Re: Convert MAX_SAOP_ARRAY_SIZE to new guc

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От Paul Ramsey
Тема Re: Convert MAX_SAOP_ARRAY_SIZE to new guc
Дата
Msg-id CACowWR2N=-GuHtLY=6=qZ2yges-JF62xAJi1JX=Spb7ONnM_4Q@mail.gmail.com
обсуждение исходный текст
Ответ на Convert MAX_SAOP_ARRAY_SIZE to new guc  (James Coleman <jtc331@gmail.com>)
Ответы Re: Convert MAX_SAOP_ARRAY_SIZE to new guc  (Tom Lane <tgl@sss.pgh.pa.us>)
Список pgsql-hackers
Дерево обсуждения
On Fri, Nov 9, 2018 at 1:32 PM James Coleman <jtc331@gmail.com> wrote:
Summary:
Create new guc array_optimization_size_limit and use it to replace
MAX_SAOP_ARRAY_SIZE in predtest.c.

Status:
The attached patch applies cleanly to master, builds without error,
and passes tests locally.

Confirmed that it applies and builds cleaning and regresses without error in my environment (osx/clang)

My main comment is that the description of the purpose of the GUC doesn't help me understand when or why I might want to alter it from the default value. If nobody is going to alter it, because nobody understands it, it might as well remain a compile-time constant.

+       <para>
+        Sets the array size limit beyond which predicate optimization is not used.
+        The optimizer evaluates scalar array expressions to determine if they can
+        be treated as AND or OR clauses. This optimization proving is only performed
+        if the array contains at most this many items.
+        The default is <literal>100</literal>.
+       </para>

If I lower the value, what problem or use case do I solve? If I increase it, what do I solve? What gets faster or slower at different settings of the value? The description doesn't mention using the "IN" SQL clause which is the use case the parameter targets. I'd suggest alternate wording, but I'm actually still not 100% sure how a larger value would change the behaviour of "IN" in the presence of large numbers of values?

P.

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