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Update docs for 7.4 array features and polymorphic functions.

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This is Joe Conway's patch of 7-Aug plus further editorializing
of my own.
This commit is contained in:
Tom Lane 2003-08-09 22:50:22 +00:00
parent 329a1b7270
commit 5bfb0540b0
8 changed files with 724 additions and 290 deletions
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338
doc/src/sgml/array.sgml
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@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.28 2003/06/27 00:33:25 tgl Exp $ -->
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.29 2003/08/09 22:50:21 tgl Exp $ -->
<sect1 id="arrays">
<title>Arrays</title>
@ -36,6 +36,41 @@ CREATE TABLE sal_emp (
<type>text</type> (<structfield>schedule</structfield>), which
represents the employee's weekly schedule.
</para>
<para>
The syntax for <command>CREATE TABLE</command> allows the exact size of
arrays to be specified, for example:
<programlisting>
CREATE TABLE tictactoe (
squares integer[3][3]
);
</programlisting>
However, the current implementation does not enforce the array size
limits --- the behavior is the same as for arrays of unspecified
length.
</para>
<para>
Actually, the current implementation does not enforce the declared
number of dimensions either. Arrays of a particular element type are
all considered to be of the same type, regardless of size or number
of dimensions. So, declaring number of dimensions or sizes in
<command>CREATE TABLE</command> is simply documentation, it does not
affect runtime behavior.
</para>
<para>
An alternative, SQL99-standard syntax may be used for one-dimensional arrays.
<structfield>pay_by_quarter</structfield> could have been defined as:
<programlisting>
pay_by_quarter integer ARRAY[4],
</programlisting>
This syntax requires an integer constant to denote the array size.
As before, however, <productname>PostgreSQL</> does not enforce the
size restriction.
</para>
</sect2>
<sect2>
@ -43,9 +78,11 @@ CREATE TABLE sal_emp (
<para>
Now we can show some <command>INSERT</command> statements. To write an array
value, we enclose the element values within curly braces and separate them
by commas. If you know C, this is not unlike the syntax for
initializing structures. (More details appear below.)
value as a literal constant, we enclose the element values within curly
braces and separate them by commas. (If you know C, this is not unlike the
C syntax for initializing structures.) We may put double quotes around any
element value, and must do so if it contains commas or curly braces.
(More details appear below.)
<programlisting>
INSERT INTO sal_emp
@ -90,7 +127,7 @@ SELECT * FROM sal_emp;
</note>
<para>
The <command>ARRAY</command> expression syntax may also be used:
The <literal>ARRAY</literal> expression syntax may also be used:
<programlisting>
INSERT INTO sal_emp
VALUES ('Bill',
@ -109,29 +146,27 @@ SELECT * FROM sal_emp;
(2 rows)
</programlisting>
Note that with this syntax, multidimensional arrays must have matching
extents for each dimension. This eliminates the missing-array-elements
problem above. For example:
extents for each dimension. A mismatch causes an error report, rather than
silently discarding values as in the previous case.
For example:
<programlisting>
INSERT INTO sal_emp
VALUES ('Carol',
ARRAY[20000, 25000, 25000, 25000],
ARRAY[['talk', 'consult'], ['meeting']]);
ERROR: Multidimensional arrays must have array expressions with matching dimensions
ERROR: multidimensional arrays must have array expressions with matching dimensions
</programlisting>
Also notice that string literals are single quoted instead of double quoted.
Also notice that the array elements are ordinary SQL constants or
expressions; for instance, string literals are single quoted, instead of
double quoted as they would be in an array literal. The <literal>ARRAY</>
expression syntax is discussed in more detail in <xref
linkend="sql-syntax-array-constructors">.
</para>
<note>
<para>
The examples in the rest of this section are based on the
<command>ARRAY</command> expression syntax <command>INSERT</command>s.
</para>
</note>
</sect2>
<sect2>
<title>Array Value References</title>
<title>Accessing Arrays</title>
<para>
Now, we can run some queries on the table.
@ -195,7 +230,7 @@ SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
represent an array slice if any of the subscripts are written in the form
<literal><replaceable>lower</replaceable>:<replaceable>upper</replaceable></literal>.
A lower bound of 1 is assumed for any subscript where only one value
is specified. Another example follows:
is specified, as in this example:
<programlisting>
SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
schedule
@ -206,17 +241,38 @@ SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
</para>
<para>
Additionally, we can also access a single arbitrary array element of
a one-dimensional array with the <function>array_subscript</function>
function:
The current dimensions of any array value can be retrieved with the
<function>array_dims</function> function:
<programlisting>
SELECT array_subscript(pay_by_quarter, 2) FROM sal_emp WHERE name = 'Bill';
array_subscript
-----------------
10000
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
array_dims
------------
[1:2][1:1]
(1 row)
</programlisting>
<function>array_dims</function> produces a <type>text</type> result,
which is convenient for people to read but perhaps not so convenient
for programs. Dimensions can also be retrieved with
<function>array_upper</function> and <function>array_lower</function>,
which return the upper and lower bound of a
specified array dimension, respectively.
