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$Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.73 2003/08/31 17:32:20 petere Exp $
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<sect1 id="xfunc">
<title>User-Defined Functions</title>
<indexterm zone="xfunc">
<primary>function</primary>
<secondary>user-defined</secondary>
</indexterm>
<para>
<productname>PostgreSQL</productname> provides four kinds of
functions:
<itemizedlist>
<listitem>
<para>
query language functions (functions written in
<acronym>SQL</acronym>) (<xref linkend="xfunc-sql">)
</para>
</listitem>
<listitem>
<para>
procedural language functions (functions written in, for
example, <application>PL/Tcl</> or <application>PL/pgSQL</>)
(<xref linkend="xfunc-pl">)
</para>
</listitem>
<listitem>
<para>
internal functions (<xref linkend="xfunc-internal">)
</para>
</listitem>
<listitem>
<para>
C-language functions (<xref linkend="xfunc-c">)
</para>
</listitem>
</itemizedlist>
</para>
<para>
Every kind
of function can take base types, composite types, or
combinations of these as arguments (parameters). In addition,
every kind of function can return a base type or
a composite type.
</para>
<para>
Many kinds of functions can take or return certain pseudo-types
(such as polymorphic types), but the available facilities vary.
Consult the description of each kind of function for more details.
</para>
<para>
It's easiest to define <acronym>SQL</acronym>
functions, so we'll start by discussing those.
</para>
<para>
Throughout this chapter, it can be useful to look at the reference
page of the <command>CREATE FUNCTION</command> command to
understand the examples better.
Some examples from this chapter
can be found in <filename>funcs.sql</filename>
and <filename>funcs.c</filename> in the tutorial directory.
</para>
</sect1>
<sect1 id="xfunc-sql">
<title>Query Language (<acronym>SQL</acronym>) Functions</title>
<indexterm zone="xfunc-sql">
<primary>function</primary>
<secondary>user-defined</secondary>
<tertiary>in SQL</tertiary>
</indexterm>
<para>
SQL functions execute an arbitrary list of SQL statements, returning
the result of the last query in the list.
In the simple (non-set)
case, the first row of the last query's result will be returned.
(Bear in mind that <quote>the first row</quote> of a multirow
result is not well-defined unless you use <literal>ORDER BY</>.)
If the last query happens
to return no rows at all, the null value will be returned.
</para>
<para>
<indexterm><primary>SETOF</><seealso>function</></> Alternatively,
an SQL function may be declared to return a set, by specifying the
function's return type as <literal>SETOF
<replaceable>sometype</></literal>.<indexterm><primary>SETOF</></>
In this case all rows of the last query's result are returned.
Further details appear below.
</para>
<para>
The body of an SQL function should be a list of one or more SQL
statements separated by semicolons. Note that because the syntax
of the <command>CREATE FUNCTION</command> command requires the body of the
function to be enclosed in single quotes, single quote marks
(<literal>'</>) used
in the body of the function must be escaped, by writing two single
quotes (<literal>''</>) or a backslash (<literal>\'</>) where each
quote is desired.
</para>
<para>
Arguments to the SQL function may be referenced in the function
body using the syntax <literal>$<replaceable>n</></>: <literal>$1</> refers to
the first argument, <literal>$2</> to the second, and so on. If an argument
is of a composite type, then the dot notation,
e.g., <literal>$1.name</literal>, may be used to access attributes
of the argument.
</para>
<sect2>
<title><acronym>SQL</acronym> Functions on Base Types</title>
<para>
The simplest possible <acronym>SQL</acronym> function has no arguments and
simply returns a base type, such as <type>integer</type>:
<screen>
CREATE FUNCTION one() RETURNS integer AS '
SELECT 1 AS result;
' LANGUAGE SQL;
SELECT one();
one
-----
1
</screen>
</para>
<para>
Notice that we defined a column alias within the function body for the result of the function
(with the name <literal>result</>), but this column alias is not visible
outside the function. Hence, the result is labeled <literal>one</>
instead of <literal>result</>.
</para>
<para>
It is almost as easy to define <acronym>SQL</acronym> functions
that take base types as arguments. In the example below, notice
how we refer to the arguments within the function as <literal>$1</>
and <literal>$2</>.
<screen>
CREATE FUNCTION add_em(integer, integer) RETURNS integer AS '
SELECT $1 + $2;
' LANGUAGE SQL;
SELECT add_em(1, 2) AS answer;
answer
--------
3
</screen>
</para>
<para>
Here is a more useful function, which might be used to debit a
bank account:
<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS integer AS '
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1;
SELECT 1;
' LANGUAGE SQL;
</programlisting>
A user could execute this function to debit account 17 by $100.00 as
follows:
<programlisting>
SELECT tf1(17, 100.0);
</programlisting>
</para>
<para>
In practice one would probably like a more useful result from the
function than a constant 1, so a more likely definition
is
<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS numeric AS '
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1;
SELECT balance FROM bank WHERE accountno = $1;
' LANGUAGE SQL;
</programlisting>
which adjusts the balance and returns the new balance.
</para>
<para>
Any collection of commands in the <acronym>SQL</acronym>
language can be packaged together and defined as a function.
Besides <command>SELECT</command> queries,
the commands can include data modification (i.e.,
<command>INSERT</command>, <command>UPDATE</command>, and
<command>DELETE</command>). However, the final command
must be a <command>SELECT</command> that returns whatever is
specified as the function's return type. Alternatively, if you
want to define a SQL function that performs actions but has no
useful value to return, you can define it as returning <type>void</>.
In that case, the function body must not end with a <command>SELECT</command>.
For example:
<screen>
CREATE FUNCTION clean_emp() RETURNS void AS '
DELETE FROM emp
WHERE salary &lt;= 0;
' LANGUAGE SQL;
SELECT clean_emp();
clean_emp
-----------
(1 row)
</screen>
</para>
</sect2>
<sect2>
<title><acronym>SQL</acronym> Functions on Composite Types</title>
<para>
When specifying functions with arguments of composite
types, we must not only specify which
argument we want (as we did above with <literal>$1</> and <literal>$2</literal>) but
also the attributes of that argument. For example, suppose that
<type>emp</type> is a table containing employee data, and therefore
also the name of the composite type of each row of the table. Here
is a function <function>double_salary</function> that computes what someone's
salary would be if it were doubled:
<screen>
CREATE TABLE emp (
name text,
salary integer,
age integer,
cubicle point
);
CREATE FUNCTION double_salary(emp) RETURNS integer AS '
SELECT $1.salary * 2 AS salary;
' LANGUAGE SQL;
SELECT name, double_salary(emp) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';
name | dream
------+-------
Sam | 2400
</screen>
</para>
<para>
Notice the use of the syntax <literal>$1.salary</literal>
to select one field of the argument row value. Also notice
how the calling <command>SELECT</> command uses a table name to denote
the entire current row of that table as a composite value.
</para>
<para>
It is also possible to build a function that returns a composite type.
This is an example of a function
that returns a single <type>emp</type> row:
<programlisting>
CREATE FUNCTION new_emp() RETURNS emp AS '
SELECT text ''None'' AS name,
1000 AS salary,
25 AS age,
point ''(2,2)'' AS cubicle;
' LANGUAGE SQL;
</programlisting>
In this example we have specified each of the attributes
with a constant value, but any computation
could have been substituted for these constants.
</para>
<para>
Note two important things about defining the function:
<itemizedlist>
<listitem>
<para>
The select list order in the query must be exactly the same as
that in which the columns appear in the table associated
with the composite type. (Naming the columns, as we did above,
is irrelevant to the system.)
</para>
</listitem>
<listitem>
<para>
You must typecast the expressions to match the
definition of the composite type, or you will get errors like this:
<screen>
<computeroutput>
ERROR: function declared to return emp returns varchar instead of text at column 1
</computeroutput>
</screen>
</para>
</listitem>
</itemizedlist>
</para>
<para>
A function that returns a row (composite type) can be used as a table
function, as described below. It can also be called in the context
of an SQL expression, but only when you
extract a single attribute out of the row or pass the entire row into
another function that accepts the same composite type.
