| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Bruno Wolff III <bruno(at)wolff(dot)to> |
| Cc: | Pavel Stehule <pavel(dot)stehule(at)hotmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: space for optimalization: DISTINCT without index |
| Date: | 2005-12-12 22:51:18 |
| Message-ID: | 2407.1134427878@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Bruno Wolff III <bruno(at)wolff(dot)to> writes:
> DISTINCT will require a sort step to remove duplicates. GROUP BY can use
> either a sort or hash aggregate plan. If there are few distinct values,
> the hash aggregate plan can be much faster.
The DISTINCT code hasn't been revisited in a long time. One obstacle to
improving it is that it's very tightly intertwined with ORDER BY. While
fixing that might be just a Small Matter Of Programming, it's not clear
how to decouple them without breaking DISTINCT ON.
regards, tom lane
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