<programlisting>
SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol';
array_upper
-------------
2
(1 row)
</programlisting>
</para>
</sect2>
<sect2>
<title>Modifying Arrays</title>
<para>
An array value can be replaced completely:
@ -226,22 +282,13 @@ UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
WHERE name = 'Carol';
</programlisting>
or using the <command>ARRAY</command> expression syntax:
or using the <literal>ARRAY</literal> expression syntax:
<programlisting>
UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000]
WHERE name = 'Carol';
</programlisting>
<note>
<para>
Anywhere you can use the <quote>curly braces</quote> array syntax,
you can also use the <command>ARRAY</command> expression syntax. The
remainder of this section will illustrate only one or the other, but
not both.
</para>
</note>
An array may also be updated at a single element:
<programlisting>
@ -256,34 +303,27 @@ UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
WHERE name = 'Carol';
</programlisting>
A one-dimensional array may also be updated with the
<function>array_assign</function> function:
<programlisting>
UPDATE sal_emp SET pay_by_quarter = array_assign(pay_by_quarter, 4, 15000)
WHERE name = 'Bill';
</programListing>
</para>
<para>
An array can be enlarged by assigning to an element adjacent to
A stored array value can be enlarged by assigning to an element adjacent to
those already present, or by assigning to a slice that is adjacent
to or overlaps the data already present. For example, if an array
value currently has 4 elements, it will have five elements after an
update that assigns to <literal>array[5]</>. Currently, enlargement in
this fashion is only allowed for one-dimensional arrays, not
multidimensional arrays.
to or overlaps the data already present. For example, if array
<literal>myarray</> currently has 4 elements, it will have five
elements after an update that assigns to <literal>myarray[5]</>.
Currently, enlargement in this fashion is only allowed for one-dimensional
arrays, not multidimensional arrays.
</para>
<para>
Array slice assignment allows creation of arrays that do not use one-based
subscripts. For example one might assign to <literal>array[-2:7]</> to
subscripts. For example one might assign to <literal>myarray[-2:7]</> to
create an array with subscript values running from -2 to 7.
</para>
<para>
An array can also be enlarged by using the concatenation operator,
<command>||</command>.
New array values can also be constructed by using the concatenation operator,
<literal>||</literal>.
<programlisting>
SELECT ARRAY[1,2] || ARRAY[3,4];
?column?
@ -299,7 +339,7 @@ SELECT ARRAY[5,6] || ARRAY[[1,2],[3,4]];
</programlisting>
The concatenation operator allows a single element to be pushed on to the
beginning or end of a one-dimensional array. It also allows two
beginning or end of a one-dimensional array. It also accepts two
<replaceable>N</>-dimensional arrays, or an <replaceable>N</>-dimensional
and an <replaceable>N+1</>-dimensional array. In the former case, the two
<replaceable>N</>-dimension arrays become outer elements of an
@ -307,12 +347,13 @@ SELECT ARRAY[5,6] || ARRAY[[1,2],[3,4]];
<replaceable>N</>-dimensional array is added as either the first or last
outer element of the <replaceable>N+1</>-dimensional array.
The array is extended in the direction of the push. Hence, by pushing
onto the beginning of an array with a one-based subscript, a zero-based
subscript array is created:
When extending an array by concatenation, the subscripts of its existing
elements are preserved. For example, when pushing
onto the beginning of an array with one-based subscripts, the resulting
array has zero-based subscripts:
<programlisting>
SELECT array_dims(t.f) FROM (SELECT 1 || ARRAY[2,3] AS f) AS t;
SELECT array_dims(1 || ARRAY[2,3]);
array_dims
------------
[0:2]
@ -321,7 +362,7 @@ SELECT array_dims(t.f) FROM (SELECT 1 || ARRAY[2,3] AS f) AS t;
</para>
<para>
An array can also be enlarged by using the functions
An array can also be constructed by using the functions
<function>array_prepend</function>, <function>array_append</function>,
or <function>array_cat</function>. The first two only support one-dimensional
arrays, but <function>array_cat</function> supports multidimensional arrays.
@ -362,60 +403,6 @@ SELECT array_cat(ARRAY[5,6], ARRAY[[1,2],[3,4]]);
{{5,6},{1,2},{3,4}}
</programlisting>
</para>
<para>
The syntax for <command>CREATE TABLE</command> allows fixed-length
arrays to be defined:
<programlisting>
CREATE TABLE tictactoe (
squares integer[3][3]
);
</programlisting>
However, the current implementation does not enforce the array size
limits --- the behavior is the same as for arrays of unspecified
length.
</para>
<para>
An alternative syntax for one-dimensional arrays may be used.
<structfield>pay_by_quarter</structfield> could have been defined as:
<programlisting>
pay_by_quarter integer ARRAY[4],
</programlisting>
This syntax may <emphasis>only</emphasis> be used with the integer
constant to denote the array size.
</para>
<para>
Actually, the current implementation does not enforce the declared
number of dimensions either. Arrays of a particular element type are
all considered to be of the same type, regardless of size or number
of dimensions. So, declaring number of dimensions or sizes in
<command>CREATE TABLE</command> is simply documentation, it does not
affect runtime behavior.
