Table functions

Table functions are functions that produce a set of rows. They can either be used in place of a relation in the FROM clause, or within the select list of a query.

If used within the select list, the table functions will be evaluated per row of the relations in the FROM clause, generating one or more rows which are appended to the result set. If multiple table functions with different amounts of rows are used, NULL values will be returned for the functions that are exhausted.

For example:

cr> select unnest([1, 2, 3]), unnest([1, 2]);
+-------------------+----------------+
| unnest([1, 2, 3]) | unnest([1, 2]) |
+-------------------+----------------+
|                 1 |              1 |
|                 2 |              2 |
|                 3 |           NULL |
+-------------------+----------------+
SELECT 3 rows in set (... sec)

Note

Table functions in the select list are executed after aggregations. So aggregations can be used as arguments to table functions, but the other way around is not allowed, unless sub queries are utilized.

For example:

(SELECT aggregate_func(col) FROM (SELECT table_func(...) AS col) ...)

Table of contents

Scalar functions

A scalar function, when used in the FROM clause in place of a relation, will result in a table of one row and one column, containing the scalar value returned from the function.

cr> SELECT * FROM abs(-5), initcap('hello world');
+-----+-------------+
| abs | initcap     |
+-----+-------------+
|   5 | Hello World |
+-----+-------------+
SELECT 1 row in set (... sec)

empty_row( )

empty_row doesn’t take any argument and produces a table with an empty row and no column.

cr> select * from empty_row();
SELECT OK, 1 row affected  (... sec)

unnest( array [ array , ] )

unnest takes any number of array parameters and produces a table where each provided array argument results in a column.

The columns are named colN where N is a number starting at 1.

cr> select * from unnest([1, 2, 3], ['Arthur', 'Trillian', 'Marvin']);
+------+----------+
| col1 | col2     |
+------+----------+
|    1 | Arthur   |
|    2 | Trillian |
|    3 | Marvin   |
+------+----------+
SELECT 3 rows in set (... sec)

pg_catalog.generate_series(start, stop, [step])

Generate a series of values from inclusive start to inclusive stop with step increments.

The argument can be integer or bigint, in which case step is optional and defaults to 1.

start and stop can also be of type timestamp with time zone or timestamp without time zone in which case step is required and must be of type interval.

The return value always matches the start / stop types.

cr> SELECT * FROM generate_series(1, 4);
+------+
| col1 |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
+------+
SELECT 4 rows in set (... sec)
cr> SELECT
...     x,
...     date_format('%Y-%m-%d, %H:%i', x)
...     FROM generate_series('2019-01-01 00:00'::timestamp, '2019-01-04 00:00'::timestamp, '30 hours'::interval) AS t(x);
+---------------+-----------------------------------+
|             x | date_format('%Y-%m-%d, %H:%i', x) |
+---------------+-----------------------------------+
| 1546300800000 | 2019-01-01, 00:00                 |
| 1546408800000 | 2019-01-02, 06:00                 |
| 1546516800000 | 2019-01-03, 12:00                 |
+---------------+-----------------------------------+
SELECT 3 rows in set (... sec)

pg_catalog.generate_subscripts(array, dim, [reverse])

Generate the subscripts for the specified dimension dim 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).

If reverse is true the subscripts will be returned in reverse order.

This example takes a one dimensional array of four elements, where elements at positions 1 and 3 are NULL:

cr> SELECT generate_subscripts([NULL, 1, NULL, 2], 1) AS s;
+---+
| s |
+---+
| 1 |
| 2 |
| 3 |
| 4 |
+---+
SELECT 4 rows in set (... sec)

This example returns the reversed list of subscripts for the same array:

cr> SELECT generate_subscripts([NULL, 1, NULL, 2], 1, true) AS s;
+---+
| s |
+---+
| 4 |
| 3 |
| 2 |
| 1 |
+---+
SELECT 4 rows in set (... sec)

This example works on an array of three dimensions. Each of the elements within a given level must be either NULL, or an array of the same size as the other arrays within the same level.

cr> select generate_subscripts([[[1],[2]], [[3],[4]], [[4],[5]]], 2) as s;
+---+
| s |
+---+
| 1 |
| 2 |
+---+
SELECT 2 rows in set (... sec)

regexp_matches(source, pattern [, flags])

Uses the regular expression pattern to match against the source string.

