Re: merge join killing performance

On Wed, May 19, 2010 at 8:04 PM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
> On Wed, May 19, 2010 at 7:46 PM, Matthew Wakeling <matthew@flymine.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.