On Tue, 13 Jan 2009, Tom Lane wrote:
Jeff Frost <jeff@xxxxxxxxxxxxxxxxxxxxxx> 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!
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Jeff Frost, Owner <jeff@xxxxxxxxxxxxxxxxxxxxxx>
Frost Consulting, LLC http://www.frostconsultingllc.com/
Phone: 916-647-6411 FAX: 916-405-4032
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