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Thomas G. Lockhart
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<Chapter>
<Title>Extending <Acronym>SQL</Acronym>: Functions</Title>
<Para>
As it turns out, part of defining a new type is the
definition of functions that describe its behavior.
Consequently, while it is possible to define a new
function without defining a new type, the reverse is
not true. We therefore describe how to add new functions
to <ProductName>Postgres</ProductName> before describing how to add new
types.
<ProductName>Postgres</ProductName> <Acronym>SQL</Acronym> provides two types of functions: query
language functions (functions written in <Acronym>SQL</Acronym> and
programming language functions (functions written in a
compiled programming language such as <Acronym>C</Acronym>.) Either kind
of function can take a base type, a composite type or
some combination as arguments (parameters). In addition,
both kinds of functions can return a base type or
a composite type. It's easier to define <Acronym>SQL</Acronym> functions,
so we'll start with those.
Examples in this section can also be found in <FileName>funcs.sql</FileName>
and <FileName>C-code/funcs.c</FileName>.
</Para>
<Sect1>
<Title>Query Language (<Acronym>SQL</Acronym>) Functions</Title>
<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 <Acronym>int4</Acronym>:
<ProgramListing>
CREATE FUNCTION one() RETURNS int4
AS 'SELECT 1 as RESULT' LANGUAGE 'sql';
SELECT one() AS answer;
+-------+
|answer |
+-------+
|1 |
+-------+
</ProgramListing>
</Para>
<Para>
Notice that we defined a target list for the function
(with the name RESULT), but the target list of the
query that invoked the function overrode the function's
target list. Hence, the result is labelled answer
instead of one.
</Para>
<Para>
It's 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 $1
and $2.
<ProgramListing>
CREATE FUNCTION add_em(int4, int4) RETURNS int4
AS 'SELECT $1 + $2;' LANGUAGE 'sql';
SELECT add_em(1, 2) AS answer;
+-------+
|answer |
+-------+
|3 |
+-------+
</ProgramListing>
</Para>
<Sect2>
<Title><Acronym>SQL</Acronym> Functions on Composite Types</Title>
<Para>
When specifying functions with arguments of composite
types (such as EMP), we must not only specify which
argument we want (as we did above with $1 and $2) but
also the attributes of that argument. For example,
take the function double_salary that computes what your
salary would be if it were doubled.
<ProgramListing>
CREATE FUNCTION double_salary(EMP) RETURNS int4
AS 'SELECT $1.salary * 2 AS salary;' LANGUAGE 'sql';
SELECT name, double_salary(EMP) AS dream
FROM EMP
WHERE EMP.dept = 'toy';
+-----+-------+
|name | dream |
+-----+-------+
|Sam | 2400 |
+-----+-------+
</ProgramListing>
<Para>
Notice the use of the syntax $1.salary.
Before launching into the subject of functions that
return composite types, we must first introduce the
function notation for projecting attributes. The simple way
to explain this is that we can usually use the
notation attribute(class) and class.attribute interchangably.
<ProgramListing>
--
-- 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 |
+----------+
</ProgramListing>
<Para>
As we shall see, however, this is not always the case.
This function notation is important when we want to use
a function that returns a single instance. We do this
by assembling the entire instance within the function,
attribute by attribute. This is an example of a function
that returns a single EMP instance:
<ProgramListing>
CREATE FUNCTION new_emp() RETURNS EMP
AS 'SELECT \'None\'::text AS name,
1000 AS salary,
25 AS age,
\'none\'::char16 AS dept;'
LANGUAGE 'sql';
</ProgramListing>
</Para>
<Para>
In this case we have specified each of the attributes
with a constant value, but any computation or expression
could have been substituted for these constants.
Defining a function like this can be tricky. Some of
the more important caveats are as follows:
<ItemizedList>
<ListItem>
<Para>
The target list order must be exactly the same as
that in which the attributes appear in the CREATE
TABLE statement (or when you execute a .* query).
</Para>
<ListItem>
<Para>
You must typecast the expressions
(using ::) very carefully or you will see the following error:
<ProgramListing>
WARN::function declared to return type EMP does not retrieve (EMP.*)
</ProgramListing>
</Para>
<ListItem>
<Para>
When calling a function that returns an instance, we
cannot retrieve the entire instance. We must either
project an attribute out of the instance or pass the
entire instance into another function.
<ProgramListing>
SELECT name(new_emp()) AS nobody;
+-------+
|nobody |
+-------+
|None |
+-------+
</ProgramListing>
</Para>
<ListItem>
<Para>
The reason why, in general, we must use the function
syntax for projecting attributes of function return
values is that the parser just doesn't understand
the other (dot) syntax for projection when combined
with function calls.
