Re: Double ocurring Subplan

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От Gurjeet Singh
Тема Re: Double ocurring Subplan
Дата
Msg-id BANLkTi=FPYxKWgyXsVXYh7FW8qSqjg-HVA@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Double ocurring Subplan  (Tom Lane <tgl@sss.pgh.pa.us>)
Список pgsql-hackers
On Fri, May 13, 2011 at 6:02 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Gurjeet Singh <singh.gurjeet@gmail.com> writes:
> I am listing the query, it's explain output and explain analyze output at
> the end.

> The EXPLAIN output shows the 'Subplan 2' node only once, whereas EXPLAIN
> ANALYZE output shows that the 'Subplan 2' is being executed twice . Is that
> true? Or is it just the plan printer getting confused? Is the confusion
> because of the 2 conditions in the WHERE clause of the correlated subquery?

The reason it looks like that is that the SubPlan is referenced in the
index condition, and there are actually two copies of that (indxqual and
indxqualorig).  They both point at the same physical subplan, but there
are two entries in the parent node's subPlan list.  In EXPLAIN you only
see one because ExecInitIndexScan skips initializing the indxquals in
EXPLAIN_ONLY mode.

In short: it's cosmetic.

That's a relief
 

We could probably suppress the duplicate printout when both references
are in the same plan node, but in bitmap scans the indxqual and
indxqualorig expressions are actually in different plan nodes (the
indexscan and heapscan respectively).  I'm not sure how we could
suppress the duplicate printout in that case, or whether it would even
make sense to do so --- after all, the subplan can in fact get called by
both nodes.

As long as it's not being re-evaluated, it's not a big deal.

It does confuse the casual onlooker, though; and if there's any automatic tool to parse and report explain analyze output, it might get its numbers quite wrong.

Regards,
--
Gurjeet Singh
EnterpriseDB Corporation
The Enterprise PostgreSQL Company

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