and fields can
be selected from it, for example <literal>$1.user_id</literal>.
</para>
<para>
Only the user-defined columns of a table row are accessible in a
row-type variable, not the OID or other system columns (because the
row could be from a view). The fields of the row type inherit the
table's field size or precision for data types such as
<type>char(<replaceable>n</>)</type>.
</para>
<para>
Here is an example of using composite types:
<programlisting>
CREATE FUNCTION use_two_tables(tablename) RETURNS text AS '
DECLARE
in_t ALIAS FOR $1;
use_t table2name%ROWTYPE;
BEGIN
SELECT * INTO use_t FROM table2name WHERE ... ;
RETURN in_t.f1 || use_t.f3 || in_t.f5 || use_t.f7;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-declaration-records">
<title>Record Types</title>
<para>
<synopsis>
<replaceable>name</replaceable> RECORD;
</synopsis>
</para>
<para>
Record variables are similar to row-type variables, but they have no
predefined structure. They take on the actual row structure of the
row they are assigned during a <command>SELECT</> or <command>FOR</> command. The substructure
of a record variable can change each time it is assigned to.
A consequence of this is that until a record variable is first assigned
to, it has no substructure, and any attempt to access a
field in it will draw a run-time error.
</para>
<para>
Note that <literal>RECORD</> is not a true data type, only a placeholder.
</para>
</sect2>
<sect2 id="plpgsql-declaration-renaming-vars">
<title><literal>RENAME</></title>
<para>
<synopsis>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
</synopsis>
Using the RENAME declaration you can change the name of a variable,
record or row. This is primarily useful if NEW or OLD should be
referenced by another name inside a trigger procedure. See also ALIAS.
</para>
<para>
Examples:
<programlisting>
RENAME id TO user_id;
RENAME this_var TO that_var;
</programlisting>
</para>
<note>
<para>
RENAME appears to be broken as of <productname>PostgreSQL</>
7.3. Fixing this is of low priority, since ALIAS covers most of
the practical uses of RENAME.
</para>
</note>
</sect2>
</sect1>
<sect1 id="plpgsql-expressions">
<title>Expressions</title>
<para>
All expressions used in <application>PL/pgSQL</application>
statements are processed using the server's regular
<acronym>SQL</acronym> executor. Expressions that appear to
contain constants may in fact require run-time evaluation
(e.g., <literal>'now'</literal> for the <type>timestamp</type>
type) so it is impossible for the
<application>PL/pgSQL</application> parser to identify real
constant values other than the key word <literal>NULL</>. All expressions are
evaluated internally by executing a query
<synopsis>
SELECT <replaceable>expression</replaceable>
</synopsis>
using the <acronym>SPI</acronym> manager. For evaluation,
occurrences of <application>PL/pgSQL</application> variable
identifiers are replaced by parameters, and the actual values from
the variables are passed to the executor in the parameter array.
This allows the query plan for the <command>SELECT</command> to
be prepared just once and then reused for subsequent
evaluations.
</para>
<para>
The evaluation done by the <productname>PostgreSQL</productname>
main parser has some side
effects on the interpretation of constant values. In detail there
is a difference between what these two functions do:
<programlisting>
CREATE FUNCTION logfunc1(text) RETURNS timestamp AS '
DECLARE
logtxt ALIAS FOR $1;
BEGIN
INSERT INTO logtable VALUES (logtxt, ''now'');
RETURN ''now'';
END;
' LANGUAGE plpgsql;
</programlisting>
and
<programlisting>
CREATE FUNCTION logfunc2(text) RETURNS timestamp AS '
DECLARE
logtxt ALIAS FOR $1;
curtime timestamp;
BEGIN
curtime := ''now'';
INSERT INTO logtable VALUES (logtxt, curtime);
RETURN curtime;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
<para>
In the case of <function>logfunc1</function>, the
<productname>PostgreSQL</productname> main parser knows when
preparing the plan for the <command>INSERT</command>, that the string
<literal>'now'</literal> should be interpreted as
<type>timestamp</type> because the target column of <classname>logtable</classname>
is of that type. Thus, it will make a constant from it at this
time and this constant value is then used in all invocations of
<function>logfunc1</function> during the lifetime of the
session. Needless to say that this isn't what the
programmer wanted.
</para>
<para>
In the case of <function>logfunc2</function>, the
<productname>PostgreSQL</productname> main parser does not know
what type <literal>'now'</literal> should become and therefore
it returns a data value of type <type>text</type> containing the string
<literal>now</literal>. During the ensuing assignment
to the local variable <varname>curtime</varname>, the
<application>PL/pgSQL</application> interpreter casts this
string to the <type>timestamp</type> type by calling the
<function>text_out</function> and <function>timestamp_in</function>
functions for the conversion. So, the computed time stamp is updated
on each execution as the programmer expects.
</para>
<para>
The mutable nature of record variables presents a problem in this
connection. When fields of a record variable are used in
expressions or statements, the data types of the fields must not
change between calls of one and the same expression, since the
expression will be planned using the data type that is present
when the expression is first reached. Keep this in mind when
writing trigger procedures that handle events for more than one
table. (<command>EXECUTE</command> can be used to get around
this problem when necessary.)
</para>
</sect1>
<sect1 id="plpgsql-statements">
<title>Basic Statements</title>
<para>
In this section and the following ones, we describe all the statement
types that are explicitly understood by
<application>PL/pgSQL</application>.
Anything not recognized as one of these statement types is presumed
to be an SQL command and is sent to the main database engine to execute
(after substitution of any <application>PL/pgSQL</application> variables
used in the statement). Thus,
for example, the SQL commands <command>INSERT</>, <command>UPDATE</>, and
<command>DELETE</> may be considered to be statements of
<application>PL/pgSQL</application>, but they are not specifically
listed here.
</para>
<sect2 id="plpgsql-statements-assignment">
<title>Assignment</title>
<para>
An assignment of a value to a variable or row/record field is
written as:
<synopsis>
<replaceable>identifier</replaceable> := <replaceable>expression</replaceable>;
</synopsis>
As explained above, the expression in such a statement is evaluated
by means of an SQL <command>SELECT</> command sent to the main
database engine. The expression must yield a single value.
</para>
<para>
If the expression's result data type doesn't match the variable's
data type, or the variable has a specific size/precision
(like <type>char(20)</type>), the result value will be implicitly
converted by the <application>PL/pgSQL</application> interpreter using
the result type's output-function and
the variable type's input-function. Note that this could potentially
result in run-time errors generated by the input function, if the
string form of the result value is not acceptable to the input function.
</para>
<para>
Examples:
<programlisting>
user_id := 20;
tax := subtotal * 0.06;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-select-into">
<title><command>SELECT INTO</command></title>
<para>
The result of a <command>SELECT</command> command yielding multiple columns (but
only one row) can be assigned to a record variable, row-type
variable, or list of scalar variables. This is done by:
<synopsis>
SELECT INTO <replaceable>target</replaceable> <replaceable>expressions</replaceable> FROM ...;
</synopsis>
where <replaceable>target</replaceable> can be a record variable, a row
variable, or a comma-separated list of simple variables and
record/row fields.
</para>
<para>
Note that this is quite different from
<productname>PostgreSQL</>'s normal interpretation of
<command>SELECT INTO</command>, where the <literal>INTO</> target
is a newly created table. If you want to create a table from a
<command>SELECT</> result inside a
<application>PL/pgSQL</application> function, use the syntax
<command>CREATE TABLE ... AS SELECT</command>.
</para>
<para>
If a row or a variable list is used as target, the selected values
must exactly match the structure of the target, or a run-time error
occurs. When a record variable is the target, it automatically
configures itself to the row type of the query result columns.
