Re: Why does the number of rows are different in actual and estimated.

On Dec 14, 2012, at 3:36 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Evgeny Shishkin <itparanoia@gmail.com> writes:
>> On Dec 14, 2012, at 3:09 AM, Andrew Dunstan <andrew@dunslane.net> wrote:
>>> Well, it looks like it's choosing a join order that's quite a bit different from the way the query is expressed, so
theOP might need to play around with forcing the join order some. 
>
>> OP joins 8 tables, and i suppose join collapse limit is set to default 8. I thought postgresql's optimiser is not
mysql's.
>
> It's not obvious to me that there's anything very wrong with the plan.
> An 8-way join that produces 150K rows is unlikely to run in milliseconds
> no matter what the plan.  The planner would possibly have done the last
> join step differently if it had had a better rowcount estimate, but even
> if that were free the query would still have been 7 seconds (vs 8.5).
>

May be in this case it is. I once wrote to this list regarding similar problem - joining 4 tables, result set are off
by2257 times - 750ms vs less then 1ms. Unfortunately the question was not accepted to the list. 

I spoke to Bruce Momjian about that problem on one local conference, he said shit happens :)

>             regards, tom lane