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

On Thu, Dec 13, 2012 at 7:36 PM, Andrew Dunstan <andrew@dunslane.net> wrote:
> On 12/13/2012 05:12 PM, AI Rumman wrote:
>>
>> Why does the number of rows are different in actual and estimated?
>>
>
>
> Isn't that in the nature of estimates? An estimate is a heuristic guess at
> the number of rows it will find for the given query or part of a query. It's
> not uncommon for estimates to be out by several orders of magnitude.
> Guaranteeing estimates within bounded accuracy and in a given short amount
> of time (you don't want your planning time to overwhelm your execution time)
> isn't possible.

Although this kind of difference could be indeed a problem:
> Nested Loop  (cost=18560.97..26864.83 rows=24871 width=535) (actual time=1335.157..8492.414 rows=157953 loops=1)
>    ->  Hash Left Join  (cost=18560.97..26518.91 rows=116 width=454) (actual time=1335.117..6996.585 rows=205418
loops=1)

It usually is due to some unrecognized correlation between the joined tables.

And it looks like it all may be starting to go south here:
>                                        ->  Hash Join  (cost=9337.97..18115.71 rows=34489 width=244) (actual
time=418.054..1156.453rows=205420 loops=1) 
>                                              Hash Cond: (customerdetails.customerid = entity.id)