</para>
<para>
Except for the <literal>INTO</> clause, the <command>SELECT</>
statement is the same as a normal SQL <command>SELECT</> command
and can use its full power.
</para>
<para>
If the query returns zero rows, null values are assigned to the
target(s). If the query returns multiple rows, the first
row is assigned to the target(s) and the rest are discarded.
(Note that <quote>the first row</> is not well-defined unless you've
used <literal>ORDER BY</>.)
</para>
<para>
At present, the <literal>INTO</> clause can appear almost anywhere in the <command>SELECT</command>
statement, but it is recommended to place it immediately after the <literal>SELECT</literal>
key word as depicted above. Future versions of
<application>PL/pgSQL</application> may be less forgiving about
placement of the <literal>INTO</literal> clause.
</para>
<para>
You can use <literal>FOUND</literal> immediately after a <command>SELECT
INTO</command> statement to determine whether the assignment was successful
(that is, at least one row was was returned by the query). For example:
<programlisting>
SELECT INTO myrec * FROM emp WHERE empname = myname;
IF NOT FOUND THEN
RAISE EXCEPTION ''employee % not found'', myname;
END IF;
</programlisting>
</para>
<para>
To test for whether a record/row result is null, you can use the
<literal>IS NULL</literal> conditional. There is, however, no
way to tell whether any additional rows might have been
discarded. Here is an example that handles the case where no
rows have been returned:
<programlisting>
DECLARE
users_rec RECORD;
full_name varchar;
BEGIN
SELECT INTO users_rec * FROM users WHERE user_id=3;
IF users_rec.homepage IS NULL THEN
-- user entered no homepage, return "http://"
RETURN ''http://'';
END IF;
END;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-statements-perform">
<title>Executing an Expression or Query With No Result</title>
<para>
Sometimes one wishes to evaluate an expression or query but
discard the result (typically because one is calling a function
that has useful side-effects but no useful result value). To do
this in <application>PL/pgSQL</application>, use the
<command>PERFORM</command> statement:
<synopsis>
PERFORM <replaceable>query</replaceable>;
</synopsis>
This executes <replaceable>query</replaceable>, which must be a
<command>SELECT</command> statement, and discards the
result. <application>PL/pgSQL</application> variables are
substituted in the query as usual. Also, the special variable
<literal>FOUND</literal> is set to true if the query produced at
least one row or false if it produced no rows.
</para>
<note>
<para>
One might expect that <command>SELECT</command> with no
<literal>INTO</> clause would accomplish this result, but at
present the only accepted way to do it is
<command>PERFORM</command>.
</para>
</note>
<para>
An example:
<programlisting>
PERFORM create_mv(''cs_session_page_requests_mv'', my_query);
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-statements-executing-dyn">
<title>Executing Dynamic Commands</title>
<para>
Oftentimes you will want to generate dynamic commands inside your
<application>PL/pgSQL</application> functions, that is, commands
that will involve different tables or different data types each
time they are executed. <application>PL/pgSQL</application>'s
normal attempts to cache plans for commands will not work in such
scenarios. To handle this sort of problem, the
<command>EXECUTE</command> statement is provided:
<synopsis>
EXECUTE <replaceable class="command">command-string</replaceable>;
</synopsis>
where <replaceable>command-string</replaceable> is an expression
yielding a string (of type
<type>text</type>) containing the command
to be executed. This string is fed literally to the SQL engine.
</para>
<para>
Note in particular that no substitution of <application>PL/pgSQL</>
variables is done on the command string. The values of variables must
be inserted in the command string as it is constructed.
</para>
<para>
When working with dynamic commands you will have to face
escaping of single quotes in <application>PL/pgSQL</>. Please refer to the
overview in <xref linkend="plpgsql-quote">,
which can save you some effort.
</para>
<para>
Unlike all other commands in <application>PL/pgSQL</>, a command
run by an <command>EXECUTE</command> statement is not prepared
and saved just once during the life of the session. Instead, the
command is prepared each time the statement is run. The command
string can be dynamically created within the function to perform
actions on variable tables and columns.
</para>
<para>
The results from <command>SELECT</command> commands are discarded
by <command>EXECUTE</command>, and <command>SELECT INTO</command>
is not currently supported within <command>EXECUTE</command>.
So, the only way to extract a result from a dynamically-created
<command>SELECT</command> is to use the <command>FOR-IN-EXECUTE</> form
described later.
</para>
<para>
An example:
<programlisting>
EXECUTE ''UPDATE tbl SET ''
|| quote_ident(colname)
|| '' = ''
|| quote_literal(newvalue)
|| '' WHERE ...'';
</programlisting>
</para>
<para>
This example shows use of the functions
<function>quote_ident(<type>text</type>)</function> and
<function>quote_literal(<type>text</type>)</function>.
Variables containing column and table identifiers should be
passed to function <function>quote_ident</function>.
Variables containing values that act as value literals in the constructed command
string should be passed to
<function>quote_literal</function>. Both take the
appropriate steps to return the input text enclosed in single
or double quotes and with any embedded special characters
properly escaped.
</para>
<para>
Here is a much larger example of a dynamic command and
<command>EXECUTE</command>:
<programlisting>
CREATE FUNCTION cs_update_referrer_type_proc() RETURNS integer AS '
DECLARE
referrer_keys RECORD; -- declare a generic record to be used in a FOR
a_output varchar(4000);
BEGIN
a_output := ''CREATE FUNCTION cs_find_referrer_type(varchar, varchar, varchar)
RETURNS varchar AS ''''
DECLARE
v_host ALIAS FOR $1;
v_domain ALIAS FOR $2;
v_url ALIAS FOR $3;
BEGIN '';
-- Notice how we scan through the results of a query in a FOR loop
-- using the FOR &lt;record&gt; construct.
FOR referrer_keys IN SELECT * FROM cs_referrer_keys ORDER BY try_order LOOP
a_output := a_output || '' IF v_'' || referrer_keys.kind || '' LIKE ''''''''''
|| referrer_keys.key_string || '''''''''' THEN RETURN ''''''
|| referrer_keys.referrer_type || ''''''; END IF;'';
END LOOP;
a_output := a_output || '' RETURN NULL; END; '''' LANGUAGE plpgsql;'';
EXECUTE a_output;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-statements-diagnostics">
<title>Obtaining the Result Status</title>
<para>
There are several ways to determine the effect of a command. The
first method is to use the <command>GET DIAGNOSTICS</command>,
which has the form:
<synopsis>
GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replaceable> <optional> , ... </optional> ;
</synopsis>
This command allows retrieval of system status indicators. Each
<replaceable>item</replaceable> is a key word identifying a state
value to be assigned to the specified variable (which should be
of the right data type to receive it). The currently available
status items are <varname>ROW_COUNT</>, the number of rows
processed by the last <acronym>SQL</acronym> command sent down to
the <acronym>SQL</acronym> engine, and <varname>RESULT_OID</>,
the OID of the last row inserted by the most recent
<acronym>SQL</acronym> command. Note that <varname>RESULT_OID</>
is only useful after an <command>INSERT</command> command.
</para>
<para>
An example:
<programlisting>
GET DIAGNOSTICS var_integer = ROW_COUNT;
</programlisting>
</para>
<para>
The second method to determine the effects of a command is the
special variable named <literal>FOUND</literal> of
type <type>boolean</type>. <literal>FOUND</literal> starts out
false within each <application>PL/pgSQL</application> function.
It is set by each of the following types of statements:
<itemizedlist>
<listitem>
<para>
A <command>SELECT INTO</command> statement sets
<literal>FOUND</literal> true if it returns a row, false if no
row is returned.