The result rows have one column:

Column name Description
groups array(text)

If pattern matches source, an array of the matched regular expression groups is returned.

If no regular expression group was used, the whole pattern is used as a group.

A regular expression group is formed by a subexpression that is surrounded by parentheses. The position of a group is determined by the position of its opening parenthesis.

For example when matching the pattern \b([A-Z]) a match for the subexpression ([A-Z]) would create group No. 1. If you want to group items with parentheses, but without grouping, use (?...).

For example matching the regular expression ([Aa](.+)z) against alcatraz, results in these groups:

  • group 1: alcatraz (from first to last parenthesis or whole pattern)
  • group 2: lcatra (beginning at second parenthesis)

The regexp_matches function will return all groups as a text array:

cr> select regexp_matches('alcatraz', '(a(.+)z)') as matched;
+------------------------+
| matched                |
+------------------------+
| ["alcatraz", "lcatra"] |
+------------------------+
SELECT 1 row in set (... sec)
cr> select regexp_matches('alcatraz', 'traz') as matched;
+----------+
| matched  |
+----------+
| ["traz"] |
+----------+
SELECT 1 row in set (... sec)

Through array element access functionality, a group can be selected directly. See Arrays within objects for details.

cr> select regexp_matches('alcatraz', '(a(.+)z)')[2] as second_group;
+--------------+
| second_group |
+--------------+
| lcatra       |
+--------------+
SELECT 1 row in set (... sec)

Flags

This function takes a number of flags as optional third parameter. These flags are given as a string containing any of the characters listed below. Order does not matter.

Flag Description
i enable case insensitive matching
u enable unicode case folding when used together with i
U enable unicode support for character classes like \W
s make . match line terminators, too
m make ^ and $ match on the beginning or end of a line too.
x permit whitespace and line comments starting with #
d only \n is considered a line-terminator when using ^, $ and .
g keep matching until the end of source, instead of stopping at the first match.

Examples

In this example the pattern does not match anything in the source and the result is an empty table:

cr> select regexp_matches('foobar', '^(a(.+)z)$') as matched;
+---------+
| matched |
+---------+
+---------+
SELECT 0 rows in set (... sec)

In this example we find the term that follows two digits:

cr> select regexp_matches('99 bottles of beer on the wall', '\d{2}\s(\w+).*', 'ixU')
... as matched;
+-------------+
| matched     |
+-------------+
| ["bottles"] |
+-------------+
SELECT 1 row in set (... sec)

This example shows the use of flag g, splitting source into a set of arrays, each containing two entries:

cr>  select regexp_matches('#abc #def #ghi #jkl', '(#[^\s]*) (#[^\s]*)', 'g') as matched;
+------------------+
| matched          |
+------------------+
| ["#abc", "#def"] |
| ["#ghi", "#jkl"] |
+------------------+
SELECT 2 rows in set (... sec)

pg_catalog.pg_get_keywords()

Returns a list of SQL keywords and their categories.

The result rows have three columns:

Column name Description
word The SQL keyword
catcode Code for the category (R for reserved keywords, U for unreserved keywords)
catdesc The description of the category
cr> SELECT * FROM pg_catalog.pg_get_keywords() ORDER BY 1 LIMIT 4;
+----------+---------+------------+
| word     | catcode | catdesc    |
+----------+---------+------------+
| add      | R       | reserved   |
| alias    | U       | unreserved |
| all      | R       | reserved   |
| allocate | U       | unreserved |
+----------+---------+------------+
SELECT 4 rows in set (... sec)

information_schema._pg_expandarray(array)

Takes an array and returns a set of value and an index into the array.

Column name Description
x Value within the array
n Index of the value within the array
cr> SELECT information_schema._pg_expandarray(ARRAY['a', 'b']) AS result;
+----------+
| result   |
+----------+
| ["a", 1] |
| ["b", 2] |
+----------+
SELECT 2 rows in set (... sec)
cr> SELECT * from information_schema._pg_expandarray(ARRAY['a', 'b']);
+---+---+
| x | n |
+---+---+
| a | 1 |
| b | 2 |
+---+---+
SELECT 2 rows in set (... sec)

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