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Generated columns
This is an SQL-standard feature that allows creating columns that are computed from expressions rather than assigned, similar to a view or materialized view but on a column basis. This implements one kind of generated column: stored (computed on write). Another kind, virtual (computed on read), is planned for the future, and some room is left for it. Reviewed-by: Michael Paquier <michael@paquier.xyz> Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com> Discussion: https://www.postgresql.org/message-id/flat/b151f851-4019-bdb1-699e-ebab07d2f40a@2ndquadrant.com
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‎contrib/postgres_fdw/expected/postgres_fdw.out‎

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Original file line numberDiff line numberDiff line change
@@ -6431,6 +6431,31 @@ select * from rem1;
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11 | bye remote
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(4 rows)
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-- ===================================================================
6435+
-- test generated columns
6436+
-- ===================================================================
6437+
create table gloc1 (a int, b int);
6438+
alter table gloc1 set (autovacuum_enabled = 'false');
6439+
create foreign table grem1 (
6440+
a int,
6441+
b int generated always as (a * 2) stored)
6442+
server loopback options(table_name 'gloc1');
6443+
insert into grem1 (a) values (1), (2);
6444+
update grem1 set a = 22 where a = 2;
6445+
select * from gloc1;
6446+
a | b
6447+
----+----
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1 | 2
6449+
22 | 44
6450+
(2 rows)
6451+
6452+
select * from grem1;
6453+
a | b
6454+
----+----
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1 | 2
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22 | 44
6457+
(2 rows)
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64346459
-- ===================================================================
64356460
-- test local triggers
64366461
-- ===================================================================

‎contrib/postgres_fdw/postgres_fdw.c‎

Lines changed: 2 additions & 1 deletion
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@@ -1644,9 +1644,10 @@ postgresPlanForeignModify(PlannerInfo *root,
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else if (operation == CMD_UPDATE)
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{
16461646
int col;
1647+
Bitmapset *allUpdatedCols = bms_union(rte->updatedCols, rte->extraUpdatedCols);
16471648

16481649
col = -1;
1649-
while ((col = bms_next_member(rte->updatedCols, col)) >= 0)
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while ((col = bms_next_member(allUpdatedCols, col)) >= 0)
16501651
{
16511652
/* bit numbers are offset by FirstLowInvalidHeapAttributeNumber */
16521653
AttrNumber attno = col + FirstLowInvalidHeapAttributeNumber;

‎contrib/postgres_fdw/sql/postgres_fdw.sql‎

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@@ -1363,6 +1363,20 @@ insert into rem1(f2) values('bye remote');
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select * from loc1;
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select * from rem1;
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-- ===================================================================
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-- test generated columns
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-- ===================================================================
1369+
create table gloc1 (a int, b int);
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alter table gloc1 set (autovacuum_enabled = 'false');
1371+
create foreign table grem1 (
1372+
a int,
1373+
b int generated always as (a * 2) stored)
1374+
server loopback options(table_name 'gloc1');
1375+
insert into grem1 (a) values (1), (2);
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update grem1 set a = 22 where a = 2;
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select * from gloc1;
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select * from grem1;
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-- ===================================================================
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-- test local triggers
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-- ===================================================================

‎doc/src/sgml/catalogs.sgml‎

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Original file line numberDiff line numberDiff line change
@@ -1129,9 +1129,11 @@
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<entry><type>bool</type></entry>
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<entry></entry>
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<entry>
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This column has a default value, in which case there will be a
1133-
corresponding entry in the <structname>pg_attrdef</structname>
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catalog that actually defines the value.
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This column has a default expression or generation expression, in which
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case there will be a corresponding entry in the
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<structname>pg_attrdef</structname> catalog that actually defines the
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expression. (Check <structfield>attgenerated</structfield> to
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determine whether this is a default or a generation expression.)
11351137
</entry>
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</row>
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@@ -1159,6 +1161,17 @@
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</entry>
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</row>
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<row>
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<entry><structfield>attgenerated</structfield></entry>
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<entry><type>char</type></entry>
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<entry></entry>
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<entry>
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If a zero byte (<literal>''</literal>), then not a generated column.
1170+
Otherwise, <literal>s</literal> = stored. (Other values might be added
1171+
in the future.)
1172+
</entry>
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</row>
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11621175
<row>
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<entry><structfield>attisdropped</structfield></entry>
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<entry><type>bool</type></entry>

‎doc/src/sgml/ddl.sgml‎

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Original file line numberDiff line numberDiff line change
@@ -233,6 +233,124 @@ CREATE TABLE products (
233233
</para>
234234
</sect1>
235235

