From: | Jeff Frost <jeff(at)frostconsultingllc(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: strange index behaviour with different statistics target |
Date: | 2009-01-13 23:23:08 |
Message-ID: | Pine.LNX.4.64.0901131513480.5588@discord.home.frostconsultingllc.com |
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Lists: | pgsql-performance |
On Tue, 13 Jan 2009, Tom Lane wrote:
> Jeff Frost <jeff(at)frostconsultingllc(dot)com> writes:
>> So, my question is, should changing the stats target on the shape column
>> affect the stats for the content_id and content_type columns?
>
> It would change the size of the sample for the table, which might
> improve the accuracy of the stats. IIRC you'd still get the same number
> of histogram entries and most-common-values for the other columns, but
> they might be more accurate.
Why would they be more accurate? Do they somehow correlate with the other
column's histogram and most-common-values when the stats target is increased
on that column?
The planner is choosing a plan I like for the query, I'm just trying to
understand why it's doing that since the planner thinks the gist index is
going to give it a single row (vs the 2827 rows it actually gets) and the fact
that the cost didn't change for perusing the gist index. I guess I was
expecting the estimated rowcount and cost for perusing the gist index to go up
and when it didn't I was pleasantly surprised to find I got a plan I wanted
anyway.
>
>> Also, why does the index on content_id win out over the compound index
>> on (content_type, content_id)?
>
> It's deciding (apparently correctly, from the explain results) that the
> larger index isn't increasing the selectivity enough to be worth its
> extra search cost. I suppose content_type = 'Story' isn't very
> selective in this table?
Ah! You're right, especially with this content_id!
--
Jeff Frost, Owner <jeff(at)frostconsultingllc(dot)com>
Frost Consulting, LLC http://www.frostconsultingllc.com/
Phone: 916-647-6411 FAX: 916-405-4032
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