本节介绍可能返回多行的函数。此类中最常用的函数是序列生成函数，如 表 9.65 和 表 9.66 中所述。本手册的其他位置介绍了其他更专业的集合返回函数。有关组合多个集合返回函数的方法，请参见 第 7.2.1.4 节。
表 9.65. 序列生成函数
|
函数 说明 |
|---|
|
从 |
|
从 |
当step 为正数时，如果start 大于stop，则返回 0 行。相反，当step 为负数时，如果start 小于stop，则返回 0 行。如果任何输入为NULL，也会返回 0 行。step 为零是错误的。下面是一些示例
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)
-- this example assumes that TimeZone is set to UTC; note the DST transition:
SELECT * FROM generate_series('2001-10-22 00:00 -04:00'::timestamptz,
'2001-11-01 00:00 -05:00'::timestamptz,
'1 day'::interval, 'America/New_York');
generate_series
------------------------
2001-10-22 04:00:00+00
2001-10-23 04:00:00+00
2001-10-24 04:00:00+00
2001-10-25 04:00:00+00
2001-10-26 04:00:00+00
2001-10-27 04:00:00+00
2001-10-28 04:00:00+00
2001-10-29 05:00:00+00
2001-10-30 05:00:00+00
2001-10-31 05:00:00+00
2001-11-01 05:00:00+00
(11 rows)
表 9.66. 下标生成函数
generate_subscripts 是一个便捷函数，可生成给定数组指定维度的有效下标集。对于没有请求的维度的数组或任何输入为NULL 的情况，将返回 0 行。下面是一些示例
-- 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)
当FROM 子句中的函数后缀为WITH ORDINALITY 时，一个bigint 列将附加到函数的输出列，该列从 1 开始，并针对函数输出的每一行递增 1。在集合返回函数（例如unnest()）的情况下，这非常有用。
-- 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)