</para>
<para>
The current dimensions of any array value can be retrieved with the
<function>array_dims</function> function:
<programlisting>
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
array_dims
------------
[1:2][1:1]
(1 row)
</programlisting>
<function>array_dims</function> produces a <type>text</type> result,
which is convenient for people to read but perhaps not so convenient
for programs. <function>array_upper</function> and <function>
array_lower</function> return the upper/lower bound of the
given array dimension, respectively.
</para>
</sect2>
<sect2>
@ -423,7 +410,7 @@ SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
<para>
To search for a value in an array, you must check each value of the
array. This can be done by hand (if you know the size of the array).
array. This can be done by hand, if you know the size of the array.
For example:
<programlisting>
@ -434,41 +421,30 @@ SELECT * FROM sal_emp WHERE pay_by_quarter[1] = 10000 OR
</programlisting>
However, this quickly becomes tedious for large arrays, and is not
helpful if the size of the array is unknown. Although it is not built
into <productname>PostgreSQL</productname>,
there is an extension available that defines new functions and
operators for iterating over array values. Using this, the above
query could be:
helpful if the size of the array is uncertain. An alternative method is
described in <xref linkend="functions-comparisons">. The above
query could be replaced by:
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter[1:4] *= 10000;
</programlisting>
To search the entire array (not just specified slices), you could
use:
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter *= 10000;
SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);
</programlisting>
In addition, you could find rows where the array had all values
equal to 10 000 with:
equal to 10000 with:
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter **= 10000;
SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter);
</programlisting>
To install this optional module, look in the
<filename>contrib/array</filename> directory of the
<productname>PostgreSQL</productname> source distribution.
</para>
<tip>
<para>
Arrays are not sets; using arrays in the manner described in the
previous paragraph is often a sign of database misdesign. The
array field should generally be split off into a separate table.
Tables can obviously be searched easily.
Arrays are not sets; searching for specific array elements
may be a sign of database misdesign. Consider
using a separate table with a row for each item that would be an
array element. This will be easier to search, and is likely to
scale up better to large numbers of elements.
</para>
</tip>
</sect2>
@ -477,7 +453,7 @@ SELECT * FROM sal_emp WHERE pay_by_quarter **= 10000;
<title>Array Input and Output Syntax</title>
<para>
The external representation of an array value consists of items that
The external text representation of an array value consists of items that
are interpreted according to the I/O conversion rules for the array's
element type, plus decoration that indicates the array structure.
The decoration consists of curly braces (<literal>{</> and <literal>}</>)
@ -497,95 +473,18 @@ SELECT * FROM sal_emp WHERE pay_by_quarter **= 10000;
</para>
<para>
As illustrated earlier in this chapter, arrays may also be represented
using the <command>ARRAY</command> expression syntax. This representation
of an array value consists of items that are interpreted according to the
I/O conversion rules for the array's element type, plus decoration that
indicates the array structure. The decoration consists of the keyword
<command>ARRAY</command> and square brackets (<literal>[</> and
<literal>]</>) around the array values, plus delimiter characters between
adjacent items. The delimiter character is always a comma (<literal>,</>).
When representing multidimensional arrays, the keyword
<command>ARRAY</command> is only necessary for the outer level. For example,
<literal>'{{"hello world", "happy birthday"}}'</literal> could be written as:
<programlisting>
SELECT ARRAY[['hello world', 'happy birthday']];
array
------------------------------------
{{"hello world","happy birthday"}}
(1 row)
</programlisting>
or it also could be written as:
<programlisting>
SELECT ARRAY[ARRAY['hello world', 'happy birthday']];
array
------------------------------------
{{"hello world","happy birthday"}}
(1 row)
</programlisting>
</para>
<para>
A final method to represent an array, is through an
<command>ARRAY</command> sub-select expression. For example:
<programlisting>
SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
?column?
-------------------------------------------------------------
{2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31}
(1 row)
</programlisting>
The sub-select may <emphasis>only</emphasis> return a single column. The
resulting one-dimensional array will have an element for each row in the
sub-select result, with an element type matching that of the sub-select's
target column.
</para>
<para>
Arrays may be cast from one type to another in similar fashion to other
data types:
<programlisting>
SELECT ARRAY[1,2,3]::oid[];
array
---------
{1,2,3}
(1 row)
SELECT CAST(ARRAY[1,2,3] AS float8[]);
array
---------
{1,2,3}
(1 row)
</programlisting>
</para>
</sect2>
<sect2>
<title>Quoting Array Elements</title>
<para>
As shown above, when writing an array value you may write double
As shown previously, when writing an array value you may write double
quotes around any individual array
element. You <emphasis>must</> do so if the element value would otherwise
confuse the array-value parser. For example, elements containing curly
braces, commas (or whatever the delimiter character is), double quotes,
backslashes, or leading white space must be double-quoted. To put a double
quote or backslash in an array element value, precede it with a backslash.
quote or backslash in a quoted array element value, precede it with a
backslash.
Alternatively, you can use backslash-escaping to protect all data characters
that would otherwise be taken as array syntax or ignorable white space.
