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 |
|
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 |
---|---|
|
enable case insensitive matching |
|
enable unicode case folding when used together with |
|
enable unicode support for character classes like |
|
make |
|
make |
|
permit whitespace and line comments starting with |
|
only |
|
keep matching until the end of |
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 |
---|---|
|
The SQL keyword |
|
Code for the category (R for reserved keywords, U for unreserved keywords) |
|
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)