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 |
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Lists: | pgsql-hackers pgsql-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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