</para>
<note>
<para>
The discussion in the preceding paragraph with respect to double quoting does
not pertain to the <command>ARRAY</command> expression syntax. In that case,
each element is quoted exactly as any other literal value of the element type.
</para>
</note>
<para>
The array output routine will put double quotes around element values
if they are empty strings or contain curly braces, delimiter characters,
@ -615,6 +514,15 @@ INSERT ... VALUES ('{"\\\\","\\""}');
in the command to get one backslash into the stored array element.)
</para>
</note>
<tip>
<para>
The <literal>ARRAY</> constructor syntax is often easier to work with
than the array-literal syntax when writing array values in SQL commands.
In <literal>ARRAY</>, individual element values are written the same way
they would be written when not members of an array.
</para>
</tip>
</sect2>
</sect1>

21
doc/src/sgml/datatype.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/datatype.sgml,v 1.121 2003/07/29 00:03:17 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/datatype.sgml,v 1.122 2003/08/09 22:50:21 tgl Exp $
-->
<chapter id="datatype">
@ -2993,6 +2993,10 @@ SELECT * FROM test;
<primary>anyarray</primary>
</indexterm>
<indexterm zone="datatype-pseudo">
<primary>anyelement</primary>
</indexterm>
<indexterm zone="datatype-pseudo">
<primary>void</primary>
</indexterm>
@ -3053,7 +3057,14 @@ SELECT * FROM test;
<row>
<entry><type>anyarray</></entry>
<entry>Indicates that a function accepts any array data type.</entry>
<entry>Indicates that a function accepts any array data type
(see <xref linkend="types-polymorphic">).</entry>
</row>
<row>
<entry><type>anyelement</></entry>
<entry>Indicates that a function accepts any data type
(see <xref linkend="types-polymorphic">).</entry>
</row>
<row>
@ -3101,8 +3112,10 @@ SELECT * FROM test;
Functions coded in procedural languages may use pseudo-types only as
allowed by their implementation languages. At present the procedural
languages all forbid use of a pseudo-type as argument type, and allow
only <type>void</> as a result type (plus <type>trigger</> when the
function is used as a trigger).
only <type>void</> and <type>record</> as a result type (plus
<type>trigger</> when the function is used as a trigger). Some also
support polymorphic functions using the types <type>anyarray</> and
<type>anyelement</>.
</para>
<para>

124
doc/src/sgml/extend.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v 1.22 2003/04/13 09:57:35 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v 1.23 2003/08/09 22:50:21 tgl Exp $
-->
<chapter id="extend">
@ -20,6 +20,11 @@ $Header: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v 1.22 2003/04/13 09:57:35 pete
functions (starting in <xref linkend="xfunc">)
</para>
</listitem>
<listitem>
<para>
aggregates (starting in <xref linkend="xaggr">)
</para>
</listitem>
<listitem>
<para>
data types (starting in <xref linkend="xtypes">)
@ -32,7 +37,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v 1.22 2003/04/13 09:57:35 pete
</listitem>
<listitem>
<para>
aggregates (starting in <xref linkend="xaggr">)
operator classes for indexes (starting in <xref linkend="xindex">)
</para>
</listitem>
</itemizedlist>
@ -47,7 +52,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v 1.22 2003/04/13 09:57:35 pete
relational database systems, you know that they store information
about databases, tables, columns, etc., in what are
commonly known as system catalogs. (Some systems call
this the data dictionary). The catalogs appear to the
this the data dictionary.) The catalogs appear to the
user as tables like any other, but the <acronym>DBMS</acronym> stores
its internal bookkeeping in them. One key difference
between <productname>PostgreSQL</productname> and standard relational database systems is
@ -88,24 +93,113 @@ $Header: /cvsroot/pgsql/doc/src/sgml/extend.sgml,v 1.22 2003/04/13 09:57:35 pete
</indexterm>
<para>
Data types are divided into base types and composite types.
Base types are those, like <type>int4</type>, that are implemented
in a language such as C. They generally correspond to
what are often known as abstract data types. <productname>PostgreSQL</productname>
can only operate on such types through methods provided
by the user and only understands the behavior of such
types to the extent that the user describes them.
Composite types are created whenever the user creates a
table. The
user can <quote>look inside</quote> at the attributes of these types
from the query language.
<productname>PostgreSQL</productname> data types are divided into base
types, composite types, domain types, and pseudo-types.
</para>
<para>
Base types are those, like <type>int4</type>, that are implemented
below the level of the <acronym>SQL</> language (typically in a low-level
language such as C). They generally correspond to
what are often known as abstract data types.
<productname>PostgreSQL</productname>
can only operate on such types through functions provided
by the user and only understands the behavior of such
types to the extent that the user describes them. Base types are
further subdivided into scalar and array types. For each scalar type,
a corresponding array type is automatically created that can hold
variable-size arrays of that scalar type.
</para>
<para>
Composite types, or row types, are created whenever the user creates a
table; it's also possible to define a <quote>stand-alone</> composite
type with no associated table. A composite type is simply a list of
base types with associated field names. A value of a composite type
is a row or record of field values. The user can access the component
fields from <acronym>SQL</> queries.
