9.24. 集合回傳函式

This section describes functions that possibly return more than one row. The most widely used functions in this class are series generating functions, as detailed inTable 9.58andTable 9.59. Other, more specialized set-returning functions are described elsewhere in this manual. SeeSection 7.2.1.4for ways to combine multiple set-returning functions.

Table 9.58. Series Generating Functions

Function

Argument Type

Return Type

Description

generate_series(start,stop)

int,bigintornumeric

setof int,setof bigint, orsetof numeric(same as argument type)

Generate a series of values, fromstart_tostop_with a step size of one

generate_series(start,stop,step)

int,bigintornumeric

setof int,setof bigintorsetof numeric(same as argument type)

Generate a series of values, fromstart_tostopwith a step size ofstep_

generate_series(start,stop,stepinterval)

timestamportimestamp with time zone

setof timestamporsetof timestamp with time zone(same as argument type)

Generate a series of values, fromstart_tostopwith a step size ofstep_

Whenstep_is positive, zero rows are returned ifstartis greater thanstop. Conversely, whenstepis negative, zero rows are returned ifstartis less thanstop. Zero rows are also returned forNULLinputs. It is an error forstep_to be zero. Some examples follow:

SELECT * FROM generate_series(2,4);
generate_series
-----------------
2
3
4
(3 rows)
SELECT * FROM generate_series(5,1,-2);
generate_series
-----------------
5
3
1
(3 rows)
SELECT * FROM generate_series(4,3);
generate_series
-----------------
(0 rows)
SELECT generate_series(1.1, 4, 1.3);
generate_series
-----------------
1.1
2.4
3.7
(3 rows)
-- this example relies on the date-plus-integer operator
SELECT current_date + s.a AS dates FROM generate_series(0,14,7) AS s(a);
dates
------------
2004-02-05
2004-02-12
2004-02-19
(3 rows)
SELECT * FROM generate_series('2008-03-01 00:00'::timestamp,
'2008-03-04 12:00', '10 hours');
generate_series
---------------------
2008-03-01 00:00:00
2008-03-01 10:00:00
2008-03-01 20:00:00
2008-03-02 06:00:00
2008-03-02 16:00:00
2008-03-03 02:00:00
2008-03-03 12:00:00
2008-03-03 22:00:00
2008-03-04 08:00:00
(9 rows)

Table 9.59. Subscript Generating Functions

Function

Return Type

Description

generate_subscripts(array anyarray,dim int)

setof int

Generate a series comprising the given array's subscripts.

generate_subscripts(array anyarray,dim int,reverse boolean)

setof int

Generate a series comprising the given array's subscripts. When_reverse_is true, the series is returned in reverse order.

generate_subscriptsis a convenience function that generates the set of valid subscripts for the specified dimension of the given array. Zero rows are returned for arrays that do not have the requested dimension, or for NULL arrays (but valid subscripts are returned for NULL array elements). Some examples follow:

-- basic usage
SELECT generate_subscripts('{NULL,1,NULL,2}'::int[], 1) AS s;
s
---
1
2
3
4
(4 rows)
-- presenting an array, the subscript and the subscripted
-- value requires a subquery
SELECT * FROM arrays;
a
--------------------
{-1,-2}
{100,200,300}
(2 rows)
SELECT a AS array, s AS subscript, a[s] AS value
FROM (SELECT generate_subscripts(a, 1) AS s, a FROM arrays) foo;
array | subscript | value
---------------+-----------+-------
{-1,-2} | 1 | -1
{-1,-2} | 2 | -2
{100,200,300} | 1 | 100
{100,200,300} | 2 | 200
{100,200,300} | 3 | 300
(5 rows)
-- unnest a 2D array
CREATE OR REPLACE FUNCTION unnest2(anyarray)
RETURNS SETOF anyelement AS $$
select $1[i][j]
from generate_subscripts($1,1) g1(i),
generate_subscripts($1,2) g2(j);
$$ LANGUAGE sql IMMUTABLE;
CREATE FUNCTION
SELECT * FROM unnest2(ARRAY[[1,2],[3,4]]);
unnest2
---------
1
2
3
4
(4 rows)

When a function in theFROMclause is suffixed byWITH ORDINALITY, abigintcolumn is appended to the output which starts from 1 and increments by 1 for each row of the function's output. This is most useful in the case of set returning functions such asunnest().

-- set returning function WITH ORDINALITY
SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
ls | n
-----------------+----
pg_serial | 1
pg_twophase | 2
postmaster.opts | 3
pg_notify | 4
postgresql.conf | 5
pg_tblspc | 6
logfile | 7
base | 8
postmaster.pid | 9
pg_ident.conf | 10
global | 11
pg_xact | 12
pg_snapshots | 13
pg_multixact | 14
PG_VERSION | 15
pg_wal | 16
pg_hba.conf | 17
pg_stat_tmp | 18
pg_subtrans | 19
(19 rows)