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Re: 8.x index insert performance

From: mark(at)mark(dot)mielke(dot)cc
To: Merlin Moncure <merlin(dot)moncure(at)rcsonline(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: 8.x index insert performance
Date: 2005-10-31 23:57:09
Message-ID: 20051031235709.GA6393@mark.mielke.cc (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, Oct 31, 2005 at 03:27:31PM -0500, Merlin Moncure wrote:
> > On Mon, Oct 31, 2005 at 12:32:03PM -0500, Merlin Moncure wrote:
> > > if that index is causing the problem, you may want to consider setting
> > > up partial index to exclude null values.
> > Hey all.
> > Pardon my ignorance. :-)
> > I've been trying to figure out whether null values are indexed or not from
> > the documentation. I was under the impression, that null values are not
> > stored in the index. Occassionally, though, I then see a suggestion such
> > as the above, that seems to indicate to me that null values *are* stored
> > in the index, allowing for the 'exclude null values' to have effect?
> > Which is it? :-)
> I think I'm the ignorant one...do explain on any lookup on an indexed
> field where the field value is null and you get a seqscan.

Nahhh... I think the documentation could use more explicit or obvious
explanation. Or, I could have checked the source code to see. In any case,
I expect we aren't the only ones that lacked confidence.

Tom was kind enough to point out that null values are stored. I expect
that the seqscan is used if the null values are not selective enough,
the same as any other value that isn't selective enough.

Now we can both have a little more confidence! :-)

Cheers,
mark

-- 
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