From: | Stephen Frost <sfrost(at)snowman(dot)net> |
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To: | Merlin Moncure <mmoncure(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Johnston <polobo(at)yahoo(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Planner hints in Postgresql |
Date: | 2014-03-17 17:58:46 |
Message-ID: | 20140317175846.GM26394@tamriel.snowman.net |
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Lists: | pgsql-hackers |
* Merlin Moncure (mmoncure(at)gmail(dot)com) wrote:
> Yeah -- the most common case I see is outlier culling where several
> repeated low non-deterministic selectivity quals stack reducing the
> row count estimate to 1. For example:
> SELECT * FROM foo WHERE length(bar) <= 1000 AND length(bar) >= 2;
This is exactly the issue that I've seen also- where we end up picking a
Nested Loop because we think only one row is going to be returned and
instead we end up getting a bunch and it takes forever.
There was also some speculation on trying to change plans mid-stream to
address a situation like that, once we realize what's happening. Not
sure that's really practical but it would be nice to find some solution.
Thanks,
Stephen
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