</para>
<para>
This is an example of extracting an attribute out of a row type:
<screen>
SELECT (new_emp()).name;
name
------
None
</screen>
We need the extra parentheses to keep the parser from getting confused:
<screen>
SELECT new_emp().name;
ERROR: syntax error at or near "." at character 17
</screen>
</para>
<para>
Another option is to use
functional notation for extracting an attribute. The simple way
to explain this is that we can use the
notations <literal>attribute(table)</> and <literal>table.attribute</>
interchangeably.
<screen>
SELECT name(new_emp());
name
------
None
</screen>
<screen>
-- This is the same as:
-- SELECT emp.name AS youngster FROM emp WHERE emp.age &lt; 30
SELECT name(emp) AS youngster
FROM emp
WHERE age(emp) &lt; 30;
youngster
-----------
Sam
</screen>
</para>
<para>
The other way to use a function returning a row result is to declare a
second function accepting a row type argument and pass the
result of the first function to it:
<screen>
CREATE FUNCTION getname(emp) RETURNS text AS
'SELECT $1.name;'
LANGUAGE SQL;
SELECT getname(new_emp());
getname
---------
None
(1 row)
</screen>
</para>
</sect2>
<sect2>
<title><acronym>SQL</acronym> Functions as Table Sources</title>
<para>
All SQL functions may be used in the <literal>FROM</> clause of a query,
but it is particularly useful for functions returning composite types.
If the function is defined to return a base type, the table function
produces a one-column table. If the function is defined to return
a composite type, the table function produces a column for each attribute
of the composite type.
</para>
<para>
Here is an example:
<screen>
CREATE TABLE foo (fooid int, foosubid int, fooname text);
INSERT INTO foo VALUES (1, 1, 'Joe');
INSERT INTO foo VALUES (1, 2, 'Ed');
INSERT INTO foo VALUES (2, 1, 'Mary');
CREATE FUNCTION getfoo(int) RETURNS foo AS '
SELECT * FROM foo WHERE fooid = $1;
' LANGUAGE SQL;
SELECT *, upper(fooname) FROM getfoo(1) AS t1;
fooid | foosubid | fooname | upper
-------+----------+---------+-------
1 | 1 | Joe | JOE
(2 rows)
</screen>
As the example shows, we can work with the columns of the function's
result just the same as if they were columns of a regular table.
</para>
<para>
Note that we only got one row out of the function. This is because
we did not use <literal>SETOF</>. This is described in the next section.
</para>
</sect2>
<sect2>
<title><acronym>SQL</acronym> Functions Returning Sets</title>
<para>
When an SQL function is declared as returning <literal>SETOF
<replaceable>sometype</></literal>, the function's final
<command>SELECT</> query is executed to completion, and each row it
outputs is returned as an element of the result set.
</para>
<para>
This feature is normally used when calling the function in the <literal>FROM</>
clause. In this case each row returned by the function becomes
a row of the table seen by the query. For example, assume that
table <literal>foo</> has the same contents as above, and we say:
<programlisting>
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS '
SELECT * FROM foo WHERE fooid = $1;
' LANGUAGE SQL;
SELECT * FROM getfoo(1) AS t1;
</programlisting>
Then we would get:
<screen>
fooid | foosubid | fooname
-------+----------+---------
1 | 1 | Joe
1 | 2 | Ed
(2 rows)
</screen>
</para>
<para>
Currently, functions returning sets may also be called in the select list
of a query. For each row that the query
generates by itself, the function returning set is invoked, and an output
row is generated for each element of the function's result set. Note,
however, that this capability is deprecated and may be removed in future
releases. The following is an example function returning a set from the
select list:
<screen>
CREATE FUNCTION listchildren(text) RETURNS SETOF text AS
'SELECT name FROM nodes WHERE parent = $1'
LANGUAGE SQL;
SELECT * FROM nodes;
name | parent
-----------+--------
Top |
Child1 | Top
Child2 | Top
Child3 | Top
SubChild1 | Child1
SubChild2 | Child1
(6 rows)
SELECT listchildren('Top');
listchildren
--------------
Child1
Child2
Child3
(3 rows)
SELECT name, listchildren(name) FROM nodes;
name | listchildren
--------+--------------
Top | Child1
Top | Child2
Top | Child3
Child1 | SubChild1
Child1 | SubChild2
(5 rows)
</screen>
In the last <command>SELECT</command>,
notice that no output row appears for <literal>Child2</>, <literal>Child3</>, etc.
This happens because <function>listchildren</function> returns an empty set
for those arguments, so no result rows are generated.
</para>
</sect2>
<sect2>
<title>Polymorphic <acronym>SQL</acronym> Functions</title>
<para>
<acronym>SQL</acronym> functions may be declared to accept and
return the <quote>polymorphic</> types
<type>anyelement</type> and <type>anyarray</type>.
See <xref linkend="extend-types-polymorphic"> for a more detailed explanation
of polymorphic functions. Here is a polymorphic function
<function>make_array</function> that builds up an array from two
arbitrary data type elements:
<screen>
CREATE FUNCTION make_array(anyelement, anyelement) RETURNS anyarray AS '
SELECT ARRAY[$1, $2];
' LANGUAGE SQL;
SELECT make_array(1, 2) AS intarray, make_array('a'::text, 'b') AS textarray;
intarray | textarray
----------+-----------
{1,2} | {a,b}
(1 row)
</screen>
</para>
<para>
Notice the use of the typecast <literal>'a'::text</literal>
to specify that the argument is of type <type>text</type>. This is
required if the argument is just a string literal, since otherwise
it would be treated as type
<type>unknown</type>, and array of <type>unknown</type> is not a valid
type.
Without the typecast, you will get errors like this:
<screen>
<computeroutput>
ERROR: could not determine ANYARRAY/ANYELEMENT type because input is UNKNOWN
</computeroutput>
</screen>
</para>
<para>
It is permitted to have polymorphic arguments with a deterministic
return type, but the converse is not. For example:
<screen>
CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS bool AS '
SELECT $1 > $2;
' LANGUAGE SQL;
SELECT is_greater(1, 2);
is_greater
------------
f
(1 row)
CREATE FUNCTION invalid_func() RETURNS anyelement AS '
SELECT 1;
' LANGUAGE SQL;
ERROR: cannot determine result datatype
DETAIL: A function returning ANYARRAY or ANYELEMENT must have at least one argument of either type.
</screen>
</para>
</sect2>
</sect1>
<sect1 id="xfunc-pl">
<title>Procedural Language Functions</title>
<para>
Procedural languages aren't built into the <productname>PostgreSQL</productname> server; they are offered
by loadable modules. Please refer to the documentation of the
procedural language in question for details about the syntax and how the function body
is interpreted for each language.
</para>
<para>
There are currently four procedural languages available in the
standard <productname>PostgreSQL</productname> distribution:
<application>PL/pgSQL</application>, <application>PL/Tcl</application>,
<application>PL/Perl</application>, and <application>PL/Python</application>. Other languages can be
defined by users. Refer to <xref linkend="xplang"> for more
information. The basics of developing a new procedural language are covered in <xref linkend="xfunc-plhandler">.
</para>
</sect1>
<sect1 id="xfunc-internal">
<title>Internal Functions</title>
<indexterm zone="xfunc-internal"><primary>function</><secondary>internal</></>
<para>
Internal functions are functions written in C that have been statically
linked into the <productname>PostgreSQL</productname> server.
The <quote>body</quote> of the function definition
specifies the C-language name of the function, which need not be the
same as the name being declared for SQL use.
(For reasons of backwards compatibility, an empty body
is accepted as meaning that the C-language function name is the
same as the SQL name.)
</para>
<para>
Normally, all internal functions present in the
server are declared during the initialization of the database cluster (<command>initdb</command>),
but a user could use <command>CREATE FUNCTION</command>
to create additional alias names for an internal function.
Internal functions are declared in <command>CREATE FUNCTION</command>
with language name <literal>internal</literal>. For instance, to
create an alias for the <function>sqrt</function> function:
<programlisting>
CREATE FUNCTION square_root(double precision) RETURNS double precision
AS 'dsqrt'
LANGUAGE internal
STRICT;
</programlisting>
(Most internal functions expect to be declared <quote>strict</quote>.)