</para>
</listitem>
<listitem>
<para>
A <command>PERFORM</> statement sets <literal>FOUND</literal>
true if it produces (discards) a row, false if no row is
produced.
</para>
</listitem>
<listitem>
<para>
<command>UPDATE</>, <command>INSERT</>, and <command>DELETE</>
statements set <literal>FOUND</literal> true if at least one
row is affected, false if no row is affected.
</para>
</listitem>
<listitem>
<para>
A <command>FETCH</> statement sets <literal>FOUND</literal>
true if it returns a row, false if no row is returned.
</para>
</listitem>
<listitem>
<para>
A <command>FOR</> statement sets <literal>FOUND</literal> true
if it iterates one or more times, else false. This applies to
all three variants of the <command>FOR</> statement (integer
<command>FOR</> loops, record-set <command>FOR</> loops, and
dynamic record-set <command>FOR</>
loops). <literal>FOUND</literal> is only set when the
<command>FOR</> loop exits: inside the execution of the loop,
<literal>FOUND</literal> is not modified by the
<command>FOR</> statement, although it may be changed by the
execution of other statements within the loop body.
</para>
</listitem>
</itemizedlist>
<literal>FOUND</literal> is a local variable; any changes
to it affect only the current <application>PL/pgSQL</application>
function.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-control-structures">
<title>Control Structures</title>
<para>
Control structures are probably the most useful (and
important) part of <application>PL/pgSQL</>. With
<application>PL/pgSQL</>'s control structures,
you can manipulate <productname>PostgreSQL</> data in a very
flexible and powerful way.
</para>
<sect2 id="plpgsql-statements-returning">
<title>Returning From a Function</title>
<para>
There are two commands available that allow you to return data
from a function: <command>RETURN</command> and <command>RETURN
NEXT</command>.
</para>
<sect3>
<title><command>RETURN</></title>
<synopsis>
RETURN <replaceable>expression</replaceable>;
</synopsis>
<para>
<command>RETURN</command> with an expression terminates the
function and returns the value of
<replaceable>expression</replaceable> to the caller. This form
is to be used for <application>PL/pgSQL</> functions that does
not return a set.
</para>
<para>
When returning a scalar type, any expression can be used. The
expression's result will be automatically cast into the
function's return type as described for assignments. To return a
composite (row) value, you must write a record or row variable
as the <replaceable>expression</replaceable>.
</para>
<para>
The return value of a function cannot be left undefined. If
control reaches the end of the top-level block of the function
without hitting a <command>RETURN</command> statement, a run-time
error will occur. Note that if you have declared the function to
return <type>void</type>, a <command>RETURN</command> statement
must still be specified; the expression following
<command>RETURN</command> is, however, optional and will be ignored in
any case.
</para>
</sect3>
<sect3>
<title><command>RETURN NEXT</></title>
<synopsis>
RETURN NEXT <replaceable>expression</replaceable>;
</synopsis>
<para>
When a <application>PL/pgSQL</> function is declared to return
<literal>SETOF <replaceable>sometype</></literal>, the procedure
to follow is slightly different. In that case, the individual
items to return are specified in <command>RETURN NEXT</command>
commands, and then a final <command>RETURN</command> command
with no arguments is used to indicate that the function has
finished executing. <command>RETURN NEXT</command> can be used
with both scalar and composite data types; in the later case, an
entire <quote>table</quote> of results will be returned.
</para>
<para>
Functions that use <command>RETURN NEXT</command> should be
called in the following fashion:
<programlisting>
SELECT * FROM some_func();
</programlisting>
That is, the function is used as a table source in a <literal>FROM</literal>
clause.
</para>
<para>
<command>RETURN NEXT</command> does not actually return from the
function; it simply saves away the value of the expression (or
record or row variable, as appropriate for the data type being
returned). Execution then continues with the next statement in
the <application>PL/pgSQL</> function. As successive
<command>RETURN NEXT</command> commands are executed, the result
set is built up. A final <command>RETURN</command>, which should
have no argument, causes control to exit the function.
</para>
<note>
<para>
The current implementation of <command>RETURN NEXT</command>
for <application>PL/pgSQL</> stores the entire result set
before returning from the function, as discussed above. That
means that if a <application>PL/pgSQL</> function produces a
very large result set, performance may be poor: data will be
written to disk to avoid memory exhaustion, but the function
itself will not return until the entire result set has been
generated. A future version of <application>PL/pgSQL</> may
allow users to allow users to define set-returning functions
that do not have this limitation. Currently, the point at
which data begins being written to disk is controlled by the
<varname>sort_mem</> configuration variable. Administrators
who have sufficient memory to store larger result sets in
memory should consider increasing this parameter.
</para>
</note>
</sect3>
</sect2>
<sect2 id="plpgsql-conditionals">
<title>Conditionals</title>
<para>
<literal>IF</> statements let you execute commands based on
certain conditions. <application>PL/pgSQL</> has four forms of
<literal>IF</>:
<itemizedlist>
<listitem>
<para><literal>IF ... THEN</></>
</listitem>
<listitem>
<para><literal>IF ... THEN ... ELSE</></>
</listitem>
<listitem>
<para><literal>IF ... THEN ... ELSE IF</></>
</listitem>
<listitem>
<para><literal>IF ... THEN ... ELSIF ... THEN ... ELSE</></>
</listitem>
</itemizedlist>
</para>
<sect3>
<title><literal>IF-THEN</></title>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
END IF;
</synopsis>
<para>
<literal>IF-THEN</literal> statements are the simplest form of
<literal>IF</literal>. The statements between
<literal>THEN</literal> and <literal>END IF</literal> will be
executed if the condition is true. Otherwise, they are
skipped.
</para>
<para>
Example:
<programlisting>
IF v_user_id &lt;&gt; 0 THEN
UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>IF-THEN-ELSE</></title>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
ELSE
<replaceable>statements</replaceable>
END IF;
</synopsis>
<para>
<literal>IF-THEN-ELSE</literal> statements add to
<literal>IF-THEN</literal> by letting you specify an
alternative set of statements that should be executed if the
condition evaluates to false.
</para>
<para>
Examples:
<programlisting>
IF parentid IS NULL OR parentid = ''''
THEN
RETURN fullname;
ELSE
RETURN hp_true_filename(parentid) || ''/'' || fullname;
END IF;
</programlisting>
<programlisting>
IF v_count > 0 THEN
INSERT INTO users_count (count) VALUES (v_count);
RETURN ''t'';
ELSE
RETURN ''f'';
END IF;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>IF-THEN-ELSE IF</></title>
<para>
<literal>IF</literal> statements can be nested, as in the
following example:
<programlisting>
IF demo_row.sex = ''m'' THEN
pretty_sex := ''man'';
ELSE
IF demo_row.sex = ''f'' THEN
pretty_sex := ''woman'';
END IF;
END IF;
</programlisting>
</para>
<para>
When you use this form, you are actually nesting an
<literal>IF</literal> statement inside the
<literal>ELSE</literal> part of an outer <literal>IF</literal>
statement. Thus you need one <literal>END IF</literal>
statement for each nested <literal>IF</literal> and one for the parent
<literal>IF-ELSE</literal>. This is workable but grows
tedious when there are many alternatives to be checked.
Hence the next form.
</para>
</sect3>
<sect3>
<title><literal>IF-THEN-ELSIF-ELSE</></title>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
...
</optional>
</optional>
<optional> ELSE
<replaceable>statements</replaceable> </optional>
END IF;
</synopsis>
<para>
<literal>IF-THEN-ELSIF-ELSE</> provides a more convenient
method of checking many alternatives in one statement.
Formally it is equivalent to nested
<literal>IF-THEN-ELSE-IF-THEN</> commands, but only one
<literal>END IF</> is needed.