</para>
<para>
A domain type is based on a particular base
type and for many purposes is interchangeable with its base type.
However, a domain may have constraints that restrict its valid values
to a subset of what the underlying base type would allow. Domains can
be created by simple <acronym>SQL</> commands.
</para>
<para>
Finally, there are a few <quote>pseudo-types</> for special purposes.
Pseudo-types cannot appear as fields of tables or composite types, but
they can be used to declare the argument and result types of functions.
This provides a mechanism within the type system to identify special
classes of functions. <xref
linkend="datatype-pseudotypes-table"> lists the existing
pseudo-types.
</para>
<sect2 id="types-polymorphic">
<title>Polymorphic Types and Functions</title>
<indexterm>
<primary>polymorphic types</primary>
</indexterm>
<indexterm>
<primary>polymorphic functions</primary>
</indexterm>
<para>
Two pseudo-types of special interest are <type>anyelement</> and
<type>anyarray</>, which are collectively called <firstterm>polymorphic
types</>. Any function declared using these types is said to be
a <firstterm>polymorphic function</>. A polymorphic function can
operate on many different data types, with the specific data type(s)
being determined by the data types actually passed to it in a particular
call.
</para>
<para>
Polymorphic arguments and results are tied to each other and are resolved
to a specific data type when a query calling a polymorphic function is
parsed. Each position (either argument or return value) declared as
<type>anyelement</type> is allowed to have any specific actual
data type, but in any given call they must all be the
<emphasis>same</emphasis> actual type. Each
position declared as <type>anyarray</type> can have any array data type,
but similarly they must all be the same type. If there are
positions declared <type>anyarray</type> and others declared
<type>anyelement</type>, the actual array type in the
<type>anyarray</type> positions must be an array whose elements are
the same type appearing in the <type>anyelement</type> positions.
</para>
<para>
Thus, when more than one argument position is declared with a polymorphic
type, the net effect is that only certain combinations of actual argument
types are allowed. For example, a function declared as
<literal>foo(anyelement, anyelement)</> will take any two input values,
so long as they are of the same data type.
</para>
<para>
When the return value of a function is declared as a polymorphic type,
there must be at least one argument position that is also polymorphic,
and the actual data type supplied as the argument determines the actual
result type for that call. For example, if there were not already
an array subscripting mechanism, one could define a function that
implements subscripting as <literal>subscript(anyarray, integer)
returns anyelement</>. This declaration constrains the actual first
argument to be an array type, and allows the parser to infer the correct
result type from the actual first argument's type.
</para>
</sect2>
</sect1>
&xfunc;
&xaggr;
&xtypes;
&xoper;
&xaggr;
&xindex;
</chapter>

43
doc/src/sgml/func.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.164 2003/08/04 14:00:13 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.165 2003/08/09 22:50:21 tgl Exp $
PostgreSQL documentation
-->
@ -7044,28 +7044,67 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
<tbody>
<row>
<entry> <literal>=</literal> </entry>
<entry>equals</entry>
<entry>equal</entry>
<entry><literal>ARRAY[1.1,2.1,3.1]::int[] = ARRAY[1,2,3]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>&lt;&gt;</literal> </entry>
<entry>not equal</entry>
<entry><literal>ARRAY[1,2,3] &lt;&gt; ARRAY[1,2,4]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>&lt;</literal> </entry>
<entry>less than</entry>
<entry><literal>ARRAY[1,2,3] &lt; ARRAY[1,2,4]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>&gt;</literal> </entry>
<entry>greater than</entry>
<entry><literal>ARRAY[1,4,3] &gt; ARRAY[1,2,4]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>&lt;=</literal> </entry>
<entry>less than or equal</entry>
<entry><literal>ARRAY[1,2,3] &lt;= ARRAY[1,2,3]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>&gt;=</literal> </entry>
<entry>greater than or equal</entry>
<entry><literal>ARRAY[1,4,3] &gt;= ARRAY[1,4,3]</literal></entry>
<entry><literal>t</literal></entry>
</row>
<row>
<entry> <literal>||</literal> </entry>
<entry>array-to-array concatenation</entry>
<entry><literal>ARRAY[1,2,3] || ARRAY[4,5,6]</literal></entry>
<entry><literal>{{1,2,3},{4,5,6}}</literal></entry>
</row>
<row>
<entry> <literal>||</literal> </entry>
<entry>array-to-array concatenation</entry>
<entry><literal>ARRAY[1,2,3] || ARRAY[[4,5,6],[7,8,9]]</literal></entry>
<entry><literal>{{1,2,3},{4,5,6},{7,8,9}}</literal></entry>
</row>
<row>
<entry> <literal>||</literal> </entry>
<entry>element-to-array concatenation</entry>
<entry><literal>3 || ARRAY[4,5,6]</literal></entry>
<entry><literal>{3,4,5,6}</literal></entry>
</row>
<row>
<entry> <literal>||</literal> </entry>
<entry>array-to-element concatenation</entry>

168
doc/src/sgml/plpgsql.sgml
View File

@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/plpgsql.sgml,v 1.19 2003/05/28 16:03:55 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/plpgsql.sgml,v 1.20 2003/08/09 22:50:22 tgl Exp $
-->
<chapter id="plpgsql">
@ -177,16 +177,54 @@ END;
</para>
</sect2>
<sect2 id="plpgsql-overview-developing-in-plpgsql">
<title>Developing in <application>PL/pgSQL</application></title>
<sect2 id="plpgsql-args-results">
<title>Supported Argument and Result Datatypes</title>
<para>
Functions written in <application>PL/pgSQL</application> can accept
as arguments any scalar or array datatype supported by the server,
and they can return a result of any of these types. They can also
accept or return any composite type (row type) specified by name.
