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集合返回函数

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本节描述那些可能返回多于一行的函数。目前这个类中被使用最广泛的是级数生成函数, 如系列生成函数表和下标生成函数表所述。其他更特殊的集合返回函数在本手册的其他地方描述。

系列生成函数

generate_series ( start integer, stop integer [, step integer ] ) → setof integer

generate_series ( start bigint, stop bigint [, step bigint ] ) → setof bigint

generate_series ( start numeric, stop numeric [, step numeric ] ) → setof numeric

startstop生成一系列的值,步长为stepstep默认为1。

generate_series ( start timestamp, stop timestamp, step interval ) → setof timestamp

generate_series ( start timestamp with time zone, stop timestamp with time zone, step interval ) → setof timestamp with time zone

startstop生成一系列的值,步长为step

step为正时,如果start大于stop则返回零行。 相反,当step为负时,如果start小于stop则返回零行。 如果任何输入为NULL也会返回零行。step为零是一个错误。下面是一些例子:

    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)

下标生成函数

generate_subscripts ( array anyarray, dim integer ) → setof integer

生成一个包含给定数组第dim维度的有效下标的序列。

generate_subscripts ( array anyarray, dim integer, reverse boolean ) → setof integer

生成一个包含给定数组第dim维度的有效下标的序列。当reverse为真时,以相反的顺序返回序列。

generate_subscripts是一个快捷函数,它为给定数组的指定维度生成一组合法的下标。 对于不具有请求维度的数组返回零行,对于任何输入为NULL数组也返回零行。下面是一些例子:

    -- 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)

FROM子句中的函数以WITH ORDINALITY作为后缀时,将在函数的输出列上附加一个bigint列,该列从 1 开始,函数输出的每一行加 1。 这在 unnest()等集合返回函数的情况下最有用。

    -- 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)