<ProgramListing>
SELECT new_emp().name AS nobody;
WARN:parser: syntax error at or near "."
</ProgramListing>
</Para>
</ItemizedList>
<Para>
Any collection of commands in the <Acronym>SQL</Acronym> query language
can be packaged together and defined as a function.
The commands can include updates (i.e., <Acronym>insert</Acronym>, <Acronym>update</Acronym>
and <Acronym>delete</Acronym>) as well as <Acronym>select</Acronym> queries. However, the
final command must be a <Acronym>select</Acronym> that returns whatever is
specified as the function's returntype.
<ProgramListing>
CREATE FUNCTION clean_EMP () RETURNS int4
AS 'DELETE FROM EMP WHERE EMP.salary &lt;= 0;
SELECT 1 AS ignore_this'
LANGUAGE 'sql';
SELECT clean_EMP();
+--+
|x |
+--+
|1 |
+--+
</ProgramListing>
</Para>
<Sect1>
<Title>Programming Language Functions</Title>
<Sect2>
<Title>Programming Language Functions on Base Types</Title>
<Para>
Internally, <ProductName>Postgres</ProductName> regards a base type as a "blob of
memory." The user-defined functions that you define
over a type in turn define the way that <ProductName>Postgres</ProductName> can
operate on it. That is, <ProductName>Postgres</ProductName> will only store and
retrieve the data from disk and use your user-defined
functions to input, process, and output the data.
Base types can have one of three internal formats:
<ItemizedList>
<ListItem><Para>pass by value, fixed-length</Para>
<ListItem><Para>pass by reference, fixed-length</Para>
<ListItem><Para>pass by reference, variable-length</Para>
</ItemizedList>
</Para>
<Para>
By-value types can only be 1, 2 or 4 bytes in length
(even if your computer supports by-value types of other
sizes). <ProductName>Postgres</ProductName> itself only passes integer types by
value. You should be careful to define your types such
that they will be the same size (in bytes) on all
architectures. For example, the <Acronym>long</Acronym> type is dangerous
because it is 4 bytes on some machines and 8 bytes on
others, whereas <Acronym>int</Acronym> type is 4 bytes on most <Acronym>UNIX</Acronym>
machines (though not on most personal computers). A
reasonable implementation of the <Acronym>int4</Acronym> type on <Acronym>UNIX</Acronym>
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 the <ProductName>Postgres</ProductName> char16 type:
<ProgramListing>
/* 16-byte structure, passed by reference */
typedef struct {
char data[16];
} char16;
</ProgramListing>
</Para>
<Para>
Only pointers to such types can be used when passing
them in and out of <ProductName>Postgres</ProductName> functions.
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 is the total length of the structure
(i.e., it includes the size of the length field
itself). We can define the text type as follows:
</Para>
<Para>
<ProgramListing>
typedef struct {
int4 length;
char data[1];
} text;
</ProgramListing>
</Para>
<Para>
Obviously, the data field is not long enough to hold
all possible strings -- it's impossible to declare such
a structure in <Acronym>C</Acronym>. When manipulating variable-length
types, we must be careful to allocate the correct
amount of memory and initialize the length field. For
example, if we wanted to store 40 bytes in a text
structure, we might use a code fragment like this:
<ProgramListing>
#include "postgres.h"
#include "utils/palloc.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
destination-&gt;length = VARHDRSZ + 40;
memmove(destination-&gt;data, buffer, 40);
...
</ProgramListing>
</Para>
<Para>
Now that we've gone over all of the possible structures
for base types, we can show some examples of real functions.
Suppose <FileName>funcs.c</FileName> look like:
<ProgramListing>
#include &lt;string.h&gt;
#include "postgres.h" /* for char16, etc. */
#include "utils/palloc.h" /* for palloc */
int
add_one(int arg)
{
return(arg + 1);
}
char16 *
concat16(char16 *arg1, char16 *arg2)
{
char16 *new_c16 = (char16 *) palloc(sizeof(char16));
memset((void *) new_c16, 0, sizeof(char16));
(void) strncpy(new_c16, arg1, 16);
return (char16 *)(strncat(new_c16, arg2, 16));
}
text *
copytext(text *t)
{
/*
* VARSIZE is the total size of the struct in bytes.
*/
text *new_t = (text *) palloc(VARSIZE(t));
memset(new_t, 0, VARSIZE(t));
VARSIZE(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);
}
</ProgramListing>
</Para>
<Para>
On <Acronym>OSF/1</Acronym> we would type:
<ProgramListing>
CREATE FUNCTION add_one(int4) RETURNS int4
AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
CREATE FUNCTION concat16(char16, char16) RETURNS char16
AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
CREATE FUNCTION copytext(text) RETURNS text
AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
</ProgramListing>
</Para>
<Para>
On other systems, we might have to make the filename
end in .sl (to indicate that it's a shared library).