It is also possible to declare a <application>PL/pgSQL</application>
function as returning <type>record</>, which means that the result
is a row type whose columns are determined by specification in the
calling query, as discussed in <xref linkend="queries-tablefunctions">.
</para>
<para>
<application>PL/pgSQL</> functions may also be declared to accept
and return the <quote>polymorphic</> types
<type>anyelement</type> and <type>anyarray</type>. The actual
datatypes handled by a polymorphic function can vary from call to
call, as discussed in <xref linkend="types-polymorphic">.
An example is shown in <xref linkend="plpgsql-declaration-aliases">.
</para>
<para>
<application>PL/pgSQL</> functions can also be declared to return
a <quote>set</>, or table, of any datatype they can return a single
instance of. Such a function generates its output by executing
<literal>RETURN NEXT</> for each desired element of the result set.
</para>
<para>
Finally, a <application>PL/pgSQL</> function may be declared to return
<type>void</> if it has no useful return value.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-development-tips">
<title>Tips for 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
<application>PL/pgSQL</> is to 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
<application>psql</application> to load and test 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
REPLACE FUNCTION</>. That way you can just reload the file to update
the function definition. For example:
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS '
@ -197,8 +235,8 @@ end;
</para>
<para>
While running <command>psql</command>, you can load or reload such a
function definition file with
While running <application>psql</application>, you can load or reload such
a function definition file with
<programlisting>
\i filename.sql
</programlisting>
@ -213,39 +251,40 @@ end;
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>
<sect2 id="plpgsql-quote-tips">
<title>Handling of Quote Marks</title>
<para>
Since the code of any procedural language function is specified
Since the code of a <application>PL/pgSQL</> function is specified in
<command>CREATE FUNCTION</command> as a string literal, single
quotes inside the function body must be escaped. This can lead to
quotes inside the function body must be escaped by doubling them.
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.
linkend="plpgsql-statements-executing-dyn">. This chart may be useful
as a summary of the needed numbers of quote marks in
various situations.
</para>
<variablelist>
<varlistentry>
<term>1 quotation mark</term>
<term>1 quote mark</term>
<listitem>
<para>
To begin/end function bodies, for example:
To begin and end the function body, for example:
<programlisting>
CREATE FUNCTION foo() RETURNS integer AS '...'
LANGUAGE plpgsql;
</programlisting>
Anywhere within the function body, quote marks <emphasis>must</>
appear in pairs.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>2 quotation marks</term>
<term>2 quote marks</term>
<listitem>
<para>
For string literals inside the function body, for example:
@ -253,7 +292,7 @@ CREATE FUNCTION foo() RETURNS integer AS '...'
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
</programlisting>
The second line is interpreted as
The second line is seen by <application>PL/pgSQL</> as
<programlisting>
SELECT * FROM users WHERE f_name='foobar';
</programlisting>
@ -262,22 +301,22 @@ SELECT * FROM users WHERE f_name='foobar';
</varlistentry>
<varlistentry>
<term>4 quotation marks</term>
<term>4 quote marks</term>
<listitem>
<para>
When you need a single quote in a string inside the function
When you need a single quote in a string constant 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>.
The value actually appended to <literal>a_output</literal> would be:
<literal> AND name LIKE 'foobar' AND xyz</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>6 quotation marks</term>
<term>6 quote marks</term>
<listitem>
<para>
When a single quote in a string inside the function body is
@ -285,14 +324,14 @@ a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar''''''
</programlisting>
The value of <literal>a_output</literal> would then be:
The value appended to <literal>a_output</literal> would then be:
<literal> AND name LIKE 'foobar'</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>10 quotation marks</term>
<term>10 quote marks</term>
<listitem>
<para>
When you want two single quotes in a string constant (which
@ -315,6 +354,15 @@ if v_... like ''...'' then return ''...''; end if;
</listitem>
</varlistentry>
</variablelist>
<para>
A different approach is to escape quote marks in the function body
with a backslash rather than by doubling them. With this method
you'll find yourself writing things like <literal>\'\'</> instead
of <literal>''''</>. Some find this easier to keep track of, some
do not.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-structure">
@ -474,7 +522,8 @@ user_id CONSTANT integer := 10;
<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>
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:
@ -498,13 +547,43 @@ END;
' LANGUAGE plpgsql;
CREATE FUNCTION use_many_fields(tablename) RETURNS text AS '
CREATE FUNCTION concat_selected_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>
<para>
When the return type of a <application>PL/pgSQL</application>
function is declared as a polymorphic type (<type>anyelement</type>
or <type>anyarray</type>), a special parameter <literal>$0</literal>
is created. Its datatype is the actual return type of the function,
as deduced from the actual input types (see <xref
linkend="types-polymorphic">).
