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Re: Bug? 8.0 does not use partial index

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Palle Girgensohn <girgen(at)pingpong(dot)net>
Cc: John Hansen <john(at)geeknet(dot)com(dot)au>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Bug? 8.0 does not use partial index
Date: 2005-01-14 01:27:21
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Lists: pgsql-hackers
Palle Girgensohn <girgen(at)pingpong(dot)net> writes:
> Interesting. Yes, I might be that desperate, actually. As desperate as 30 s 
> vs 30 ms can get me... :)

> And this one would actually accumulate stats alright? Looks kinda hairy to 
> me. It's not really a partial index anymore, but I guess that doesn't 
> matter...

Yeah, it would.  The trick is finding a mapping function that will map
all the cases you care about to distinct values.

> How is the behaviour on 7.4 for this stuff? It seems 7.4 does use the 
> partial index, but not always, as shown in a previous mail. Will this 
> example work on both versions, or just for 8.0?

7.4 doesn't keep stats on functional indexes, so the hack is a
nonstarter there.

I am not sure why you are seeing different results on 7.4 than 8.0 for
the original example --- 7.4 is certainly not smarter than 8.0, and we
seem to have ruled out the idea that some sort of glitch is confusing
the 8.0 planner.

			regards, tom lane

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