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doc: clarify handling of ON CONFLICT with triggers 

The previous wording was confusing.  Also move partitioning mention to a
more logical location.

Reported-by: neil@fairwindsoft.com

Discussion: https://postgr.es/m/20170703200710.27956.64565@wrigleys.postgresql.org

Backpatch-through: master
This commit is contained in:
Bruce Momjian 2023-12-07 21:35:29 -05:00
parent e5b8c4f68f
commit 2cc2d02dd0
1 changed files with 18 additions and 20 deletions
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38
doc/src/sgml/trigger.sgml
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@ -132,29 +132,19 @@
</para>
<para>
A statement that targets a parent table in an inheritance or partitioning
hierarchy does not cause the statement-level triggers of affected child
tables to be fired; only the parent table's statement-level triggers are
fired. However, row-level triggers of any affected child tables will be
fired.
If an <command>INSERT</command> contains an <literal>ON CONFLICT
DO UPDATE</literal> clause, it is possible for row-level
<literal>BEFORE</literal> <command>INSERT</command> and then
<literal>BEFORE</literal> <command>UPDATE</command> triggers
to be executed on triggered rows. Such interactions can be
complex if the triggers are not idempotent because change made by
<literal>BEFORE</literal> <command>INSERT</command> triggers will be
seen by <literal>BEFORE</literal> <command>UPDATE</command> triggers,
including changes to <varname>EXCLUDED</varname> columns.
</para>
<para>
If an <command>INSERT</command> contains an <literal>ON CONFLICT
DO UPDATE</literal> clause, it is possible that the effects of
row-level <literal>BEFORE</literal> <command>INSERT</command> triggers and
row-level <literal>BEFORE</literal> <command>UPDATE</command> triggers can
both be applied in a way that is apparent from the final state of
the updated row, if an <varname>EXCLUDED</varname> column is referenced.
There need not be an <varname>EXCLUDED</varname> column reference for
both sets of row-level <literal>BEFORE</literal> triggers to execute,
though. The
possibility of surprising outcomes should be considered when there
are both <literal>BEFORE</literal> <command>INSERT</command> and
<literal>BEFORE</literal> <command>UPDATE</command> row-level triggers
that change a row being inserted/updated (this can be
problematic even if the modifications are more or less equivalent, if
they're not also idempotent). Note that statement-level
Note that statement-level
<command>UPDATE</command> triggers are executed when <literal>ON
CONFLICT DO UPDATE</literal> is specified, regardless of whether or not
any rows were affected by the <command>UPDATE</command> (and
@ -169,6 +159,14 @@
triggers.
</para>
<para>
A statement that targets a parent table in an inheritance or partitioning
hierarchy does not cause the statement-level triggers of affected child
tables to be fired; only the parent table's statement-level triggers are
fired. However, row-level triggers of any affected child tables will be
fired.
</para>
<para>
If an <command>UPDATE</command> on a partitioned table causes a row to move
to another partition, it will be performed as a <command>DELETE</command>