Re: Huge overestimation in rows expected results in bad plan

On Tue, Nov 9, 2010 at 3:55 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> bricklen <bricklen@gmail.com> writes:
>> On Tue, Nov 9, 2010 at 3:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> The query doesn't seem to match the plan.  Where is that OR (c.id =
>>> 38441828354::bigint) condition coming from?
>
>> Ah sorry, I was testing it with and without that part. Here is the
>> corrected query, with that as part of the join condition:
>
>> explain analyze
>> select c.id, c.transactionid, c.clickgenerated, c.confirmed,
>> c.rejected, cr.rejectedreason
>> from conversion c
>> inner join conversionrejected cr on cr.idconversion = c.id or c.id = 38441828354
>> where date = '2010-11-06'
>> and idaction = 12906
>> and idaffiliate = 198338
>> order by transactionid;
>
> Hm.  Well, the trouble with that query is that if there is any
> conversion row with c.id = 38441828354, it will join to *every* row of
> conversionrejected.  The planner not unreasonably assumes there will be
> at least one such row, so it comes up with a join size estimate that's
>>= size of conversionrejected; and it also tends to favor a seqscan
> since it thinks it's going to have to visit every row of
> conversionrejected anyway.
>
> If you have reason to think the c.id = 38441828354 test is usually dead
> code, you might see if you can get rid of it, or at least rearrange the
> query as a UNION of two independent joins.
>
>                        regards, tom lane
>

Okay, thanks. I'll talk to the developer that wrote that query and see
what he has to say about it.


Cheers,

Bricklen