</para>
<note>
<para>
Not all <quote>predefined</quote> functions are
<quote>internal</quote> in the above sense. Some predefined
functions are written in SQL.
</para>
</note>
</sect1>
<sect1 id="xfunc-c">
<title>C-Language Functions</title>
<indexterm zone="xfunc-sql">
<primary>function</primary>
<secondary>user-defined</secondary>
<tertiary>in C</tertiary>
</indexterm>
<para>
User-defined functions can be written in C (or a language that can
be made compatible with C, such as C++). Such functions are
compiled into dynamically loadable objects (also called shared
libraries) and are loaded by the server on demand. The dynamic
loading feature is what distinguishes <quote>C language</> functions
from <quote>internal</> functions --- the actual coding conventions
are essentially the same for both. (Hence, the standard internal
function library is a rich source of coding examples for user-defined
C functions.)
</para>
<para>
Two different calling conventions are currently used for C functions.
The newer <quote>version 1</quote> calling convention is indicated by writing
a <literal>PG_FUNCTION_INFO_V1()</literal> macro call for the function,
as illustrated below. Lack of such a macro indicates an old-style
(<quote>version 0</quote>) function. The language name specified in <command>CREATE FUNCTION</command>
is <literal>C</literal> in either case. Old-style functions are now deprecated
because of portability problems and lack of functionality, but they
are still supported for compatibility reasons.
</para>
<sect2 id="xfunc-c-dynload">
<title>Dynamic Loading</title>
<indexterm zone="xfunc-c-dynload">
<primary>dynamic loading</primary>
</indexterm>
<para>
The first time a user-defined function in a particular
loadable object file is called in a session,
the dynamic loader loads that object file into memory so that the
function can be called. The <command>CREATE FUNCTION</command>
for a user-defined C function must therefore specify two pieces of
information for the function: the name of the loadable
object file, and the C name (link symbol) of the specific function to call
within that object file. If the C name is not explicitly specified then
it is assumed to be the same as the SQL function name.
</para>
<para>
The following algorithm is used to locate the shared object file
based on the name given in the <command>CREATE FUNCTION</command>
command:
<orderedlist>
<listitem>
<para>
If the name is an absolute path, the given file is loaded.
</para>
</listitem>
<listitem>
<para>
If the name starts with the string <literal>$libdir</literal>,
that part is replaced by the <productname>PostgreSQL</> package
library directory
name, which is determined at build time.<indexterm><primary>$libdir</></>
</para>
</listitem>
<listitem>
<para>
If the name does not contain a directory part, the file is
searched for in the path specified by the configuration variable
<varname>dynamic_library_path</varname>.<indexterm><primary>dynamic_library_path</></>
</para>
</listitem>
<listitem>
<para>
Otherwise (the file was not found in the path, or it contains a
non-absolute directory part), the dynamic loader will try to
take the name as given, which will most likely fail. (It is
unreliable to depend on the current working directory.)
</para>
</listitem>
</orderedlist>
If this sequence does not work, the platform-specific shared
library file name extension (often <filename>.so</filename>) is
appended to the given name and this sequence is tried again. If
that fails as well, the load will fail.
</para>
<note>
<para>
The user ID the <productname>PostgreSQL</productname> server runs
as must be able to traverse the path to the file you intend to
load. Making the file or a higher-level directory not readable
and/or not executable by the <systemitem>postgres</systemitem> user is a
common mistake.
</para>
</note>
<para>
In any case, the file name that is given in the
<command>CREATE FUNCTION</command> command is recorded literally
in the system catalogs, so if the file needs to be loaded again
the same procedure is applied.
</para>
<note>
<para>
<productname>PostgreSQL</productname> will not compile a C function
automatically. The object file must be compiled before it is referenced
in a <command>CREATE
FUNCTION</> command. See <xref linkend="dfunc"> for additional
information.
</para>
</note>
<note>
<para>
After it is used for the first time, a dynamically loaded object
file is retained in memory. Future calls in the same session to the
function(s) in that file will only incur the small overhead of a symbol
table lookup. If you need to force a reload of an object file, for
example after recompiling it, use the <command>LOAD</> command or
begin a fresh session.
</para>
</note>
<para>
It is recommended to locate shared libraries either relative to
<literal>$libdir</literal> or through the dynamic library path.
This simplifies version upgrades if the new installation is at a
different location. The actual directory that
<literal>$libdir</literal> stands for can be found out with the
command <literal>pg_config --pkglibdir</literal>.
</para>
<note>
<para>
Before <productname>PostgreSQL</productname> release 7.2, only exact
absolute paths to object files could be specified in <command>CREATE
FUNCTION</>. This approach is now deprecated since it makes the
function definition unnecessarily unportable. It's best to specify
just the shared library name with no path nor extension, and let
the search mechanism provide that information instead.
</para>
</note>
</sect2>
<sect2 id="xfunc-c-basetype">
<title>Base Types in C-Language Functions</title>
<indexterm zone="xfunc-c-basetype">
<primary>data type</primary>
<secondary>internal organisation</secondary>
</indexterm>
<para>
To know how to write C-language functions, you need to know how
PostgreSQL internally represents base data types and how they can
be passed to and from functions.
Internally, <productname>PostgreSQL</productname> regards a
base type as a <quote>blob of memory</quote>. The user-defined
functions that you define over a type in turn define the
way that <productname>PostgreSQL</productname> can operate
on it. That is, <productname>PostgreSQL</productname> will
only store and retrieve the data from disk and use your
user-defined functions to input, process, and output the data.
</para>
<para>
Base types can have one of three internal formats:
<itemizedlist>
<listitem>
<para>
pass by value, fixed-length
</para>
</listitem>
<listitem>
<para>
pass by reference, fixed-length
</para>
</listitem>
<listitem>
<para>
pass by reference, variable-length
</para>
</listitem>
</itemizedlist>
</para>
<para>
By-value types can only be 1, 2, or 4 bytes in length
(also 8 bytes, if <literal>sizeof(Datum)</literal> is 8 on your machine).
You should be careful
to define your types such that they will be the same
size (in bytes) on all architectures. For example, the
<literal>long</literal> type is dangerous because it
is 4 bytes on some machines and 8 bytes on others, whereas
<type>int</type> type is 4 bytes on most
Unix machines. A reasonable implementation of
the <type>int4</type> type on Unix
machines might be:
<programlisting>
/* 4-byte integer, passed by value */
typedef int int4;
</programlisting>
</para>
<para>
On the other hand, fixed-length types of any size may
be passed by-reference. For example, here is a sample
implementation of a <productname>PostgreSQL</productname> type:
<programlisting>
/* 16-byte structure, passed by reference */
typedef struct
{
double x, y;
} Point;
</programlisting>
Only pointers to such types can be used when passing
them in and out of <productname>PostgreSQL</productname> functions.
To return a value of such a type, allocate the right amount of
memory with <literal>palloc</literal>, fill in the allocated memory,
and return a pointer to it. (You can also return an input value
that has the same type as the return value directly by returning
the pointer to the input value. <emphasis>Never</> modify the
contents of a pass-by-reference input value, however.)
</para>
<para>
Finally, all variable-length types must also be passed
by reference. All variable-length types must begin
with a length field of exactly 4 bytes, and all data to
be stored within that type must be located in the memory
immediately following that length field. The
length field contains the total length of the structure,
that is, it includes the size of the length field
itself.
</para>
<para>
As an example, we can define the type <type>text</type> as
follows:
<programlisting>
typedef struct {
int4 length;
char data[1];
} text;
</programlisting>
Obviously, the data field declared here is not long enough to hold
all possible strings. Since it's impossible to declare a variable-size
structure in <acronym>C</acronym>, we rely on the knowledge that the
<acronym>C</acronym> compiler won't range-check array subscripts. We
just allocate the necessary amount of space and then access the array as
if it were declared the right length. (This is a common trick, which
you can read about in many textbooks about C.)
</para>
<para>
When manipulating
variable-length types, we must be careful to allocate
the correct amount of memory and set the length field correctly.
For example, if we wanted to store 40 bytes in a <structname>text</>
structure, we might use a code fragment like this:
<programlisting>
#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
destination-&gt;length = VARHDRSZ + 40;
memcpy(destination-&gt;data, buffer, 40);
...