</para>
<para>
Here is an example:
<programlisting>
IF number = 0 THEN
result := ''zero'';
ELSIF number &gt; 0 THEN
result := ''positive'';
ELSIF number &lt; 0 THEN
result := ''negative'';
ELSE
-- hmm, the only other possibility is that number is null
result := ''NULL'';
END IF;
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-control-structures-loops">
<title>Simple Loops</title>
<para>
With the <literal>LOOP</>, <literal>EXIT</>, <literal>WHILE</>,
and <literal>FOR</> statements, you can arrange for your
<application>PL/pgSQL</application> function to repeat a series
of commands.
</para>
<sect3>
<title><literal>LOOP</></title>
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
LOOP
<replaceable>statements</replaceable>
END LOOP;
</synopsis>
<para>
<literal>LOOP</> defines an unconditional loop that is repeated indefinitely
until terminated by an <literal>EXIT</> or <command>RETURN</command>
statement. The optional label can be used by <literal>EXIT</> statements in
nested loops to specify which level of nesting should be
terminated.
</para>
</sect3>
<sect3>
<title><literal>EXIT</></title>
<synopsis>
EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>expression</replaceable> </optional>;
</synopsis>
<para>
If no <replaceable>label</replaceable> is given,
the innermost loop is terminated and the
statement following <literal>END LOOP</> is executed next.
If <replaceable>label</replaceable> is given, it
must be the label of the current or some outer level of nested loop
or block. Then the named loop or block is terminated and control
continues with the statement after the loop's/block's corresponding
<literal>END</>.
</para>
<para>
If <literal>WHEN</> is present, loop exit occurs only if the specified condition
is true, otherwise control passes to the statement after <literal>EXIT</>.
</para>
<para>
Examples:
<programlisting>
LOOP
-- some computations
IF count > 0 THEN
EXIT; -- exit loop
END IF;
END LOOP;
LOOP
-- some computations
EXIT WHEN count > 0;
END LOOP;
BEGIN
-- some computations
IF stocks > 100000 THEN
EXIT; -- invalid; cannot use EXIT outside of LOOP
END IF;
END;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>WHILE</></title>
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
WHILE <replaceable>expression</replaceable> LOOP
<replaceable>statements</replaceable>
END LOOP;
</synopsis>
<para>
The <literal>WHILE</> statement repeats a
sequence of statements so long as the condition expression
evaluates to true. The condition is checked just before
each entry to the loop body.
</para>
<para>
For example:
<programlisting>
WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
-- some computations here
END LOOP;
WHILE NOT boolean_expression LOOP
-- some computations here
END LOOP;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>FOR</> (integer variant)</title>
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>name</replaceable> IN <optional> REVERSE </optional> <replaceable>expression</replaceable> .. <replaceable>expression</replaceable> LOOP
<replaceable>statements</replaceable>
END LOOP;
</synopsis>
<para>
This form of <literal>FOR</> creates a loop that iterates over a range of integer
values. The variable
<replaceable>name</replaceable> is automatically defined as type
<type>integer</> and exists only inside the loop. The two expressions giving
the lower and upper bound of the range are evaluated once when entering
the loop. The iteration step is normally 1, but is -1 when <literal>REVERSE</> is
specified.
</para>
<para>
Some examples of integer <literal>FOR</> loops:
<programlisting>
FOR i IN 1..10 LOOP
-- some expressions here
RAISE NOTICE ''i is %'', i;
END LOOP;
FOR i IN REVERSE 10..1 LOOP
-- some expressions here
END LOOP;
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-records-iterating">
<title>Looping Through Query Results</title>
<para>
Using a different type of <literal>FOR</> loop, you can iterate through
the results of a query and manipulate that data
accordingly. The syntax is:
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>record_or_row</replaceable> IN <replaceable>query</replaceable> LOOP
<replaceable>statements</replaceable>
END LOOP;
</synopsis>
The record or row variable is successively assigned each row
resulting from the query (a <command>SELECT</command> command) and the loop
body is executed for each row. Here is an example:
<programlisting>
CREATE FUNCTION cs_refresh_mviews() RETURNS integer AS '
DECLARE
mviews RECORD;
BEGIN
PERFORM cs_log(''Refreshing materialized views...'');
FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP
-- Now "mviews" has one record from cs_materialized_views
PERFORM cs_log(''Refreshing materialized view '' || quote_ident(mviews.mv_name) || ''...'');
EXECUTE ''TRUNCATE TABLE '' || quote_ident(mviews.mv_name);
EXECUTE ''INSERT INTO '' || quote_ident(mviews.mv_name) || '' '' || mviews.mv_query;
END LOOP;
PERFORM cs_log(''Done refreshing materialized views.'');
RETURN 1;
END;
' LANGUAGE plpgsql;
</programlisting>
If the loop is terminated by an <literal>EXIT</> statement, the last
assigned row value is still accessible after the loop.
</para>
<para>
The <literal>FOR-IN-EXECUTE</> statement is another way to iterate over
records:
<synopsis>
<optional>&lt;&lt;<replaceable>label</replaceable>&gt;&gt;</optional>
FOR <replaceable>record_or_row</replaceable> IN EXECUTE <replaceable>text_expression</replaceable> LOOP
<replaceable>statements</replaceable>
END LOOP;
</synopsis>
This is like the previous form, except that the source
<command>SELECT</command> statement is specified as a string
expression, which is evaluated and replanned on each entry to
the <literal>FOR</> loop. This allows the programmer to choose the speed of
a preplanned query or the flexibility of a dynamic query, just
as with a plain <command>EXECUTE</command> statement.
</para>
<note>
<para>
The <application>PL/pgSQL</> parser presently distinguishes the
two kinds of <literal>FOR</> loops (integer or query result) by checking
whether the target variable mentioned just after <literal>FOR</> has been
declared as a record or row variable. If not, it's presumed to be
an integer <literal>FOR</> loop. This can cause rather nonintuitive error
messages when the true problem is, say, that one has
misspelled the variable name after the <literal>FOR</>.
</para>
</note>
</sect2>
</sect1>
<sect1 id="plpgsql-cursors">
<title>Cursors</title>
<para>
Rather than executing a whole query at once, it is possible to set
up a <firstterm>cursor</> that encapsulates the query, and then read
the query result a few rows at a time. One reason for doing this is
to avoid memory overrun when the result contains a large number of
rows. (However, <application>PL/pgSQL</> users do not normally need
to worry about that, since <literal>FOR</> loops automatically use a cursor
internally to avoid memory problems.) A more interesting usage is to
return a reference to a cursor that it has created, allowing the
caller to read the rows. This provides an efficient way to return
large row sets from functions.
</para>
<sect2 id="plpgsql-cursor-declarations">
<title>Declaring Cursor Variables</title>
<para>
All access to cursors in <application>PL/pgSQL</> goes through
cursor variables, which are always of the special data type
<type>refcursor</>. One way to create a cursor variable
is just to declare it as a variable of type <type>refcursor</>.
Another way is to use the cursor declaration syntax,
which in general is:
<synopsis>
<replaceable>name</replaceable> CURSOR <optional> ( <replaceable>arguments</replaceable> ) </optional> FOR <replaceable>query</replaceable> ;
</synopsis>
(<literal>FOR</> may be replaced by <literal>IS</> for
<productname>Oracle</productname> compatibility.)
<replaceable>arguments</replaceable>, if specified, is a
comma-separated list of pairs <literal><replaceable>name</replaceable>
<replaceable>datatype</replaceable></literal> that define names to be
replaced by parameter values in the given query. The actual
values to substitute for these names will be specified later,
when the cursor is opened.
