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Re: dum query plan

From: Jean-Luc Lachance <jllachan(at)nsd(dot)ca>
To: Jonathan Moore <moore(at)discern(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: dum query plan
Date: 2003-04-17 14:12:35
Message-ID: 3E9EB653.B6E200B4@nsd.ca (view raw or flat)
Thread:
Lists: pgsql-performance
Your WHERE clause is the reason you get O(N^2).

How about describing in words what you want.

maybe what you really want is:

select left_entry from entry_pairs A where not exists ( 
  select 1 from entry_pairs B where A.right_entry = B.left_entry)

JLL

Jonathan Moore wrote:
> 
> I am wondering why it uses the O(n^2) nested loop when there is a O(N)
> methoud using btree indexes for a merg join. I am using  7.2.1 would
> upgrading fix my problime or is it somthing else?
> 
> Given the schema:
> 
> drop   table Entry_Pairs;
> create table Entry_Pairs (
>     left_entry  int              REFERENCES Entry   ON DELETE RESTRICT,
>     right_entry int              REFERENCES Entry   ON DELETE RESTRICT,
>     relation    int     NOT NULL                                      ,
>     subtract    bool    NOT NULL                                      ,
>     comment     int         NULL REFERENCES Comment ON DELETE SET NULL,
>     UNIQUE (left_entry, right_entry, relation)
> );
> CREATE INDEX entry_pairs_left_index ON entry_pairs (left_entry);
> CREATE INDEX entry_pairs_right_index ON entry_pairs (right_entry);
> --
> 
> You get this"
> 
> dblex=> explain select A.left_entry from entry_pairs A, entry_pairs B
> where A.right_entry != B.left_entry;
> NOTICE:  QUERY PLAN:
> 
> Nested Loop  (cost=100000000.00..102876671.17 rows=97545252 width=12)
>   ->  Seq Scan on entry_pairs a  (cost=0.00..167.77 rows=9877 width=8)
>   ->  Seq Scan on entry_pairs b  (cost=0.00..167.77 rows=9877 width=4)
> 
> EXPLAIN
> 
> That is dum. If you just walk both B-Tree indexes there is a O(n)
> search. I tryed to turn off netsed loops but it still did it. (the
> reason the cost is 100000000.00 is a artifact from turing off loops)
> 
> -Jonathan
> 
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Subject: Re: dum query plan: more info.
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