</programlisting>
<literal>VARHDRSZ</> is the same as <literal>sizeof(int4)</>, but
it's considered good style to use the macro <literal>VARHDRSZ</>
to refer to the size of the overhead for a variable-length type.
</para>
<para>
<xref linkend="xfunc-c-type-table"> specifies which C type
corresponds to which SQL type when writing a C-language function
that uses a built-in type of <productname>PostgreSQL</>.
The <quote>Defined In</quote> column gives the header file that
needs to be included to get the type definition. (The actual
definition may be in a different file that is included by the
listed file. It is recommended that users stick to the defined
interface.) Note that you should always include
<filename>postgres.h</filename> first in any source file, because
it declares a number of things that you will need anyway.
</para>
<table tocentry="1" id="xfunc-c-type-table">
<title>Equivalent C Types for Built-In SQL Types</title>
<tgroup cols="3">
<thead>
<row>
<entry>
SQL Type
</entry>
<entry>
C Type
</entry>
<entry>
Defined In
</entry>
</row>
</thead>
<tbody>
<row>
<entry><type>abstime</type></entry>
<entry><type>AbsoluteTime</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
<row>
<entry><type>boolean</type></entry>
<entry><type>bool</type></entry>
<entry><filename>postgres.h</filename> (maybe compiler built-in)</entry>
</row>
<row>
<entry><type>box</type></entry>
<entry><type>BOX*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>bytea</type></entry>
<entry><type>bytea*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>"char"</type></entry>
<entry><type>char</type></entry>
<entry>(compiler built-in)</entry>
</row>
<row>
<entry><type>character</type></entry>
<entry><type>BpChar*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>cid</type></entry>
<entry><type>CommandId</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>date</type></entry>
<entry><type>DateADT</type></entry>
<entry><filename>utils/date.h</filename></entry>
</row>
<row>
<entry><type>smallint</type> (<type>int2</type>)</entry>
<entry><type>int2</type> or <type>int16</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>int2vector</type></entry>
<entry><type>int2vector*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>integer</type> (<type>int4</type>)</entry>
<entry><type>int4</type> or <type>int32</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>real</type> (<type>float4</type>)</entry>
<entry><type>float4*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>double precision</type> (<type>float8</type>)</entry>
<entry><type>float8*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>interval</type></entry>
<entry><type>Interval*</type></entry>
<entry><filename>utils/timestamp.h</filename></entry>
</row>
<row>
<entry><type>lseg</type></entry>
<entry><type>LSEG*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>name</type></entry>
<entry><type>Name</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>oid</type></entry>
<entry><type>Oid</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>oidvector</type></entry>
<entry><type>oidvector*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>path</type></entry>
<entry><type>PATH*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>point</type></entry>
<entry><type>POINT*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>regproc</type></entry>
<entry><type>regproc</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>reltime</type></entry>
<entry><type>RelativeTime</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
<row>
<entry><type>text</type></entry>
<entry><type>text*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>tid</type></entry>
<entry><type>ItemPointer</type></entry>
<entry><filename>storage/itemptr.h</filename></entry>
</row>
<row>
<entry><type>time</type></entry>
<entry><type>TimeADT</type></entry>
<entry><filename>utils/date.h</filename></entry>
</row>
<row>
<entry><type>time with time zone</type></entry>
<entry><type>TimeTzADT</type></entry>
<entry><filename>utils/date.h</filename></entry>
</row>
<row>
<entry><type>timestamp</type></entry>
<entry><type>Timestamp*</type></entry>
<entry><filename>utils/timestamp.h</filename></entry>
</row>
<row>
<entry><type>tinterval</type></entry>
<entry><type>TimeInterval</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
<row>
<entry><type>varchar</type></entry>
<entry><type>VarChar*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>xid</type></entry>
<entry><type>TransactionId</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Now that we've gone over all of the possible structures
for base types, we can show some examples of real functions.
</para>
</sect2>
<sect2>
<title>Calling Conventions Version 0 for C-Language Functions</title>
<para>
We present the <quote>old style</quote> calling convention first --- although
this approach is now deprecated, it's easier to get a handle on
initially. In the version-0 method, the arguments and result
of the C function are just declared in normal C style, but being
careful to use the C representation of each SQL data type as shown
above.
</para>
<para>
Here are some examples:
<programlisting>
#include "postgres.h"
#include &lt;string.h&gt;
/* by value */
int
add_one(int arg)
{
return arg + 1;
}
/* by reference, fixed length */
float8 *
add_one_float8(float8 *arg)
{
float8 *result = (float8 *) palloc(sizeof(float8));
*result = *arg + 1.0;
return result;
}
Point *
makepoint(Point *pointx, Point *pointy)
{
Point *new_point = (Point *) palloc(sizeof(Point));
new_point->x = pointx->x;
new_point->y = pointy->y;
return new_point;
}
/* by reference, variable length */
text *
copytext(text *t)
{
/*
* VARSIZE is the total size of the struct in bytes.
*/
text *new_t = (text *) palloc(VARSIZE(t));
VARATT_SIZEP(new_t) = VARSIZE(t);
/*
* VARDATA is a pointer to the data region of the struct.
*/
memcpy((void *) VARDATA(new_t), /* destination */
(void *) VARDATA(t), /* source */
VARSIZE(t)-VARHDRSZ); /* how many bytes */
return new_t;
}
text *
concat_text(text *arg1, text *arg2)
{
int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
text *new_text = (text *) palloc(new_text_size);
VARATT_SIZEP(new_text) = new_text_size;
memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
return new_text;
}
</programlisting>
</para>
<para>
Supposing that the above code has been prepared in file
<filename>funcs.c</filename> and compiled into a shared object,
we could define the functions to <productname>PostgreSQL</productname>
with commands like this:
<programlisting>
CREATE FUNCTION add_one(integer) RETURNS integer
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one'
LANGUAGE C STRICT;
-- note overloading of SQL function name "add_one"
CREATE FUNCTION add_one(double precision) RETURNS double precision
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one_float8'
LANGUAGE C STRICT;
CREATE FUNCTION makepoint(point, point) RETURNS point
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'makepoint'
LANGUAGE C STRICT;
CREATE FUNCTION copytext(text) RETURNS text
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'copytext'
LANGUAGE C STRICT;
CREATE FUNCTION concat_text(text, text) RETURNS text
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'concat_text',
LANGUAGE C STRICT;
</programlisting>
</para>
<para>
Here, <replaceable>DIRECTORY</replaceable> stands for the
directory of the shared library file (for instance the PostgreSQL
tutorial directory, which contains the code for the examples used
in this section). (Better style would be to use just
<literal>'funcs'</> in the <literal>AS</> clause, after having
added <replaceable>DIRECTORY</replaceable> to the search path.
In any case, we may omit the system-specific extension for a
shared library, commonly <literal>.so</literal> or
<literal>.sl</literal>.)
</para>
<para>
Notice that we have specified the functions as <quote>strict</quote>,
meaning that
the system should automatically assume a null result if any input
value is null. By doing this, we avoid having to check for null inputs
in the function code. Without this, we'd have to check for null values
explicitly, by checking for a null pointer for each
pass-by-reference argument. (For pass-by-value arguments, we don't
even have a way to check!)
</para>
<para>
Although this calling convention is simple to use,
it is not very portable; on some architectures there are problems
with passing data types that are smaller than <type>int</type> this way. Also, there is
no simple way to return a null result, nor to cope with null arguments
in any way other than making the function strict. The version-1
convention, presented next, overcomes these objections.
</para>
</sect2>
<sect2>
<title>Calling Conventions Version 1 for C-Language Functions</title>
<para>
The version-1 calling convention relies on macros to suppress most
of the complexity of passing arguments and results. The C declaration
of a version-1 function is always
<programlisting>
Datum funcname(PG_FUNCTION_ARGS)
</programlisting>
In addition, the macro call
<programlisting>
PG_FUNCTION_INFO_V1(funcname);
</programlisting>
must appear in the same source file. (Conventionally. it's
written just before the function itself.) This macro call is not
needed for <literal>internal</>-language functions, since
<productname>PostgreSQL</> assumes that all internal functions
use the version-1 convention. It is, however, required for
dynamically-loaded functions.