</para>
<para>
Some examples:
<programlisting>
DECLARE
curs1 refcursor;
curs2 CURSOR FOR SELECT * FROM tenk1;
curs3 CURSOR (key integer) IS SELECT * FROM tenk1 WHERE unique1 = key;
</programlisting>
All three of these variables have the data type <type>refcursor</>,
but the first may be used with any query, while the second has
a fully specified query already <firstterm>bound</> to it, and the last
has a parameterized query bound to it. (<literal>key</> will be
replaced by an integer parameter value when the cursor is opened.)
The variable <literal>curs1</>
is said to be <firstterm>unbound</> since it is not bound to
any particular query.
</para>
</sect2>
<sect2 id="plpgsql-cursor-opening">
<title>Opening Cursors</title>
<para>
Before a cursor can be used to retrieve rows, it must be
<firstterm>opened</>. (This is the equivalent action to the SQL
command <command>DECLARE CURSOR</>.) <application>PL/pgSQL</> has
three forms of the <command>OPEN</> statement, two of which use unbound cursor
variables and the other uses a bound cursor variable.
</para>
<sect3>
<title><command>OPEN FOR SELECT</command></title>
<synopsis>
OPEN <replaceable>unbound-cursor</replaceable> FOR SELECT ...;
</synopsis>
<para>
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor (that is, as a simple
<type>refcursor</> variable). The <command>SELECT</command> query
is treated in the same way as other <command>SELECT</command>
statements in <application>PL/pgSQL</>: <application>PL/pgSQL</>
variable names are substituted, and the query plan is cached for
possible reuse.
</para>
<para>
An example:
<programlisting>
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
</programlisting>
</para>
</sect3>
<sect3>
<title><command>OPEN FOR EXECUTE</command></title>
<synopsis>
OPEN <replaceable>unbound-cursor</replaceable> FOR EXECUTE <replaceable class="command">query-string</replaceable>;
</synopsis>
<para>
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor (that is, as a simple
<type>refcursor</> variable). The query is specified as a string
expression in the same way as in the <command>EXECUTE</command>
command. As usual, this gives flexibility so the query can vary
from one run to the next.
</para>
<para>
An example:
<programlisting>
OPEN curs1 FOR EXECUTE ''SELECT * FROM '' || quote_ident($1);
</programlisting>
</para>
</sect3>
<sect3>
<title>Opening a Bound Cursor</title>
<synopsis>
OPEN <replaceable>bound-cursor</replaceable> <optional> ( <replaceable>argument_values</replaceable> ) </optional>;
</synopsis>
<para>
This form of <command>OPEN</command> is used to open a cursor
variable whose query was bound to it when it was declared. The
cursor cannot be open already. A list of actual argument value
expressions must appear if and only if the cursor was declared to
take arguments. These values will be substituted in the query.
The query plan for a bound cursor is always considered cacheable;
there is no equivalent of <command>EXECUTE</command> in this case.
</para>
<para>
Examples:
<programlisting>
OPEN curs2;
OPEN curs3(42);
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-cursor-using">
<title>Using Cursors</title>
<para>
Once a cursor has been opened, it can be manipulated with the
statements described here.
</para>
<para>
These manipulations need not occur in the same function that
opened the cursor to begin with. You can return a <type>refcursor</>
value out of a function and let the caller operate on the cursor.
(Internally, a <type>refcursor</> value is simply the string name
of a so-called portal containing the active query for the cursor. This name
can be passed around, assigned to other <type>refcursor</> variables,
and so on, without disturbing the portal.)
</para>
<para>
All portals are implicitly closed at transaction end. Therefore
a <type>refcursor</> value is usable to reference an open cursor
only until the end of the transaction.
</para>
<sect3>
<title><literal>FETCH</></title>
<synopsis>
FETCH <replaceable>cursor</replaceable> INTO <replaceable>target</replaceable>;
</synopsis>
<para>
<command>FETCH</command> retrieves the next row from the
cursor into a target, which may be a row variable, a record
variable, or a comma-separated list of simple variables, just like
<command>SELECT INTO</command>. As with <command>SELECT
INTO</command>, the special variable <literal>FOUND</literal> may
be checked to see whether a row was obtained or not.
</para>
<para>
An example:
<programlisting>
FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>CLOSE</></title>
<synopsis>
CLOSE <replaceable>cursor</replaceable>;
</synopsis>
<para>
<command>CLOSE</command> closes the Portal underlying an open
cursor. This can be used to release resources earlier than end of
transaction, or to free up the cursor variable to be opened again.
</para>
<para>
An example:
<programlisting>
CLOSE curs1;
</programlisting>
</para>
</sect3>
<sect3>
<title>Returning Cursors</title>
<para>
<application>PL/pgSQL</> functions can return cursors to the
caller. This is used to return multiple rows or columns from
the function. To do this, the function opens the cursor and returns the
cursor name to the caller. The caller can then
fetch rows from the cursor. The cursor can
be closed by the caller, or it will be closed automatically
when the transaction closes.
</para>
<para>
The cursor name returned by the function can be specified by the
caller or automatically generated. The following example shows
how a cursor name can be supplied by the caller:
<programlisting>
CREATE TABLE test (col text);
INSERT INTO test VALUES ('123');
CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
BEGIN
OPEN $1 FOR SELECT col FROM test;
RETURN $1;
END;
' LANGUAGE plpgsql;
BEGIN;
SELECT reffunc('funccursor');
FETCH ALL IN funccursor;
COMMIT;
</programlisting>
</para>
<para>
The following example uses automatic cursor name generation:
<programlisting>
CREATE FUNCTION reffunc2() RETURNS refcursor AS '
DECLARE
ref refcursor;
BEGIN
OPEN ref FOR SELECT col FROM test;
RETURN ref;
END;
' LANGUAGE plpgsql;
BEGIN;
SELECT reffunc2();
reffunc2
--------------------
&lt;unnamed cursor 1&gt;
(1 row)
FETCH ALL IN "&lt;unnamed cursor 1&gt;";
COMMIT;
</programlisting>
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="plpgsql-errors-and-messages">
<title>Errors and Messages</title>
<para>
Use the <command>RAISE</command> statement to report messages and
raise errors.
<synopsis>
RAISE <replaceable class="parameter">level</replaceable> '<replaceable class="parameter">format</replaceable>' <optional>, <replaceable class="parameter">variable</replaceable> <optional>, ...</optional></optional>;
</synopsis>
Possible levels are <literal>DEBUG</literal> (write the message to
the server log), <literal>LOG</literal> (write the message to the
server log with a higher priority), <literal>INFO</literal>,
<literal>NOTICE</literal> and <literal>WARNING</literal> (write
the message to the server log and send it to the client, with
respectively higher priorities), and <literal>EXCEPTION</literal>
(raise an error and abort the current transaction). Whether
messages of a particular priority are reported to the client,
written to the server log, or both is controlled by the
<option>log_min_messages</option> and
<option>client_min_messages</option> configuration variables. See
<xref linkend="runtime-config"> for more information.
</para>
<para>
Inside the format string, <literal>%</literal> is replaced by the
next optional argument's string representation. Write
<literal>%%</literal> to emit a literal <literal>%</literal>. Note
that the optional arguments must presently be simple variables,
not expressions, and the format must be a simple string literal.
</para>
<!--
This example should work, but does not:
RAISE NOTICE ''Id number '' || key || '' not found!'';
Put it back when we allow non-string-literal formats.
-->
<para>
In this example, the value of <literal>v_job_id</> will replace the
<literal>%</literal> in the string.
<programlisting>
RAISE NOTICE ''Calling cs_create_job(%)'', v_job_id;
</programlisting>
</para>
<para>
This example will abort the transaction with the given error message.
