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Generated columns

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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
This commit is contained in:
Peter Eisentraut
2019-03-30 08:13:09 +01:00
parent 6b8b5364dd
commit fc22b6623b
84 changed files with 3067 additions and 157 deletions
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18
doc/src/sgml/trigger.sgml
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@@ -243,6 +243,24 @@
operation, and so they can return <symbol>NULL</symbol>.
</para>
<para>
Some considerations apply for generated
columns.<indexterm><primary>generated column</primary><secondary>in
triggers</secondary></indexterm> Stored generated columns are computed after
<literal>BEFORE</literal> triggers and before <literal>AFTER</literal>
triggers. Therefore, the generated value can be inspected in
<literal>AFTER</literal> triggers. In <literal>BEFORE</literal> triggers,
the <literal>OLD</literal> row contains the old generated value, as one
would expect, but the <literal>NEW</literal> row does not yet contain the
new generated value and should not be accessed. In the C language
interface, the content of the column is undefined at this point; a
higher-level programming language should prevent access to a stored
generated column in the <literal>NEW</literal> row in a
<literal>BEFORE</literal> trigger. Changes to the value of a generated
column in a <literal>BEFORE</literal> trigger are ignored and will be
overwritten.
</para>
<para>
If more than one trigger is defined for the same event on the same
relation, the triggers will be fired in alphabetical order by