This allows the function to access its actual return type
as shown in <xref linkend="plpgsql-declaration-type">.
<literal>$0</literal> is initialized to NULL and can be modified by
the function, so it can be used to hold the return value if desired,
though that is not required. <literal>$0</literal> can also be
given an alias. For example, this function works on any datatype
that has a <literal>+</> operator:
<programlisting>
CREATE FUNCTION add_three_values(anyelement, anyelement, anyelement)
RETURNS anyelement AS '
DECLARE
result ALIAS FOR $0;
first ALIAS FOR $1;
second ALIAS FOR $2;
third ALIAS FOR $3;
BEGIN
result := first + second + third;
RETURN result;
END;
' LANGUAGE plpgsql;
</programlisting>
</para>
</sect2>
@ -536,6 +615,15 @@ user_id users.user_id%TYPE;
from <type>integer</type> to <type>real</type>), you may not need
to change your function definition.
</para>
<para>
<literal>%TYPE</literal> is particularly valuable in polymorphic
functions, since the data types needed for internal variables may
change from one call to the next. Appropriate variables can be
created by applying <literal>%TYPE</literal> to the function's
arguments or result placeholders.
</para>
</sect2>
<sect2 id="plpgsql-declaration-rowtypes">
@ -620,6 +708,14 @@ END;
<para>
Note that <literal>RECORD</> is not a true data type, only a placeholder.
One should also realize that when a <application>PL/pgSQL</application>
function is declared to return type <type>record</>, this is not quite the
same concept as a record variable, even though such a function may well
use a record variable to hold its result. In both cases the actual row
structure is unknown when the function is written, but for a function
returning <type>record</> the actual structure is determined when the
calling query is parsed, whereas a record variable can change its row
structure on-the-fly.
</para>
</sect2>
@ -965,7 +1061,7 @@ EXECUTE <replaceable class="command">command-string</replaceable>;
<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">,
overview in <xref linkend="plpgsql-quote-tips">,
which can save you some effort.
</para>
@ -1004,11 +1100,11 @@ EXECUTE ''UPDATE tbl SET ''
<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
Variables containing values that should be literal strings in the
constructed command 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
appropriate steps to return the input text enclosed in double
or single quotes respectively, with any embedded special characters
properly escaped.
</para>
@ -2253,7 +2349,7 @@ CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
<listitem>
<para>
In <productname>PostgreSQL</> you need to escape single
quotes in the function body. See <xref linkend="plpgsql-quote">.
quotes in the function body. See <xref linkend="plpgsql-quote-tips">.
</para>
</listitem>

91
doc/src/sgml/syntax.sgml
View File

@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.80 2003/08/04 14:00:14 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.81 2003/08/09 22:50:22 tgl Exp $
-->
<chapter id="sql-syntax">
@ -867,7 +867,8 @@ SELECT 3 OPERATOR(pg_catalog.+) 4;
<listitem>
<para>
A positional parameter reference, in the body of a function definition.
A positional parameter reference, in the body of a function definition
or prepared statement.
</para>
</listitem>
@ -901,6 +902,12 @@ SELECT 3 OPERATOR(pg_catalog.+) 4;
</para>
</listitem>
<listitem>
<para>
An array constructor.
</para>
</listitem>
<listitem>
<para>
Another value expression in parentheses, useful to group subexpressions and override precedence.
@ -1216,8 +1223,86 @@ SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
</para>
</sect2>
<sect2 id="sql-syntax-array-constructors">
<title>Array Constructors</title>
<indexterm>
<primary>arrays</primary>
<secondary>constructors</secondary>
</indexterm>
<para>
An <firstterm>array constructor</> is an expression that builds an
array value from values for its member elements. A simple array
constructor
consists of the keyword <literal>ARRAY</literal>, a left square bracket
<literal>[</>, one or more expressions (separated by commas) for the
array element values, and finally a right square bracket <literal>]</>.
For example,
<programlisting>
SELECT ARRAY[1,2,3+4];
array
---------
{1,2,7}
(1 row)
</programlisting>
The array element type is the common type of the member expressions,
determined using the same rules as for <literal>UNION</> or
<literal>CASE</> constructs (see <xref linkend="typeconv-union-case">).
</para>
<para>
Multidimensional array values can be built by nesting array
constructors.
In the inner constructors, the keyword <literal>ARRAY</literal> may
be omitted. For example, these produce the same result:
<programlisting>
SELECT ARRAY[ARRAY[1,2], ARRAY[3,4]];
array
---------------
{{1,2},{3,4}}
(1 row)
SELECT ARRAY[[1,2],[3,4]];
array
---------------
{{1,2},{3,4}}
(1 row)
</programlisting>
Since multidimensional arrays must be rectangular, inner constructors
at the same level must produce sub-arrays of identical dimensions.
</para>
<para>
It is also possible to construct an array from the results of a
subquery. In this form, the array constructor is written with the
keyword <literal>ARRAY</literal> followed by a parenthesized (not
bracketed) subquery. For example:
<programlisting>
SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
?column?