<programlisting>
RAISE EXCEPTION ''Inexistent ID --> %'', user_id;
</programlisting>
</para>
<para>
<productname>PostgreSQL</productname> does not have a very smart
exception handling model. Whenever the parser, planner/optimizer
or executor decide that a statement cannot be processed any longer,
the whole transaction gets aborted and the system jumps back
into the main loop to get the next command from the client application.
</para>
<para>
It is possible to hook into the error mechanism to notice that this
happens. But currently it is impossible to tell what really
caused the abort (data type format error, floating-point
error, parse error, etc.). And it is possible that the database server
is in an inconsistent state at this point so returning to the upper
executor or issuing more commands might corrupt the whole database.
</para>
<para>
Thus, the only thing <application>PL/pgSQL</application>
currently does when it encounters an abort during execution of a
function or trigger procedure is to write some additional
<literal>NOTICE</literal> level log messages telling in which
function and where (line number and type of statement) this
happened. The error always stops execution of the function.
</para>
</sect1>
<sect1 id="plpgsql-trigger">
<title>Trigger Procedures</title>
<para>
<application>PL/pgSQL</application> can be used to define trigger
procedures. A trigger procedure is created with the
<command>CREATE FUNCTION</> command as a function with no
arguments and a return type of <type>trigger</type>. Note that
the function must be declared with no arguments even if it expects
to receive arguments specified in <command>CREATE TRIGGER</> ---
trigger arguments are passed via <varname>TG_ARGV</>, as described
below.
</para>
<para>
When a <application>PL/pgSQL</application> function is called as a
trigger, several special variables are created automatically in the
top-level block. They are:
<variablelist>
<varlistentry>
<term><varname>NEW</varname></term>
<listitem>
<para>
Data type <type>RECORD</type>; variable holding the new
database row for <command>INSERT</>/<command>UPDATE</> operations in row-level
triggers. This variable is null in statement-level triggers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>OLD</varname></term>
<listitem>
<para>
Data type <type>RECORD</type>; variable holding the old
database row for <command>UPDATE</>/<command>DELETE</> operations in row-level
triggers. This variable is null in statement-level triggers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_NAME</varname></term>
<listitem>
<para>
Data type <type>name</type>; variable that contains the name of the trigger actually
fired.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_WHEN</varname></term>
<listitem>
<para>
Data type <type>text</type>; a string of either
<literal>BEFORE</literal> or <literal>AFTER</literal>
depending on the trigger's definition.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_LEVEL</varname></term>
<listitem>
<para>
Data type <type>text</type>; a string of either
<literal>ROW</literal> or <literal>STATEMENT</literal>
depending on the trigger's definition.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_OP</varname></term>
<listitem>
<para>
Data type <type>text</type>; a string of
<literal>INSERT</literal>, <literal>UPDATE</literal>, or
<literal>DELETE</literal> telling for which operation the
trigger was fired.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_RELID</varname></term>
<listitem>
<para>
Data type <type>oid</type>; the object ID of the table that caused the
trigger invocation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_RELNAME</varname></term>
<listitem>
<para>
Data type <type>name</type>; the name of the table that caused the trigger
invocation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_NARGS</varname></term>
<listitem>
<para>
Data type <type>integer</type>; the number of arguments given to the trigger
procedure in the <command>CREATE TRIGGER</command> statement.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_ARGV[]</varname></term>
<listitem>
<para>
Data type array of <type>text</type>; the arguments from
the <command>CREATE TRIGGER</command> statement.
The index counts from 0. Invalid
indices (less than 0 or greater than or equal to <varname>tg_nargs</>) result in a null value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
A trigger function must return either null or a record/row value
having exactly the structure of the table the trigger was fired
for. The return value of a <literal>BEFORE</> or <literal>AFTER</> statement-level
trigger or an <literal>AFTER</> row-level trigger is ignored; it may as well
be null. However, any of these types of triggers can still
abort the entire trigger operation by raising an error.
</para>
<para>
Row-level triggers fired <literal>BEFORE</> may return null to signal the
trigger manager to skip the rest of the operation for this row
(i.e., subsequent triggers are not fired, and the
<command>INSERT</>/<command>UPDATE</>/<command>DELETE</> does not occur for this row). If a nonnull
value is returned then the operation proceeds with that row value.
Returning a row value different from the original value
of <varname>NEW</> alters the row that will be inserted or updated. It is
possible to replace single values directly in <varname>NEW</> and return <varname>NEW</>,
or to build a complete new record/row to return.
</para>
<para>
<xref linkend="plpgsql-trigger-example"> shows an example of a
trigger procedure in <application>PL/pgSQL</application>.
</para>
<example id="plpgsql-trigger-example">
<title>A <application>PL/pgSQL</application> Trigger Procedure</title>
<para>
This example trigger ensures that any time a row is inserted or updated
in the table, the current user name and time are stamped into the
row. And it ensures that an employee's name is given and that the
salary is a positive value.
</para>
<programlisting>
CREATE TABLE emp (
empname text,
salary integer,
last_date timestamp,
last_user text
);
CREATE FUNCTION emp_stamp() RETURNS trigger AS '
BEGIN
-- Check that empname and salary are given
IF NEW.empname IS NULL THEN
RAISE EXCEPTION ''empname cannot be null'';
END IF;
IF NEW.salary IS NULL THEN
RAISE EXCEPTION ''% cannot have null salary'', NEW.empname;
END IF;
-- Who works for us when she must pay for it?
IF NEW.salary &lt; 0 THEN
RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname;
END IF;
-- Remember who changed the payroll when
NEW.last_date := ''now'';
NEW.last_user := current_user;
RETURN NEW;
END;
' LANGUAGE plpgsql;
CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
</programlisting>
</example>
</sect1>
<!-- **** Porting from Oracle PL/SQL **** -->
<sect1 id="plpgsql-porting">
<title>Porting from <productname>Oracle</productname> PL/SQL</title>
<indexterm zone="plpgsql-porting">
<primary>Oracle</primary>
</indexterm>
<indexterm zone="plpgsql-porting">
<primary>PL/SQL</primary>
</indexterm>
<para>
This section explains differences between
<productname>PostgreSQL</>'s <application>PL/pgSQL</application>
language and Oracle's <application>PL/SQL</application> language,
to help developers that port applications from Oracle to
<productname>PostgreSQL</>.
</para>
<para>
<application>PL/pgSQL</application> is similar to PL/SQL in many
aspects. It is a block-structured, imperative language, and all
variables have to be declared. Assignments, loops, conditionals
are similar. The main differences you should keep in mind when
porting from <application>PL/SQL</> to
<application>PL/pgSQL</application> are:
<itemizedlist>
<listitem>
<para>
There are no default values for parameters in <productname>PostgreSQL</>.
</para>
</listitem>
<listitem>
<para>
You can overload functions in <productname>PostgreSQL</>. This is often used to work
around the lack of default parameters.
</para>
</listitem>
<listitem>
<para>
No need for cursors in <application>PL/pgSQL</>, just put the
query in the <literal>FOR</literal> statement. (See <xref
linkend="plpgsql-porting-ex2">.)
</para>
</listitem>
<listitem>
<para>
In <productname>PostgreSQL</> you need to escape single
quotes in the function body. See <xref linkend="plpgsql-quote">.
</para>
</listitem>
<listitem>
<para>
Instead of packages, use schemas to organize your functions
into groups.
</para>
</listitem>
</itemizedlist>
</para>
<sect2>
<title>Porting Examples</title>
<para>
<xref linkend="pgsql-porting-ex1"> shows how to port a simple
function from <application>PL/SQL</> to <application>PL/pgSQL</>.
