| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Jonathan Vanasco <postgres(at)2xlp(dot)com> |
| Cc: | PostgreSQL General <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: issue with double ordering in a wrapped distinct |
| Date: | 2014-11-18 23:43:14 |
| Message-ID: | 25752.1416354194@sss.pgh.pa.us |
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| Lists: | pgsql-general |
Jonathan Vanasco <postgres(at)2xlp(dot)com> writes:
> This is what I don't understand -- notice the two order_by calls.
> If i run this with an inner and outer order_by, I get ~305ms. (I don't think I need both, but I wasn't sure if ordering is kept from a subselect )
> If i run this with only the inner, I get ~304ms.
> If I run this with only the outer, it's pushing over 10minutes again
> i'm wondering if anyone might know why that performance hit would be happening
The inner ORDER BY would prevent the planner from flattening the two
query levels into one; but as for why it gets a much worse plan after
flattening --- insufficient data.
https://wiki.postgresql.org/wiki/Slow_Query_Questions
regards, tom lane
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