236+
<sect1 id="ddl-generated-columns">
237+
<title>Generated Columns</title>
238+
239+
<indexterm zone="ddl-generated-columns">
240+
<primary>generated column</primary>
241+
</indexterm>
242+
243+
<para>
244+
A generated column is a special column that is always computed from other
245+
columns. Thus, it is for columns what a view is for tables. There are two
246+
kinds of generated columns: stored and virtual. A stored generated column
247+
is computed when it is written (inserted or updated) and occupies storage
248+
as if it were a normal column. A virtual generated column occupies no
249+
storage and is computed when it is read. Thus, a virtual generated column
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is similar to a view and a stored generated column is similar to a
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materialized view (except that it is always updated automatically).
252+
PostgreSQL currently implements only stored generated columns.
253+
</para>
254+
255+
<para>
256+
To create a generated column, use the <literal>GENERATED ALWAYS
257+
AS</literal> clause in <command>CREATE TABLE</command>, for example:
258+
<programlisting>
259+
CREATE TABLE people (
260+
...,
261+
height_cm numeric,
262+
height_in numeric <emphasis>GENERATED ALWAYS AS (height_cm * 2.54) STORED</emphasis>
263+
);
264+
</programlisting>
265+
The keyword <literal>STORED</literal> must be specified to choose the
266+
stored kind of generated column. See <xref linkend="sql-createtable"/> for
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more details.
268+
</para>
269+
270+
<para>
271+
A generated column cannot be written to directly. In
272+
<command>INSERT</command> or <command>UPDATE</command> commands, a value
273+
cannot be specified for a generated column, but the keyword
274+
<literal>DEFAULT</literal> may be specified.
275+
</para>
276+
277+
<para>
278+
Consider the differences between a column with a default and a generated
279+
column. The column default is evaluated once when the row is first
280+
inserted if no other value was provided; a generated column is updated
281+
whenever the row changes and cannot be overridden. A column default may
282+
not refer to other columns of the table; a generation expression would
283+
normally do so. A column default can use volatile functions, for example
284+
<literal>random()</literal> or functions referring to the current time;
285+
this is not allowed for generated columns.
286+
</para>
287+
288+
<para>
289+
Several restrictions apply to the definition of generated columns and
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tables involving generated columns:
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292+
<itemizedlist>
293+
<listitem>
294+
<para>
295+
The generation expression can only use immutable functions and cannot
296+
use subqueries or reference anything other than the current row in any
297+
way.
298+
</para>
299+
</listitem>
300+
<listitem>
301+
<para>
302+
A generation expression cannot reference another generated column.
303+
</para>
304+
</listitem>
305+
<listitem>
306+
<para>
307+
A generation expression cannot reference a system column, except
308+
<varname>tableoid</varname>.
309+
</para>
310+
</listitem>
311+
<listitem>
312+
<para>
313+
A generated column cannot have a column default or an identity definition.
314+
</para>
315+
</listitem>
316+
<listitem>
317+
<para>
318+
A generated column cannot be part of a partition key.
319+
</para>
320+
</listitem>
321+
<listitem>
322+
<para>
323+
Foreign tables can have generated columns. See <xref
324+
linkend="sql-createforeigntable"/> for details.
325+
</para>
326+
</listitem>
327+
</itemizedlist>
328+
</para>
329+
330+
<para>
331+
Additional considerations apply to the use of generated columns.
332+
<itemizedlist>
333+
<listitem>
334+
<para>
335+
Generated columns maintain access privileges separately from their
336+
underlying base columns. So, it is possible to arrange it so that a
337+
particular role can read from a generated column but not from the
338+
underlying base columns.
339+
</para>
340+
</listitem>
341+
<listitem>
342+
<para>
343+
Generated columns are, conceptually, updated after
344+
<literal>BEFORE</literal> triggers have run. Therefore, changes made to
345+
base columns in a <literal>BEFORE</literal> trigger will be reflected in
346+
generated columns. But conversely, it is not allowed to access
347+
generated columns in <literal>BEFORE</literal> triggers.
348+
</para>
349+
</listitem>
350+
</itemizedlist>
351+
</para>
352+
</sect1>
353+
236354
<sect1 id="ddl-constraints">
237355
<title>Constraints</title>
238356