-------------------------------------------------------------
{2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31}
(1 row)
</programlisting>
The sub-select must return a single column. The
resulting one-dimensional array will have an element for each row in the
sub-select result, with an element type matching that of the sub-select's
output column.
</para>
<para>
The subscripts of an array value built with <literal>ARRAY</literal>
always begin with one. For more information about arrays, see
<xref linkend="arrays">.
</para>
</sect2>
<sect2 id="syntax-express-eval">
<title>Expression Evaluation</title>
<title>Expression Evaluation Rules</title>
<para>
The order of evaluation of subexpressions is not defined. In

58
doc/src/sgml/xaggr.sgml
View File

@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/xaggr.sgml,v 1.20 2003/04/10 01:22:44 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/xaggr.sgml,v 1.21 2003/08/09 22:50:22 tgl Exp $
-->
<sect1 id="xaggr">
@ -72,8 +72,9 @@ SELECT complex_sum(a) FROM test_complex;
omitting the <literal>initcond</literal> phrase, so that the initial state
condition is null. Ordinarily this would mean that the <literal>sfunc</literal>
would need to check for a null state-condition input, but for
<function>sum</function> and some other simple aggregates like <function>max</> and <function>min</>,
it would be sufficient to insert the first nonnull input value into
<function>sum</function> and some other simple aggregates like
<function>max</> and <function>min</>,
it is sufficient to insert the first nonnull input value into
the state variable and then start applying the transition function
at the second nonnull input value. <productname>PostgreSQL</productname>
will do that automatically if the initial condition is null and
@ -111,8 +112,55 @@ CREATE AGGREGATE avg (
</para>
<para>
For further details see the description of the <command>CREATE
AGGREGATE</command> command in <xref linkend="reference">.
Aggregate functions may use polymorphic
state transition functions or final functions, so that the same functions
can be used to implement multiple aggregates.
See <xref linkend="types-polymorphic">
for an explanation of polymorphic functions.
Going a step further, the aggregate function itself may be specified
with a polymorphic base type and state type, allowing a single
aggregate definition to serve for multiple input data types.
Here is an example of a polymorphic aggregate:
<programlisting>
CREATE AGGREGATE array_accum (
sfunc = array_append,
basetype = anyelement,
stype = anyarray,
initcond = '{}'
);
</programlisting>
Here, the actual state type for any aggregate call is the array type
having the actual input type as elements.
</para>
<para>
Here's the output using two different actual data types as arguments:
<programlisting>
SELECT attrelid::regclass, array_accum(attname)
FROM pg_attribute WHERE attnum > 0
AND attrelid = 'pg_user'::regclass GROUP BY attrelid;
attrelid | array_accum
----------+-----------------------------------------------------------------------------
pg_user | {usename,usesysid,usecreatedb,usesuper,usecatupd,passwd,valuntil,useconfig}
(1 row)
SELECT attrelid::regclass, array_accum(atttypid)
FROM pg_attribute WHERE attnum > 0
AND attrelid = 'pg_user'::regclass GROUP BY attrelid;
attrelid | array_accum
----------+------------------------------
pg_user | {19,23,16,16,16,25,702,1009}
(1 row)
</programlisting>
</para>
<para>
For further details see the
<xref linkend="sql-createaggregate" endterm="sql-createaggregate-title">
command.
</para>
</sect1>

171
doc/src/sgml/xfunc.sgml
View File

@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.70 2003/07/25 20:17:49 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.71 2003/08/09 22:50:22 tgl Exp $
-->
<sect1 id="xfunc">
@ -41,22 +41,29 @@ $Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.70 2003/07/25 20:17:49 tgl E
<para>
Every kind
of function can take base types, composite types, or
some combination as arguments (parameters). In addition,
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 with those. Examples in this section
can also be found in <filename>funcs.sql</filename>
and <filename>funcs.c</filename> in the tutorial directory.
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>
@ -67,8 +74,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/xfunc.sgml,v 1.70 2003/07/25 20:17:49 tgl E
<para>
SQL functions execute an arbitrary list of SQL statements, returning
the result of the last query in the list, which must be a
<literal>SELECT</>.
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
@ -276,7 +282,7 @@ CREATE FUNCTION new_emp() RETURNS emp AS '
' LANGUAGE SQL;
</programlisting>
In this case we have specified each of the attributes
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>
@ -316,7 +322,7 @@ ERROR: function declared to return emp returns varchar instead of text at colum
</para>
<para>
This is an example for how to extract an attribute out of a row type:
This is an example of extracting an attribute out of a row type:
<screen>
SELECT (new_emp()).name;
@ -330,7 +336,7 @@ SELECT (new_emp()).name;
<screen>
SELECT new_emp().name;
ERROR: syntax error at or near "."
ERROR: syntax error at or near "." at character 17
</screen>
</para>
@ -509,6 +515,68 @@ SELECT name, listchildren(name) FROM nodes;
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="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">
@ -1999,6 +2067,89 @@ CREATE OR REPLACE FUNCTION testpassbyval(integer, integer) RETURNS SETOF __testp
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="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">