# 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 in[Table 9.58](https://www.postgresql.org/docs/10/static/functions-srf.html#functions-srf-series)and[Table 9.59](https://www.postgresql.org/docs/10/static/functions-srf.html#functions-srf-subscripts). Other, more specialized set-returning functions are described elsewhere in this manual. See[Section 7.2.1.4](https://www.postgresql.org/docs/10/static/queries-table-expressions.html#queries-tablefunctions)for ways to combine multiple set-returning functions.

**Table 9.58. Series Generating Functions**

| Function                                       | Argument Type                           | Return Type                                                                | Description                                                                       |
| ---------------------------------------------- | --------------------------------------- | -------------------------------------------------------------------------- | --------------------------------------------------------------------------------- |
| `generate_series(start`,`stop`)                | `int`,`bigint`or`numeric`               | `setof int`,`setof bigint`, or`setof numeric`(same as argument type)       | Generate a series of values, from`start`*\_to*`stop`\_with a step size of one     |
| `generate_series(start`,`stop`,`step`)         | `int`,`bigint`or`numeric`               | `setof int`,`setof bigint`or`setof numeric`(same as argument type)         | Generate a series of values, from`start`*\_to*`stop`*with a step size of*`step`\_ |
| `generate_series(start`,`stop`,`stepinterval`) | `timestamp`or`timestamp with time zone` | `setof timestamp`or`setof timestamp with time zone`(same as argument type) | Generate a series of values, from`start`*\_to*`stop`*with a step size of*`step`\_ |

When`step`*\_is positive, zero rows are returned if*`start`*is greater than*`stop`*. Conversely, when*`step`*is negative, zero rows are returned if*`start`*is less than*`stop`*. Zero rows are also returned for*`NULL`*inputs. It is an error for*`step`\_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_subscripts`is 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 the`FROM`clause is suffixed by`WITH ORDINALITY`, a`bigint`column 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 as`unnest()`.

```
-- 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)
```