</para>
<para>
In a version-1 function, each actual argument is fetched using a
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
macro that corresponds to the argument's data type, and the
result is returned using a
<function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
macro for the return type.
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
takes as its argument the number of the function argument to
fetch, where the count starts at 0.
<function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
takes as its argument the actual value to return.
</para>
<para>
Here we show the same functions as above, coded in version-1 style:
<programlisting>
#include "postgres.h"
#include &lt;string.h&gt;
#include "fmgr.h"
/* by value */
PG_FUNCTION_INFO_V1(add_one);
Datum
add_one(PG_FUNCTION_ARGS)
{
int32 arg = PG_GETARG_INT32(0);
PG_RETURN_INT32(arg + 1);
}
/* b reference, fixed length */
PG_FUNCTION_INFO_V1(add_one_float8);
Datum
add_one_float8(PG_FUNCTION_ARGS)
{
/* The macros for FLOAT8 hide its pass-by-reference nature. */
float8 arg = PG_GETARG_FLOAT8(0);
PG_RETURN_FLOAT8(arg + 1.0);
}
PG_FUNCTION_INFO_V1(makepoint);
Datum
makepoint(PG_FUNCTION_ARGS)
{
/* Here, the pass-by-reference nature of Point is not hidden. */
Point *pointx = PG_GETARG_POINT_P(0);
Point *pointy = PG_GETARG_POINT_P(1);
Point *new_point = (Point *) palloc(sizeof(Point));
new_point->x = pointx->x;
new_point->y = pointy->y;
PG_RETURN_POINT_P(new_point);
}
/* by reference, variable length */
PG_FUNCTION_INFO_V1(copytext);
Datum
copytext(PG_FUNCTION_ARGS)
{
text *t = PG_GETARG_TEXT_P(0);
/*
* VARSIZE is the total size of the struct in bytes.
*/
text *new_t = (text *) palloc(VARSIZE(t));
VARATT_SIZEP(new_t) = VARSIZE(t);
/*
* VARDATA is a pointer to the data region of the struct.
*/
memcpy((void *) VARDATA(new_t), /* destination */
(void *) VARDATA(t), /* source */
VARSIZE(t)-VARHDRSZ); /* how many bytes */
PG_RETURN_TEXT_P(new_t);
}
PG_FUNCTION_INFO_V1(concat_text);
Datum
concat_text(PG_FUNCTION_ARGS)
{
text *arg1 = PG_GETARG_TEXT_P(0);
text *arg2 = PG_GETARG_TEXT_P(1);
int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
text *new_text = (text *) palloc(new_text_size);
VARATT_SIZEP(new_text) = new_text_size;
memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
PG_RETURN_TEXT_P(new_text);
}
</programlisting>
</para>
<para>
The <command>CREATE FUNCTION</command> commands are the same as
for the version-0 equivalents.
</para>
<para>
At first glance, the version-1 coding conventions may appear to
be just pointless obscurantism. They do, however, offer a number
of improvements, because the macros can hide unnecessary detail.
An example is that in coding <function>add_one_float8</>, we no longer need to
be aware that <type>float8</type> is a pass-by-reference type. Another
example is that the <literal>GETARG</> macros for variable-length types allow
for more efficient fetching of <quote>toasted</quote> (compressed or
out-of-line) values.
</para>
<para>
One big improvement in version-1 functions is better handling of null
inputs and results. The macro <function>PG_ARGISNULL(<replaceable>n</>)</function>
allows a function to test whether each input is null. (Of course, doing
this is only necessary in functions not declared <quote>strict</>.)
As with the
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function> macros,
the input arguments are counted beginning at zero. Note that one
should refrain from executing
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function> until
one has verified that the argument isn't null.
To return a null result, execute <function>PG_RETURN_NULL()</function>;
this works in both strict and nonstrict functions.
</para>
<para>
Other options provided in the new-style interface are two
variants of the
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
macros. The first of these,
<function>PG_GETARG_<replaceable>xxx</replaceable>_COPY()</function>,
guarantees to return a copy of the specified argument that is
safe for writing into. (The normal macros will sometimes return a
pointer to a value that is physically stored in a table, which
must not be written to. Using the
<function>PG_GETARG_<replaceable>xxx</replaceable>_COPY()</function>
macros guarantees a writable result.)
The second variant consists of the
<function>PG_GETARG_<replaceable>xxx</replaceable>_SLICE()</function>
macros which take three arguments. The first is the number of the
function argument (as above). The second and third are the offset and
length of the segment to be returned. Offsets are counted from
zero, and a negative length requests that the remainder of the
value be returned. These macros provide more efficient access to
parts of large values in the case where they have storage type
<quote>external</quote>. (The storage type of a column can be specified using
<literal>ALTER TABLE <replaceable>tablename</replaceable> ALTER
COLUMN <replaceable>colname</replaceable> SET STORAGE
<replaceable>storagetype</replaceable></literal>. <replaceable>storagetype</replaceable> is one of
<literal>plain</>, <literal>external</>, <literal>extended</literal>,
or <literal>main</>.)
</para>
<para>
Finally, the version-1 function call conventions make it possible
to return set results (<xref linkend="xfunc-c-return-set">) and
implement trigger functions (<xref linkend="triggers">) and
procedural-language call handlers (<xref
linkend="xfunc-plhandler">). Version-1 code is also more
portable than version-0, because it does not break restrictions
on function call protocol in the C standard. For more details
see <filename>src/backend/utils/fmgr/README</filename> in the
source distribution.
</para>
</sect2>
<sect2>
<title>Writing Code</title>
<para>
Before we turn to the more advanced topics, we should discuss
some coding rules for PostgreSQL C-language functions. While it
may be possible to load functions written in languages other than
C into <productname>PostgreSQL</productname>, this is usually
difficult (when it is possible at all) because other languages,
such as C++, FORTRAN, or Pascal often do not follow the same
calling convention as C. That is, other languages do not pass
argument and return values between functions in the same way.
For this reason, we will assume that your C-language functions
are actually written in C.
</para>
<para>
The basic rules for writing and building C functions are as follows:
<itemizedlist>
<listitem>
<para>
Use <literal>pg_config
--includedir-server</literal><indexterm><primary>pg_config</><secondary>with user-defined C functions</></>
to find out where the <productname>PostgreSQL</> server header
files are installed on your system (or the system that your
users will be running on). This option is new with
<productname>PostgreSQL</> 7.2. For
<productname>PostgreSQL</> 7.1 you should use the option
<option>--includedir</option>. (<command>pg_config</command>
will exit with a non-zero status if it encounters an unknown
option.) For releases prior to 7.1 you will have to guess,
but since that was before the current calling conventions were
introduced, it is unlikely that you want to support those
releases.
</para>
</listitem>
<listitem>
<para>
When allocating memory, use the
<productname>PostgreSQL</productname> functions
<function>palloc</function><indexterm><primary>palloc</></> and <function>pfree</function><indexterm><primary>pfree</></>
instead of the corresponding C library functions
<function>malloc</function> and <function>free</function>.
The memory allocated by <function>palloc</function> will be
freed automatically at the end of each transaction, preventing
memory leaks.
</para>
</listitem>
<listitem>
<para>
Always zero the bytes of your structures using
<function>memset</function>. Without this, it's difficult to
support hash indexes or hash joins, as you must pick out only
the significant bits of your data structure to compute a hash.
Even if you initialize all fields of your structure, there may be
alignment padding (holes in the structure) that may contain
garbage values.
</para>
</listitem>
<listitem>
<para>
Most of the internal <productname>PostgreSQL</productname>
types are declared in <filename>postgres.h</filename>, while
the function manager interfaces
(<symbol>PG_FUNCTION_ARGS</symbol>, etc.) are in
<filename>fmgr.h</filename>, so you will need to include at
least these two files. For portability reasons it's best to
include <filename>postgres.h</filename> <emphasis>first</>,
before any other system or user header files. Including
<filename>postgres.h</filename> will also include
<filename>elog.h</filename> and <filename>palloc.h</filename>
for you.
