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Re: query plan with index having a btrim is different for strings of different length

From: Richard Yen <dba(at)richyen(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: query plan with index having a btrim is different for strings of different length
Date: 2008-12-11 01:44:23
Message-ID: 3EF5395F-0F95-4313-B64A-ECAAE2880C57@richyen.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Dec 10, 2008, at 4:08 PM, Tom Lane wrote:

> Richard Yen <dba(at)richyen(dot)com> writes:
>> Is there any way to tune this so that for the common last names,  
>> the query
>> run time doesn't jump from <1s to >300s?
>
> Well, as near as I can tell there's factor of a couple hundred
> difference between the frequencies of 'smith' and 'smithers', so
> you shouldn't really expect similar runtimes for the two cases.
>
> Having said that, I still think you should try to index both first
> and last name.  Also I wonder whether the index on owner is worth
> having at all.  It definitely doesn't seem worthwhile to index the
> entries with owner = -1, since there are so many; so maybe you could
> make it a partial index that excludes those entries, in order to  
> prevent
> the planner from trying to use it for this case.

Created the 2-col index, and the query runs much faster on all  
permutations.

Thanks much for all your help,
--Richard

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