Re: Optimizer wrongly picks Nested Loop Left Join

От: Tom Lane
Тема: Re: Optimizer wrongly picks Nested Loop Left Join
Дата: ,
Msg-id: 4085.1115823823@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Re: Optimizer wrongly picks Nested Loop Left Join  (John A Meinel)
Список: pgsql-performance

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Optimizer wrongly picks Nested Loop Left Join  ("Edin Kadribasic", )
 Re: Optimizer wrongly picks Nested Loop Left Join  (John A Meinel, )
  Re: Optimizer wrongly picks Nested Loop Left Join  (Tom Lane, )
 Re: Optimizer wrongly picks Nested Loop Left Join  (Tom Lane, )
 Re: Optimizer wrongly picks Nested Loop Left Join  ("Edin Kadribasic", )

John A Meinel <> writes:
> Unfortunately, because Hash Join doesn't report the number of rows
> (rows=0 always), it's hard to tell how good the estimator is.

This is only a cosmetic problem because you can just look at the number
of rows actually emitted by the Hash node's child; that's always exactly
the number loaded into the hashtable.

(But having said that, it is fixed in CVS tip.)

You may be confused though --- the Hash node is not the Hash Join node.
A zero report from Hash Join does actually mean that it emitted zero
rows.

            regards, tom lane


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