Re: Question about difference in performance of 2 queries

Tom,

Thanks. I will make the changes you suggest concerning the indexes. I
am finding partial indexes to be very handy. :-)

I canceled the explain analyze on the other query as we have found the
problem and who knows how long it would take to complete.

Thanks again.

--sean

Tom Lane wrote:

>Sean Shanny <shannyconsulting(at)earthlink(dot)net> writes:
>
>
>>Here is the pg_stats data. The explain analyze queries are still running.
>>
>>
>
>
>
>>select * from pg_stats where tablename = 'f_pageviews' and attname =
>>'content_key';
>> schemaname | tablename | attname | null_frac | avg_width |
>>n_distinct | most_common_vals | most_common_freqs
>>|
>>histogram_bounds | correlation
>>------------+-------------+-------------+-----------+-----------+------------+------------------+-----------------------+-------------------------------------------------------------------------------------+-------------
>> public | f_pageviews | content_key | 0 | 4 |
>>983 | {-1,1528483} | {0.749333,0.00166667} |
>>
>>
>
>Oh-ho, I see the problem: about 75% of your table has content_key = -1.
>
>Why is that a problem, you ask? Well, the planner realizes that
>"content_key > -1" is a pretty good restriction condition (better than
>the date condition, apparently) and so it tries to use that as the index
>scan condition. The problem is that in 7.4 and before, the btree index
>code implements a "> -1" scan starting boundary by finding the first -1
>and then advancing to the first key that's not -1. So you end up
>scanning through 75% of the index before anything useful happens :-(
>
>I just fixed this poor behavior in CVS tip a couple weeks ago:
>http://archives.postgresql.org/pgsql-committers/2003-12/msg00220.php
>but the patch seems too large and unproven to risk back-patching into
>7.4.*.
>
>If you expect that a pretty large fraction of your data will always have
>dummy content_key, it'd probably be worth changing the index to not
>index -1's at all --- that is, make it a partial index with the
>condition "WHERE content_key > -1". Another workaround is to leave the
>index as-is but phrase the query WHERE condition as "content_key >= 0"
>instead of "> -1".
>
> regards, tom lane
>
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