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

[Evgeny Shishkin <itparanoia@xxxxxxxxx>]

On Dec 14, 2012, at 2:36 AM, Andrew Dunstan <andrew@xxxxxxxxxxxx> 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.

The main question i think is what to do with it.

The problem starts here

  ->  Hash Join  (cost=9337.97..18115.71 rows=34489 width=244) (actual time=418.054..1156.453 rows=205420 loops=1)

                                             Hash Cond: (customerdetails.customerid = entity.id)

                                             ->  Seq Scan on customerdetails  (cost=0.00..4752.46 rows=327146 width=13) (actual time=0.021..176.389 rows=327328 loops=1)

                                             ->  Hash  (cost=6495.65..6495.65 rows=227386 width=231) (actual time=417.839..417.839 rows=205420 loops=1)

                                                   Buckets: 32768  Batches: 1  Memory Usage: 16056kB

                                                   ->  Index Scan using entity_setype_idx on entity  (cost=0.00..6495.65 rows=227386 width=231) (actual time=0.033..2

53.880 rows=205420 loops=1)

                                                         Index Cond: ((setype)::text = 'con_s'::text)

                           ->  Index Scan using con_address_pkey on con_address  (cost=0.00..0.27 rows=1 width=46) (actual time=0.003..0.004 rows=1 loops=2054

20)

As you see access methods estimates are ok, it is join result set which is wrong.

How to deal with it?

May be a hack with CTE can help, but is there a way to improve statistics correlation?

cheers

andrew



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