</para>
</listitem>
<listitem>
<para>
Symbol names defined within object files must not conflict
with each other or with symbols defined in the
<productname>PostgreSQL</productname> server executable. You
will have to rename your functions or variables if you get
error messages to this effect.
</para>
</listitem>
<listitem>
<para>
Compiling and linking your code so that it can be dynamically
loaded into <productname>PostgreSQL</productname> always
requires special flags. See <xref linkend="dfunc"> for a
detailed explanation of how to do it for your particular
operating system.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
&dfunc;
<sect2>
<title>Composite-Type Arguments in C-Language Functions</title>
<para>
Composite types do not have a fixed layout like C
structures. Instances of a composite type may contain
null fields. In addition, composite types that are
part of an inheritance hierarchy may have different
fields than other members of the same inheritance hierarchy.
Therefore, <productname>PostgreSQL</productname> provides
a function interface for accessing fields of composite types
from C.
</para>
<para>
Suppose we want to write a function to answer the query
<programlisting>
SELECT name, c_overpaid(emp, 1500) AS overpaid
FROM emp
WHERE name = 'Bill' OR name = 'Sam';
</programlisting>
Using call conventions version 0, we can define
<function>c_overpaid</> as:
<programlisting>
#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
bool
c_overpaid(TupleTableSlot *t, /* the current row of emp */
int32 limit)
{
bool isnull;
int32 salary;
salary = DatumGetInt32(GetAttributeByName(t, "salary", &amp;isnull));
if (isnull)
return false;
return salary &gt; limit;
}
</programlisting>
In version-1 coding, the above would look like this:
<programlisting>
#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
PG_FUNCTION_INFO_V1(c_overpaid);
Datum
c_overpaid(PG_FUNCTION_ARGS)
{
TupleTableSlot *t = (TupleTableSlot *) PG_GETARG_POINTER(0);
int32 limit = PG_GETARG_INT32(1);
bool isnull;
int32 salary;
salary = DatumGetInt32(GetAttributeByName(t, "salary", &amp;isnull));
if (isnull)
PG_RETURN_BOOL(false);
/* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary. */
PG_RETURN_BOOL(salary &gt; limit);
}
</programlisting>
</para>
<para>
<function>GetAttributeByName</function> is the
<productname>PostgreSQL</productname> system function that
returns attributes out of the specified row. It has
three arguments: the argument of type <type>TupleTableSlot*</type> passed into
the function, the name of the desired attribute, and a
return parameter that tells whether the attribute
is null. <function>GetAttributeByName</function> returns a <type>Datum</type>
value that you can convert to the proper data type by using the
appropriate <function>DatumGet<replaceable>XXX</replaceable>()</function> macro.
</para>
<para>
The following command declares the function
<function>c_overpaid</function> in SQL:
<programlisting>
CREATE FUNCTION c_overpaid(emp, integer)
RETURNS boolean
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'c_overpaid'
LANGUAGE C;
</programlisting>
</para>
</sect2>
<sect2>
<title>Returning Rows (Composite Types) from C-Language Functions</title>
<para>
To return a row or composite-type value from a C-language
function, you can use a special API that provides macros and
functions to hide most of the complexity of building composite
data types. To use this API, the source file must include:
<programlisting>
#include "funcapi.h"
</programlisting>
</para>
<para>
The support for returning composite data types (or rows) starts
with the <structname>AttInMetadata</> structure. This structure
holds arrays of individual attribute information needed to create
a row from raw C strings. The information contained in the
structure is derived from a <structname>TupleDesc</> structure,
but it is stored to avoid redundant computations on each call to
a set-returning function (see next section). In the case of a
function returning a set, the <structname>AttInMetadata</>
structure should be computed once during the first call and saved
for reuse in later calls. <structname>AttInMetadata</> also
saves a pointer to the original <structname>TupleDesc</>.
<programlisting>
typedef struct AttInMetadata
{
/* full TupleDesc */
TupleDesc tupdesc;
/* array of attribute type input function finfo */
FmgrInfo *attinfuncs;
/* array of attribute type typelem */
Oid *attelems;
/* array of attribute typmod */
int32 *atttypmods;
} AttInMetadata;
</programlisting>
</para>
<para>
To assist you in populating this structure, several functions and a macro
are available. Use
<programlisting>
TupleDesc RelationNameGetTupleDesc(const char *relname)
</programlisting>
to get a <structname>TupleDesc</> for a named relation, or
<programlisting>
TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)
</programlisting>
to get a <structname>TupleDesc</> based on a type OID. This can
be used to get a <structname>TupleDesc</> for a base or
composite type. Then
<programlisting>
AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)
</programlisting>
will return a pointer to an <structname>AttInMetadata</>,
initialized based on the given
<structname>TupleDesc</>. <structname>AttInMetadata</> can be
used in conjunction with C strings to produce a properly formed
row value (internally called tuple).
</para>
<para>
To return a tuple you must create a tuple slot based on the
<structname>TupleDesc</>. You can use
<programlisting>
TupleTableSlot *TupleDescGetSlot(TupleDesc tupdesc)
</programlisting>
to initialize this tuple slot, or obtain one through other (user provided)
means. The tuple slot is needed to create a <type>Datum</> for return by the
function. The same slot can (and should) be reused on each call.
</para>
<para>
After constructing an <structname>AttInMetadata</> structure,
<programlisting>
HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)
</programlisting>
can be used to build a <structname>HeapTuple</> given user data
in C string form. <literal>values</literal> is an array of C strings, one for
each attribute of the return row. Each C string should be in
the form expected by the input function of the attribute data
type. In order to return a null value for one of the attributes,
the corresponding pointer in the <parameter>values</> array
should be set to <symbol>NULL</>. This function will need to
be called again for each row you return.
</para>
<para>
Building a tuple via <function>TupleDescGetAttInMetadata</> and
<function>BuildTupleFromCStrings</> is only convenient if your
function naturally computes the values to be returned as text
strings. If your code naturally computes the values as a set of
<type>Datum</> values, you should instead use the underlying
function <function>heap_formtuple</> to convert the
<type>Datum</type> values directly into a tuple. You will still need
the <structname>TupleDesc</> and a <structname>TupleTableSlot</>,
but not <structname>AttInMetadata</>.
</para>
<para>
Once you have built a tuple to return from your function, it
must be converted into a <type>Datum</>. Use
<programlisting>
TupleGetDatum(TupleTableSlot *slot, HeapTuple tuple)
</programlisting>
to get a <type>Datum</> given a tuple and a slot. This
<type>Datum</> can be returned directly if you intend to return
just a single row, or it can be used as the current return value
in a set-returning function.
</para>
<para>
An example appears in the next section.
</para>
</sect2>
<sect2 id="xfunc-c-return-set">
<title>Returning Sets from C-Language Functions</title>
<para>
There is also a special API that provides support for returning
sets (multiple rows) from a C-language function. A set-returning
function must follow the version-1 calling conventions. Also,
source files must include <filename>funcapi.h</filename>, as
above.
</para>
<para>
A set-returning function (<acronym>SRF</>) is called
once for each item it returns. The <acronym>SRF</> must
therefore save enough state to remember what it was doing and
return the next item on each call.
The structure <structname>FuncCallContext</> is provided to help
control this process. Within a function, <literal>fcinfo-&gt;flinfo-&gt;fn_extra</>
is used to hold a pointer to <structname>FuncCallContext</>
across calls.
<programlisting>
typedef struct
{
/*
* Number of times we've been called before
*
* call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
* incremented for you every time SRF_RETURN_NEXT() is called.
*/
uint32 call_cntr;
/*
* OPTIONAL maximum number of calls
*
* max_calls is here for convenience only and setting it is optional.
* If not set, you must provide alternative means to know when the
* function is done.
*/
uint32 max_calls;
/*
* OPTIONAL pointer to result slot
*
* slot is for use when returning tuples (i.e., composite data types)
* and is not needed when returning base data types.
*/
TupleTableSlot *slot;
/*
* OPTIONAL pointer to miscellaneous user-provided context information
*
* user_fctx is for use as a pointer to your own data to retain
* arbitrary context information between calls of your function.
