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Re: merge join killing performance

From: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
To: Matthew Wakeling <matthew(at)flymine(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-performance(at)postgresql(dot)org
Subject: Re: merge join killing performance
Date: 2010-05-20 02:06:15
Message-ID: AANLkTil5u_ZEPtdVfsl4-XZuieLcXvd9wn5G2jc6k4Eo@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Wed, May 19, 2010 at 8:04 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
> On Wed, May 19, 2010 at 7:46 PM, Matthew Wakeling <matthew(at)flymine(dot)org> wrote:
>> On Wed, 19 May 2010, Scott Marlowe wrote:
>>>>
>>>> It's apparently estimating (wrongly) that the merge join won't have to
>>>> scan very much of "files" before it can stop because it finds an eid
>>>> value larger than any eid in the other table.  So the issue here is an
>>>> inexact stats value for the max eid.
>>
>> I wandered if it could be something like that, but I rejected that idea, as
>> it obviously wasn't the real world case, and statistics should at least get
>> that right, if they are up to date.
>>
>>> I changed stats target to 1000 for that field and still get the bad plan.
>>
>> What do the stats say the max values are?
>
> 5277063,5423043,13843899 (I think).
>
> # select count(distinct eid) from files;
>  count
> -------
>   365
> (1 row)
>
> # select count(*) from files;
>  count
> ---------
>  3793748

A followup.  of those rows,

select count(*) from files where eid is null;
  count
---------
 3793215

are null.

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