‎doc/src/sgml/information_schema.sgml‎

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@@ -952,6 +952,62 @@
952952
</table>
953953
</sect1>
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955+
<sect1 id="infoschema-column-column-usage">
956+
<title><literal>column_column_usage</literal></title>
957+
958+
<para>
959+
The view <literal>column_column_usage</literal> identifies all generated
960+
columns that depend on another base column in the same table. Only tables
961+
owned by a currently enabled role are included.
962+
</para>
963+
964+
<table>
965+
<title><literal>column_column_usage</literal> Columns</title>
966+
967+
<tgroup cols="3">
968+
<thead>
969+
<row>
970+
<entry>Name</entry>
971+
<entry>Data Type</entry>
972+
<entry>Description</entry>
973+
</row>
974+
</thead>
975+
976+
<tbody>
977+
<row>
978+
<entry><literal>table_catalog</literal></entry>
979+
<entry><type>sql_identifier</type></entry>
980+
<entry>Name of the database containing the table (always the current database)</entry>
981+
</row>
982+
983+
<row>
984+
<entry><literal>table_schema</literal></entry>
985+
<entry><type>sql_identifier</type></entry>
986+
<entry>Name of the schema containing the table</entry>
987+
</row>
988+
989+
<row>
990+
<entry><literal>table_name</literal></entry>
991+
<entry><type>sql_identifier</type></entry>
992+
<entry>Name of the table</entry>
993+
</row>
994+
995+
<row>
996+
<entry><literal>column_name</literal></entry>
997+
<entry><type>sql_identifier</type></entry>
998+
<entry>Name of the base column that a generated column depends on</entry>
999+
</row>
1000+
1001+
<row>
1002+
<entry><literal>dependent_column</literal></entry>
1003+
<entry><type>sql_identifier</type></entry>
1004+
<entry>Name of the generated column</entry>
1005+
</row>
1006+
</tbody>
1007+
</tgroup>
1008+
</table>
1009+
</sect1>
1010+
9551011
<sect1 id="infoschema-column-domain-usage">
9561012
<title><literal>column_domain_usage</literal></title>
9571013

@@ -1648,13 +1704,19 @@
16481704
<row>
16491705
<entry><literal>is_generated</literal></entry>
16501706
<entry><type>character_data</type></entry>
1651-
<entry>Applies to a feature not available in <productname>PostgreSQL</productname></entry>
1707+
<entry>
1708+
If the column is a generated column, then <literal>ALWAYS</literal>,
1709+
else <literal>NEVER</literal>.
1710+
</entry>
16521711
</row>
16531712

16541713
<row>
16551714
<entry><literal>generation_expression</literal></entry>
16561715
<entry><type>character_data</type></entry>
1657-
<entry>Applies to a feature not available in <productname>PostgreSQL</productname></entry>
1716+
<entry>
1717+
If the column is a generated column, then the generation expression,
1718+
else null.
1719+
</entry>
16581720
</row>
16591721

16601722
<row>

‎doc/src/sgml/protocol.sgml‎

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@@ -6450,7 +6450,7 @@ Relation
64506450
</listitem>
64516451
</varlistentry>
64526452
</variablelist>
6453-
Next, the following message part appears for each column:
6453+
Next, the following message part appears for each column (except generated columns):
64546454
<variablelist>
64556455
<varlistentry>
64566456
<term>
@@ -6875,7 +6875,7 @@ TupleData
68756875
</listitem>
68766876
</varlistentry>
68776877
</variablelist>
6878-
Next, one of the following submessages appears for each column:
6878+
Next, one of the following submessages appears for each column (except generated columns):
68796879
<variablelist>
68806880
<varlistentry>
68816881
<term>

‎doc/src/sgml/ref/copy.sgml‎

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@@ -103,7 +103,8 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
103103
<listitem>
104104
<para>
105105
An optional list of columns to be copied. If no column list is
106-
specified, all columns of the table will be copied.
106+
specified, all columns of the table except generated columns will be
107+
copied.
107108
</para>
108109
</listitem>
109110
</varlistentry>

‎doc/src/sgml/ref/create_foreign_table.sgml‎

Lines changed: 26 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -42,7 +42,8 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="parameter">table_name
4242
{ NOT NULL |
4343
NULL |
4444
CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
45-
DEFAULT <replaceable>default_expr</replaceable> }
45+
DEFAULT <replaceable>default_expr</replaceable> |
46+
GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED }
4647

4748
<phrase>and <replaceable class="parameter">table_constraint</replaceable> is:</phrase>
4849

@@ -258,6 +259,30 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
258259
</listitem>
259260
</varlistentry>
260261

262+
<varlistentry>
263+
<term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED</literal><indexterm><primary>generated column</primary></indexterm></term>
264+
<listitem>
265+
<para>
266+
This clause creates the column as a <firstterm>generated
267+
column</firstterm>. The column cannot be written to, and when read it
268+
will be computed from the specified expression.
269+
</para>
270+
271+
<para>
272+
The keyword <literal>STORED</literal> is required to signify that the
273+
column will be computed on write. (The computed value will be presented
274+
to the foreign-data wrapper for storage and must be returned on
275+
reading.)
276+
</para>
277+
278+
<para>
279+
The generation expression can refer to other columns in the table, but
280+
not other generated columns. Any functions and operators used must be
281+
immutable. References to other tables are not allowed.
282+
</para>
283+
</listitem>
284+
</varlistentry>
285+
261286
<varlistentry>
262287
<term><replaceable class="parameter">server_name</replaceable></term>
263288
<listitem>

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