*/
void *user_fctx;
/*
* OPTIONAL pointer to struct containing attribute type input metadata
*
* attinmeta is for use when returning tuples (i.e., composite data types)
* and is not needed when returning base data types. It
* is only needed if you intend to use BuildTupleFromCStrings() to create
* the return tuple.
*/
AttInMetadata *attinmeta;
/*
* memory context used for structures that must live for multiple calls
*
* multi_call_memory_ctx is set by SRF_FIRSTCALL_INIT() for you, and used
* by SRF_RETURN_DONE() for cleanup. It is the most appropriate memory
* context for any memory that is to be reused across multiple calls
* of the SRF.
*/
MemoryContext multi_call_memory_ctx;
} FuncCallContext;
</programlisting>
</para>
<para>
An <acronym>SRF</> uses several functions and macros that
automatically manipulate the <structname>FuncCallContext</>
structure (and expect to find it via <literal>fn_extra</>). Use
<programlisting>
SRF_IS_FIRSTCALL()
</programlisting>
to determine if your function is being called for the first or a
subsequent time. On the first call (only) use
<programlisting>
SRF_FIRSTCALL_INIT()
</programlisting>
to initialize the <structname>FuncCallContext</>. On every function call,
including the first, use
<programlisting>
SRF_PERCALL_SETUP()
</programlisting>
to properly set up for using the <structname>FuncCallContext</>
and clearing any previously returned data left over from the
previous pass.
</para>
<para>
If your function has data to return, use
<programlisting>
SRF_RETURN_NEXT(funcctx, result)
</programlisting>
to return it to the caller. (<literal>result</> must be of type
<type>Datum</>, either a single value or a tuple prepared as
described above.) Finally, when your function is finished
returning data, use
<programlisting>
SRF_RETURN_DONE(funcctx)
</programlisting>
to clean up and end the <acronym>SRF</>.
</para>
<para>
The memory context that is current when the <acronym>SRF</> is called is
a transient context that will be cleared between calls. This means
that you do not need to call <function>pfree</> on everything
you allocated using <function>palloc</>; it will go away anyway. However, if you want to allocate
any data structures to live across calls, you need to put them somewhere
else. The memory context referenced by
<structfield>multi_call_memory_ctx</> is a suitable location for any
data that needs to survive until the <acronym>SRF</> is finished running. In most
cases, this means that you should switch into
<structfield>multi_call_memory_ctx</> while doing the first-call setup.
</para>
<para>
A complete pseudo-code example looks like the following:
<programlisting>
Datum
my_set_returning_function(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
Datum result;
MemoryContext oldcontext;
<replaceable>further declarations as needed</replaceable>
if (SRF_IS_FIRSTCALL())
{
funcctx = SRF_FIRSTCALL_INIT();
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* One-time setup code appears here: */
<replaceable>user code</replaceable>
<replaceable>if returning composite</replaceable>
<replaceable>build TupleDesc, and perhaps AttInMetadata</replaceable>
<replaceable>obtain slot</replaceable>
funcctx-&gt;slot = slot;
<replaceable>endif returning composite</replaceable>
<replaceable>user code</replaceable>
MemoryContextSwitchTo(oldcontext);
}
/* Each-time setup code appears here: */
<replaceable>user code</replaceable>
funcctx = SRF_PERCALL_SETUP();
<replaceable>user code</replaceable>
/* this is just one way we might test whether we are done: */
if (funcctx-&gt;call_cntr &lt; funcctx-&gt;max_calls)
{
/* Here we want to return another item: */
<replaceable>user code</replaceable>
<replaceable>obtain result Datum</replaceable>
SRF_RETURN_NEXT(funcctx, result);
}
else
{
/* Here we are done returning items and just need to clean up: */
<replaceable>user code</replaceable>
SRF_RETURN_DONE(funcctx);
}
}
</programlisting>
</para>
<para>
A complete example of a simple <acronym>SRF</> returning a composite type looks like:
<programlisting>
PG_FUNCTION_INFO_V1(testpassbyval);
Datum
testpassbyval(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
int call_cntr;
int max_calls;
TupleDesc tupdesc;
TupleTableSlot *slot;
AttInMetadata *attinmeta;
/* stuff done only on the first call of the function */
if (SRF_IS_FIRSTCALL())
{
MemoryContext oldcontext;
/* create a function context for cross-call persistence */
funcctx = SRF_FIRSTCALL_INIT();
/* switch to memory context appropriate for multiple function calls */
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* total number of tuples to be returned */
funcctx-&gt;max_calls = PG_GETARG_UINT32(0);
/* Build a tuple description for a __testpassbyval tuple */
tupdesc = RelationNameGetTupleDesc("__testpassbyval");
/* allocate a slot for a tuple with this tupdesc */
slot = TupleDescGetSlot(tupdesc);
/* assign slot to function context */
funcctx-&gt;slot = slot;
/*
* generate attribute metadata needed later to produce tuples from raw
* C strings
*/
attinmeta = TupleDescGetAttInMetadata(tupdesc);
funcctx-&gt;attinmeta = attinmeta;
MemoryContextSwitchTo(oldcontext);
}
/* stuff done on every call of the function */
funcctx = SRF_PERCALL_SETUP();
call_cntr = funcctx-&gt;call_cntr;
max_calls = funcctx-&gt;max_calls;
slot = funcctx-&gt;slot;
attinmeta = funcctx-&gt;attinmeta;
if (call_cntr &lt; max_calls) /* do when there is more left to send */
{
char **values;
HeapTuple tuple;
Datum result;
/*
* Prepare a values array for storage in our slot.
* This should be an array of C strings which will
* be processed later by the type input functions.
*/
values = (char **) palloc(3 * sizeof(char *));
values[0] = (char *) palloc(16 * sizeof(char));
values[1] = (char *) palloc(16 * sizeof(char));
values[2] = (char *) palloc(16 * sizeof(char));
snprintf(values[0], 16, "%d", 1 * PG_GETARG_INT32(1));
snprintf(values[1], 16, "%d", 2 * PG_GETARG_INT32(1));
snprintf(values[2], 16, "%d", 3 * PG_GETARG_INT32(1));
/* build a tuple */
tuple = BuildTupleFromCStrings(attinmeta, values);
/* make the tuple into a datum */
result = TupleGetDatum(slot, tuple);
/* clean up (this is not really necessary) */
pfree(values[0]);
pfree(values[1]);
pfree(values[2]);
pfree(values);
SRF_RETURN_NEXT(funcctx, result);
}
else /* do when there is no more left */
{
SRF_RETURN_DONE(funcctx);
}
}
</programlisting>
The SQL code to declare this function is:
<programlisting>
CREATE TYPE __testpassbyval AS (f1 integer, f2 integer, f3 integer);
CREATE OR REPLACE FUNCTION testpassbyval(integer, integer) RETURNS SETOF __testpassbyval
AS '<replaceable>filename</>', 'testpassbyval'
LANGUAGE C IMMUTABLE STRICT;
</programlisting>
</para>
<para>
The directory <filename>contrib/tablefunc</> in the source
distribution contains more examples of set-returning functions.
</para>
</sect2>
<sect2>
<title>Polymorphic Arguments and Return Types</title>
<para>
C-language functions may be declared to accept and
return the <quote>polymorphic</> types
<type>anyelement</type> and <type>anyarray</type>.
See <xref linkend="extend-types-polymorphic"> for a more detailed explanation
of polymorphic functions. When function arguments or return types
are defined as polymorphic types, the function author cannot know
in advance what data type it will be called with, or
need to return. There are two routines provided in <filename>fmgr.h</>
to allow a version-1 C function to discover the actual data types
of its arguments and the type it is expected to return. The routines are
called <literal>get_fn_expr_rettype(FmgrInfo *flinfo)</> and
<literal>get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</>.
They return the result or argument type OID, or InvalidOid if the
information is not available.
The structure <literal>flinfo</> is normally accessed as
<literal>fcinfo-&gt;flinfo</>. The parameter <literal>argnum</>
is zero based.
