From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Markus Schaber <schabios(at)logi-track(dot)com> |
Cc: | PostgreSQL Performance List <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Multiple Uniques |
Date: | 2004-09-02 14:22:37 |
Message-ID: | 19516.1094134957@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Markus Schaber <schabios(at)logi-track(dot)com> writes:
> Today, we stumbled about the following query plan on PostGreSQL 7.4.1:
> logigis=# explain select count(id) from (select distinct id from (select distinct ref_in_id as id from streets union select distinct nref_in_id as id from streets) as blubb) as blabb;
> I was somehow irritated by the fact that the Query Plan contains 4 Uniques.
Well, if you write a silly query, you can get a silly plan ...
As you appear to have realized later, given the definition of UNION,
all three of the explicit DISTINCTs are redundant.
> So, now my question is, why does the query optimizer not recognize that
> it can throw away those "non-unique" Sort/Unique passes?
Because the issue doesn't come up often enough to justify expending
cycles to check for it.
regards, tom lane
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