Re: "SELECT .. WHERE NOT IN" query running for hours

On 1/10/11 12:37 PM, "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:

>Scott Carey <scott(at)richrelevance(dot)com> wrote:
>
>> Often, the best query plans result from 'LEFT JOIN WHERE right
>> side is NULL' rather than NOT EXISTS however. I often get
>> performance gains by switching NOT EXISTS queries to LEFT JOIN
>> form.
>
>Even in 8.4 and later? I would think that the anti-join that Tom
>added in 8.4 would always perform at least as well as the LEFT JOIN
>technique you describe.
>
>-Kevin

Yes, in 8.4. The query planner definitely does not treat the two as
equivalent. I don't have a concrete example at hand right now, but I've
been working exclusively on 8.4 since a month after it was released. It
does often use an anti-join for NOT EXISTS, but does not seem to explore
all avenues there. Or perhaps the statistics it has differ for some
reason at that point. All I know, is that the resulting query plan
differs sometimes and I'd say 3 out of 4 times the LEFT JOIN variant is
more optimal when they differ.

>