</para>
<example id="pgsql-porting-ex1">
<title>Porting a Simple Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>
<para>
Here is an <productname>Oracle</productname> <application>PL/SQL</> function:
<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name IN varchar, v_version IN varchar)
RETURN varchar IS
BEGIN
IF v_version IS NULL THEN
RETURN v_name;
END IF;
RETURN v_name || '/' || v_version;
END;
/
show errors;
</programlisting>
</para>
<para>
Let's go through this function and see the differences to <application>PL/pgSQL</>:
<itemizedlist>
<listitem>
<para>
<productname>PostgreSQL</productname> does not have named
parameters. You have to explicitly alias them inside your
function.
</para>
</listitem>
<listitem>
<para>
<productname>Oracle</productname> can have
<literal>IN</literal>, <literal>OUT</literal>, and
<literal>INOUT</literal> parameters passed to functions.
<literal>INOUT</literal>, for example, means that the
parameter will receive a value and return
another. <productname>PostgreSQL</> only has <literal>IN</literal>
parameters.
</para>
</listitem>
<listitem>
<para>
The <literal>RETURN</literal> key word in the function
prototype (not the function body) becomes
<literal>RETURNS</literal> in PostgreSQL.
</para>
</listitem>
<listitem>
<para>
In <productname>PostgreSQL</>, functions are created using
single quotes as the delimiters of the function body, so you
have to escape single quotes inside the function body.
</para>
</listitem>
<listitem>
<para>
The <literal>/show errors</literal> command does not exist in
<productname>PostgreSQL</>.
</para>
</listitem>
</itemizedlist>
</para>
<para>
This is how this function would look when ported to
<productname>PostgreSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(varchar, varchar)
RETURNS varchar AS '
DECLARE
v_name ALIAS FOR $1;
v_version ALIAS FOR $2;
BEGIN
IF v_version IS NULL THEN
return v_name;
END IF;
RETURN v_name || ''/'' || v_version;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</example>
<para>
<xref linkend="plpgsql-porting-ex2"> shows how to port a
function that creates another function and how to handle to
ensuing quoting problems.
</para>
<example id="plpgsql-porting-ex2">
<title>Porting a Function that Creates Another Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>
<para>
The following procedure grabs rows from a
<command>SELECT</command> statement and builds a large function
with the results in <literal>IF</literal> statements, for the
sake of efficiency. Notice particularly the differences in the
cursor and the <literal>FOR</literal> loop,
</para>
<para>
This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
CURSOR referrer_keys IS
SELECT * FROM cs_referrer_keys
ORDER BY try_order;
a_output VARCHAR(4000);
BEGIN
a_output := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR, v_domain IN VARCHAR,
v_url IN VARCHAR) RETURN VARCHAR IS BEGIN';
FOR referrer_key IN referrer_keys LOOP
a_output := a_output || ' IF v_' || referrer_key.kind || ' LIKE ''' ||
referrer_key.key_string || ''' THEN RETURN ''' || referrer_key.referrer_type ||
'''; END IF;';
END LOOP;
a_output := a_output || ' RETURN NULL; END;';
EXECUTE IMMEDIATE a_output;
END;
/
show errors;
</programlisting>
</para>
<para>
Here is how this function would end up in <productname>PostgreSQL</>:
<programlisting>
CREATE FUNCTION cs_update_referrer_type_proc() RETURNS integer AS '
DECLARE
referrer_keys RECORD; -- Declare a generic record to be used in a FOR
a_output varchar(4000);
BEGIN
a_output := ''CREATE FUNCTION cs_find_referrer_type(varchar, varchar, varchar)
RETURNS varchar AS ''''
DECLARE
v_host ALIAS FOR $1;
v_domain ALIAS FOR $2;
v_url ALIAS FOR $3;
BEGIN '';
-- Notice how we scan through the results of a query in a FOR loop
-- using the FOR &lt;record&gt; construct.
FOR referrer_keys IN SELECT * FROM cs_referrer_keys ORDER BY try_order LOOP
a_output := a_output || '' IF v_'' || referrer_keys.kind || '' LIKE ''''''''''
|| referrer_keys.key_string || '''''''''' THEN RETURN ''''''
|| referrer_keys.referrer_type || ''''''; END IF;'';
END LOOP;
a_output := a_output || '' RETURN NULL; END; '''' LANGUAGE plpgsql;'';
-- EXECUTE will work because we are not substituting any variables.
-- Otherwise it would fail. Look at PERFORM for another way to run functions.
EXECUTE a_output;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</example>
<para>
<xref linkend="plpgsql-porting-ex3"> shows how to port a function
with <literal>OUT</> parameters and string manipulation.
<productname>PostgreSQL</> does not have an
<function>instr</function> function, but you can work around it
using a combination of other functions. In <xref
linkend="plpgsql-porting-appendix"> there is a
<application>PL/pgSQL</application> implementation of
<function>instr</function> that you can use to make your porting
easier.
</para>
<example id="plpgsql-porting-ex3">
<title>Porting a Procedure With String Manipulation and
<literal>OUT</> Parameters from <application>PL/SQL</> to
<application>PL/pgSQL</></title>
<para>
The following <productname>Oracle</productname> PL/SQL procedure is used to parse a URL and
return several elements (host, path, and query).
<application>PL/pgSQL</application> functions can return only one value. In
<productname>PostgreSQL</>, one way to work around this is to split the procedure
in three different functions: one to return the host, another for
the path, and another for the query.
</para>
<para>
This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_parse_url(
v_url IN VARCHAR,
v_host OUT VARCHAR, -- This will be passed back
v_path OUT VARCHAR, -- This one too
v_query OUT VARCHAR) -- And this one
IS
a_pos1 INTEGER;
a_pos2 INTEGER;
BEGIN
v_host := NULL;
v_path := NULL;
v_query := NULL;
a_pos1 := instr(v_url, '//');
IF a_pos1 = 0 THEN
RETURN;
END IF;
a_pos2 := instr(v_url, '/', a_pos1 + 2);
IF a_pos2 = 0 THEN
v_host := substr(v_url, a_pos1 + 2);
v_path := '/';
RETURN;
END IF;
v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
a_pos1 := instr(v_url, '?', a_pos2 + 1);
IF a_pos1 = 0 THEN
v_path := substr(v_url, a_pos2);
RETURN;
END IF;
v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;
</programlisting>
</para>
<para>
Here is how the <application>PL/pgSQL</> function that returns
the host part could look like:
<programlisting>
CREATE OR REPLACE FUNCTION cs_parse_url_host(varchar) RETURNS varchar AS '
DECLARE
v_url ALIAS FOR $1;
v_host varchar;
v_path varchar;
a_pos1 integer;
a_pos2 integer;
a_pos3 integer;
BEGIN
v_host := NULL;
a_pos1 := instr(v_url, ''//'');
IF a_pos1 = 0 THEN
RETURN ''''; -- Return a blank
END IF;
a_pos2 := instr(v_url,''/'',a_pos1 + 2);
IF a_pos2 = 0 THEN
v_host := substr(v_url, a_pos1 + 2);
v_path := ''/'';
RETURN v_host;
END IF;
v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2 );
RETURN v_host;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</example>
<para>
<xref linkend="plpgsql-porting-ex4"> shows how to port a procedure
that uses numerous features that are specific to Oracle.