</Para>
</Sect2>
<Sect2>
<Title>Programming Language Functions on Composite Types</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>Postgres</ProductName> provides a procedural
interface for accessing fields of composite types from
C.
As <ProductName>Postgres</ProductName> processes a set of instances, each instance
will be passed into your function as an opaque structure of type <Acronym>TUPLE</Acronym>.
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>
In the query above, we can define c_overpaid as:
<ProgramListing>
#include "postgres.h" /* for char16, etc. */
#include "libpq-fe.h" /* for TUPLE */
bool
c_overpaid(TUPLE t,/* the current instance of EMP */
int4 limit)
{
bool isnull = false;
int4 salary;
salary = (int4) GetAttributeByName(t, "salary", &amp;isnull);
if (isnull)
return (false);
return(salary &gt; limit);
}
</ProgramListing>
</Para>
<Para>
<Acronym>GetAttributeByName</Acronym> is the <ProductName>Postgres</ProductName> system function that
returns attributes out of the current instance. It has
three arguments: the argument of type TUPLE passed into
the function, the name of the desired attribute, and a
return parameter that describes whether the attribute
is null. <Acronym>GetAttributeByName</Acronym> will align data properly
so you can cast its return value to the desired type.
For example, if you have an attribute name which is of
the type char16, the <Acronym>GetAttributeByName</Acronym> call would look
like:
<ProgramListing>
char *str;
...
str = (char *) GetAttributeByName(t, "name", &amp;isnull)
</ProgramListing>
</Para>
<Para>
The following query lets <ProductName>Postgres</ProductName> know about the
c_overpaid function:
<ProgramListing>
* CREATE FUNCTION c_overpaid(EMP, int4) RETURNS bool
AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';
</ProgramListing>
</Para>
<Para>
While there are ways to construct new instances or modify
existing instances from within a C function, these
are far too complex to discuss in this manual.
</Para>
</Sect2>
<Sect2>
<Title>Caveats</Title>
<Para>
We now turn to the more difficult task of writing
programming language functions. Be warned: this section
of the manual will not make you a programmer. You must
have a good understanding of <Acronym>C</Acronym> (including the use of
pointers and the malloc memory manager) before trying
to write <Acronym>C</Acronym> functions for use with <ProductName>Postgres</ProductName>.
While it may be possible to load functions written in
languages other than <Acronym>C</Acronym> into <ProductName>Postgres</ProductName>, this is often
difficult (when it is possible at all) because other
languages, such as <Acronym>FORTRAN</Acronym> and <Acronym>Pascal</Acronym> often do not follow
the same "calling convention" as <Acronym>C</Acronym>. 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 programming language functions
are written in <Acronym>C</Acronym>.
The basic rules for building <Acronym>C</Acronym> functions are as follows:
<ItemizedList>
<ListItem>
<Para>
Most of the header (include) files for <ProductName>Postgres</ProductName>
should already be installed in
<FileName>PGROOT/include</FileName> (see Figure 2).
You should always include
<ProgramListing>
-I$PGROOT/include
</ProgramListing>
on your cc command lines. Sometimes, you may
find that you require header files that are in
the server source itself (i.e., you need a file
we neglected to install in include). In those
cases you may need to add one or more of
<ProgramListing>
-I$PGROOT/src/backend
-I$PGROOT/src/backend/include
-I$PGROOT/src/backend/port/&lt;PORTNAME&gt;
-I$PGROOT/src/backend/obj
</ProgramListing>
(where &lt;PORTNAME&gt; is the name of the port, e.g.,
alpha or sparc).
</ListItem>
<ListItem>
<Para> When allocating memory, use the <ProductName>Postgres</ProductName>
routines palloc and pfree instead of the
corresponding <Acronym>C</Acronym> library routines malloc and free.
The memory allocated by palloc will be freed
automatically at the end of each transaction,
preventing memory leaks.
</Para>
<ListItem>
<Para> Always zero the bytes of your structures using
memset or bzero. Several routines (such as the
hash access method, hash join and the sort algorithm)
compute functions of the raw bits contained in
your structure. Even if you initialize all fields
of your structure, there may be
several bytes of alignment padding (holes in the
structure) that may contain garbage values.
</Para>
<ListItem>
<Para> Most of the internal <ProductName>Postgres</ProductName> types are declared
in postgres.h, so it's usually a good idea to
include that file as well.
</Para>
<ListItem>
<Para> Compiling and loading your object code so that
it can be dynamically loaded into <ProductName>Postgres</ProductName>
always requires special flags. See Appendix A
for a detailed explanation of how to do it for
your particular operating system.
</Para>
</ListItem>
</ItemizedList>
</Para>
</Sect2>