</para>
<para>
For example, suppose we want to write a function to accept a single
element of any type, and return a one-dimensional array of that type:
<programlisting>
PG_FUNCTION_INFO_V1(make_array);
Datum
make_array(PG_FUNCTION_ARGS)
{
ArrayType *result;
Oid element_type = get_fn_expr_argtype(fcinfo-&gt;flinfo, 0);
Datum element;
int16 typlen;
bool typbyval;
char typalign;
int ndims;
int dims[MAXDIM];
int lbs[MAXDIM];
if (!OidIsValid(element_type))
elog(ERROR, "could not determine data type of input");
/* get the provided element */
element = PG_GETARG_DATUM(0);
/* we have one dimension */
ndims = 1;
/* and one element */
dims[0] = 1;
/* and lower bound is 1 */
lbs[0] = 1;
/* get required info about the element type */
get_typlenbyvalalign(element_type, &amp;typlen, &amp;typbyval, &amp;typalign);
/* now build the array */
result = construct_md_array(&amp;element, ndims, dims, lbs,
element_type, typlen, typbyval, typalign);
PG_RETURN_ARRAYTYPE_P(result);
}
</programlisting>
</para>
<para>
The following command declares the function
<function>make_array</function> in SQL:
<programlisting>
CREATE FUNCTION make_array(anyelement)
RETURNS anyarray
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'make_array'
LANGUAGE 'C' STRICT;
</programlisting>
Note the use of STRICT; this is essential since the code is not
bothering to test for a NULL input.
</para>
</sect2>
</sect1>
<sect1 id="xfunc-overload">
<title>Function Overloading</title>
<indexterm zone="xfunc-overload">
<primary>overloading</primary>
<secondary>functions</secondary>
</indexterm>
<para>
More than one function may be defined with the same SQL name, so long
as the arguments they take are different. In other words,
function names can be <firstterm>overloaded</firstterm>. When a
query is executed, the server will determine which function to
call from the data types and the number of the provided arguments.
Overloading can also be used to simulate functions with a variable
number of arguments, up to a finite maximum number.
</para>
<para>
A function may also have the same name as an attribute. (Recall
that <literal>attribute(table)</literal> is equivalent to
<literal>table.attribute</literal>.) In the case that there is an
ambiguity between a function on a complex type and an attribute of
the complex type, the attribute will always be used.
</para>
<para>
When creating a family of overloaded functions, one should be
careful not to create ambiguities. For instance, given the
functions
<programlisting>
CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...
</programlisting>
it is not immediately clear which function would be called with
some trivial input like <literal>test(1, 1.5)</literal>. The
currently implemented resolution rules are described in
<xref linkend="typeconv">, but it is unwise to design a system that subtly
relies on this behavior.
</para>
<para>
When overloading C-language functions, there is an additional
constraint: The C name of each function in the family of
overloaded functions must be different from the C names of all
other functions, either internal or dynamically loaded. If this
rule is violated, the behavior is not portable. You might get a
run-time linker error, or one of the functions will get called
(usually the internal one). The alternative form of the
<literal>AS</> clause for the SQL <command>CREATE
FUNCTION</command> command decouples the SQL function name from
the function name in the C source code. E.g.,
<programlisting>
CREATE FUNCTION test(int) RETURNS int
AS '<replaceable>filename</>', 'test_1arg'
LANGUAGE C;
CREATE FUNCTION test(int, int) RETURNS int
AS '<replaceable>filename</>', 'test_2arg'
LANGUAGE C;
</programlisting>
The names of the C functions here reflect one of many possible conventions.
</para>
</sect1>
<sect1 id="xfunc-plhandler">
<title>Procedural Language Handlers</title>
<indexterm zone="xfunc-plhandler">
<primary>procedural language</primary>
<secondary>handler for</secondary>
</indexterm>
<para>
All calls to functions that are written in a language other than
the current <quote>version 1</quote> interface for compiled
languages (this includes functions in user-defined procedural languages,
functions written in SQL, and functions using the version 0 compiled
language interface), go through a <firstterm>call handler</firstterm>
function for the specific language. It is the responsibility of
the call handler to execute the function in a meaningful way, such
as by interpreting the supplied source text. This section
describes how a language call handler can be written. This is not
a common task, in fact, it has only been done a handful of times
in the history of <productname>PostgreSQL</productname>, but the
topic naturally belongs in this chapter, and the material might
give some insight into the extensible nature of the
<productname>PostgreSQL</productname> system.
</para>
<para>
The call handler for a procedural language is a
<quote>normal</quote> function that must be written in a compiled
language such as C, using the version-1 interface, and registered
with <productname>PostgreSQL</productname> as taking no arguments
and returning the type <type>language_handler</type>. This
special pseudotype identifies the function as a call handler and
prevents it from being called directly in SQL commands.
</para>
<para>
The call handler is called in the same way as any other function:
It receives a pointer to a
<structname>FunctionCallInfoData</structname> <type>struct</> containing
argument values and information about the called function, and it
is expected to return a <type>Datum</type> result (and possibly
set the <structfield>isnull</structfield> field of the
<structname>FunctionCallInfoData</structname> structure, if it wishes
to return an SQL null result). The difference between a call
handler and an ordinary callee function is that the
<structfield>flinfo-&gt;fn_oid</structfield> field of the
<structname>FunctionCallInfoData</structname> structure will contain
the OID of the actual function to be called, not of the call
handler itself. The call handler must use this field to determine
which function to execute. Also, the passed argument list has
been set up according to the declaration of the target function,
not of the call handler.
</para>
<para>
It's up to the call handler to fetch the entry of the function from the system table
<classname>pg_proc</classname> and to analyze the argument
and return types of the called function. The <literal>AS</> clause from the
<command>CREATE FUNCTION</command> of the function will be found
in the <literal>prosrc</literal> column of the
<classname>pg_proc</classname> row. This may be the source
text in the procedural language itself (like for PL/Tcl), a
path name to a file, or anything else that tells the call handler
what to do in detail.
</para>
<para>
Often, the same function is called many times per SQL statement.
A call handler can avoid repeated lookups of information about the
called function by using the
<structfield>flinfo-&gt;fn_extra</structfield> field. This will
initially be <symbol>NULL</>, but can be set by the call handler to point at
information about the called function. On subsequent calls, if
<structfield>flinfo-&gt;fn_extra</structfield> is already non-<symbol>NULL</>
then it can be used and the information lookup step skipped. The
call handler must make sure that
<structfield>flinfo-&gt;fn_extra</structfield> is made to point at
memory that will live at least until the end of the current query,
since an <structname>FmgrInfo</structname> data structure could be
kept that long. One way to do this is to allocate the extra data
in the memory context specified by
<structfield>flinfo-&gt;fn_mcxt</structfield>; such data will
normally have the same lifespan as the
<structname>FmgrInfo</structname> itself. But the handler could
also choose to use a longer-lived memory context so that it can cache
function definition information across queries.
</para>
<para>
When a procedural-language function is invoked as a trigger, no arguments
are passed in the usual way, but the
<structname>FunctionCallInfoData</structname>'s
<structfield>context</structfield> field points at a
<structname>TriggerData</structname> structure, rather than being <symbol>NULL</>
as it is in a plain function call. A language handler should
provide mechanisms for procedural-language functions to get at the trigger
information.
</para>
<para>
This is a template for a procedural-language handler written in C:
<programlisting>
#include "postgres.h"
#include "executor/spi.h"
#include "commands/trigger.h"
#include "fmgr.h"
#include "access/heapam.h"
#include "utils/syscache.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"
PG_FUNCTION_INFO_V1(plsample_call_handler);
Datum
plsample_call_handler(PG_FUNCTION_ARGS)
{
Datum retval;
if (CALLED_AS_TRIGGER(fcinfo))
{
/*
* Called as a trigger procedure
*/
TriggerData *trigdata = (TriggerData *) fcinfo->context;
retval = ...
}
else
{
/*
* Called as a function
*/
retval = ...
}
return retval;
}
</programlisting>
Only a few thousand lines of code have to be added instead of the
dots to complete the call handler.
</para>
<para>
After having compiled the handler function into a loadable module
(see <xref linkend="dfunc">), the following commands then
register the sample procedural language:
<programlisting>
CREATE FUNCTION plsample_call_handler() RETURNS language_handler
AS '<replaceable>filename</replaceable>'
LANGUAGE C;
CREATE LANGUAGE plsample
HANDLER plsample_call_handler;
</programlisting>
</para>
</sect1>
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