</para>
<example id="plpgsql-porting-ex4">
<title>Porting a Procedure from <application>PL/SQL</> to <application>PL/pgSQL</></title>
<para>
The Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
a_running_job_count INTEGER;
PRAGMA AUTONOMOUS_TRANSACTION;<co id="co.plpgsql-porting-pragma">
BEGIN
LOCK TABLE cs_jobs IN EXCLUSIVE MODE;<co id="co.plpgsql-porting-locktable">
SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;
IF a_running_job_count > 0 THEN
COMMIT; -- free lock<co id="co.plpgsql-porting-commit">
raise_application_error(-20000, 'Unable to create a new job: a job is currently running.');
END IF;
DELETE FROM cs_active_job;
INSERT INTO cs_active_job(job_id) VALUES (v_job_id);
BEGIN
INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, sysdate);
EXCEPTION WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists<co id="co.plpgsql-porting-exception">
END;
COMMIT;
END;
/
show errors
</programlisting>
</para>
<para>
Procedures like this can be easily converted into <productname>PostgreSQL</>
functions returning an <type>integer</type>. This procedure in
particular is interesting because it can teach us some things:
<calloutlist>
<callout arearefs="co.plpgsql-porting-pragma">
<para>
There is no <literal>PRAGMA</literal> statement in <productname>PostgreSQL</>.
</para>
</callout>
<callout arearefs="co.plpgsql-porting-locktable">
<para>
If you do a <command>LOCK TABLE</command> in <application>PL/pgSQL</>, the lock
will not be released until the calling transaction is finished.
</para>
</callout>
<callout arearefs="co.plpgsql-porting-commit">
<para>
You also cannot have transactions in <application>PL/pgSQL</application> functions. The
entire function (and other functions called from therein) is
executed in one transaction and <productname>PostgreSQL</> rolls back the transaction if
something goes wrong.
</para>
</callout>
<callout arearefs="co.plpgsql-porting-exception">
<para>
The exception when would have to be replaced by an
<literal>IF</literal> statement.
</para>
</callout>
</calloutlist>
</para>
<para>
This is how we could port this procedure to <application>PL/pgSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_create_job(integer) RETURNS integer AS '
DECLARE
v_job_id ALIAS FOR $1;
a_running_job_count integer;
a_num integer;
BEGIN
LOCK TABLE cs_jobs IN EXCLUSIVE MODE;
SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;
IF a_running_job_count > 0
THEN
RAISE EXCEPTION ''Unable to create a new job: a job is currently running.'';
END IF;
DELETE FROM cs_active_job;
INSERT INTO cs_active_job(job_id) VALUES (v_job_id);
SELECT count(*) INTO a_num FROM cs_jobs WHERE job_id=v_job_id;
IF NOT FOUND THEN -- If nothing was returned in the last query
-- This job is not in the table so lets insert it.
INSERT INTO cs_jobs(job_id, start_stamp) VALUES (v_job_id, current_timestamp);
RETURN 1;
ELSE
RAISE NOTICE ''Job already running.'';<co id="co.plpgsql-porting-raise">
END IF;
RETURN 0;
END;
' LANGUAGE plpgsql;
</programlisting>
<calloutlist>
<callout arearefs="co.plpgsql-porting-raise">
<para>
Notice how you can raise notices (or errors) in <application>PL/pgSQL</>.
</para>
</callout>
</calloutlist>
</para>
</example>
</sect2>
<sect2 id="plpgsql-porting-other">
<title>Other Things to Watch For</title>
<para>
This section explains a few other things to watch for when porting
Oracle <application>PL/SQL</> functions to PostgreSQL.
</para>
<sect3>
<title><command>EXECUTE</command></title>
<para>
The <application>PL/pgSQL</> version of
<command>EXECUTE</command> works similar to the
<application>PL/SQL</> version, but you have to remember to use
<function>quote_literal(text)</function> and
<function>quote_string(text)</function> as described in <xref
linkend="plpgsql-statements-executing-dyn">. Constructs of the
type <literal>EXECUTE ''SELECT * FROM $1'';</literal> will not
work unless you use these functions.
</para>
</sect3>
<sect3 id="plpgsql-porting-optimization">
<title>Optimizing <application>PL/pgSQL</application> Functions</title>
<para>
<productname>PostgreSQL</> gives you two function creation
modifiers to optimize execution: the volatility (whether the
function always returns the same result when given the same
arguments) and the <quote>strictness</quote> (whether the
function returns null if any argument is null). Consult the description of
<command>CREATE FUNCTION</command> for details.
</para>
<para>
To make use of these optimization attributes, your
<command>CREATE FUNCTION</command> statement could look something
like this:
<programlisting>
CREATE FUNCTION foo(...) RETURNS integer AS '
...
' LANGUAGE plpgsql STRICT IMMUTABLE;
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-porting-appendix">
<title>Appendix</title>
<para>
This section contains the code for an Oracle-compatible
<function>instr</function> function that you can use to simplify
your porting efforts.
</para>
<programlisting>
--
-- instr functions that mimic Oracle's counterpart
-- Syntax: instr(string1, string2, [n], [m]) where [] denotes optional parameters.
--
-- Searches string1 beginning at the nth character for the mth occurrence
-- of string2. If n is negative, search backwards. If m is not passed,
-- assume 1 (search starts at first character).
--
CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS '
DECLARE
pos integer;
BEGIN
pos:= instr($1, $2, 1);
RETURN pos;
END;
' LANGUAGE plpgsql;
CREATE FUNCTION instr(varchar, varchar, varchar) RETURNS integer AS '
DECLARE
string ALIAS FOR $1;
string_to_search ALIAS FOR $2;
beg_index ALIAS FOR $3;
pos integer NOT NULL DEFAULT 0;
temp_str varchar;
beg integer;
length integer;
ss_length integer;
BEGIN
IF beg_index > 0 THEN
temp_str := substring(string FROM beg_index);
pos := position(string_to_search IN temp_str);
IF pos = 0 THEN
RETURN 0;
ELSE
RETURN pos + beg_index - 1;
END IF;
ELSE
ss_length := char_length(string_to_search);
length := char_length(string);
beg := length + beg_index - ss_length + 2;
WHILE beg > 0 LOOP
temp_str := substring(string FROM beg FOR ss_length);
pos := position(string_to_search IN temp_str);
IF pos > 0 THEN
RETURN beg;
END IF;
beg := beg - 1;
END LOOP;
RETURN 0;
END IF;
END;
' LANGUAGE plpgsql;
CREATE FUNCTION instr(varchar, varchar, integer, integer) RETURNS integer AS '
DECLARE
string ALIAS FOR $1;
string_to_search ALIAS FOR $2;
beg_index ALIAS FOR $3;
occur_index ALIAS FOR $4;
pos integer NOT NULL DEFAULT 0;
occur_number integer NOT NULL DEFAULT 0;
temp_str varchar;
beg integer;
i integer;
length integer;
ss_length integer;
BEGIN
IF beg_index > 0 THEN
beg := beg_index;
temp_str := substring(string FROM beg_index);
FOR i IN 1..occur_index LOOP
pos := position(string_to_search IN temp_str);
IF i = 1 THEN
beg := beg + pos - 1;
ELSE
beg := beg + pos;
END IF;
temp_str := substring(string FROM beg + 1);
END LOOP;
IF pos = 0 THEN
RETURN 0;
ELSE
RETURN beg;
END IF;
ELSE
ss_length := char_length(string_to_search);
length := char_length(string);
beg := length + beg_index - ss_length + 2;
WHILE beg > 0 LOOP
temp_str := substring(string FROM beg FOR ss_length);
pos := position(string_to_search IN temp_str);
IF pos > 0 THEN
occur_number := occur_number + 1;
IF occur_number = occur_index THEN
RETURN beg;
END IF;
END IF;
beg := beg - 1;
END LOOP;
RETURN 0;
END IF;
END;
' LANGUAGE plpgsql;
</programlisting>
</sect2>
</sect1>
</chapter>
<!-- Keep this comment at the end of the file
Local variables:
mode:sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"./reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:("/usr/lib/sgml/catalog")
sgml-local-ecat-files:nil
End:
-->
View Git Blame Copy Permalink