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
