Message-ID: <55074424.9050506@2ndquadrant.com>
Date: Mon, 16 Mar 2015 21:59:16 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Organization: 2ndQuadrant
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To: Jim Nasby <Jim.Nasby@BlueTreble.com>, vjoshi@zetainteractive.com,
 Scott Marlowe <scott.marlowe@gmail.com>,
 Varadharajan Mukundan <srinathsmn@gmail.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Performance issues
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On 16.3.2015 20:43, Jim Nasby wrote:
> On 3/13/15 7:12 PM, Tomas Vondra wrote:
>> (4) I suspect many of the relations referenced in the views are not
>>      actually needed in the query, i.e. the join is performed but
>>      then it's just discarded because those columns are not used.
>>      Try to simplify the views as much has possible - remove all the
>>      tables that are not really necessary to run the query. If two
>>      queries need different tables, maybe defining two views is
>>      a better approach.
> 
> A better alternative with multi-purpose views is to use an outer
> join instead of an inner join. With an outer join if you ultimately
> don't refer to any of the columns in a particular table Postgres will
> remove the table from the query completely.

Really? Because a quick test suggests otherwise:

db=# create table test_a (id int);
CREATE TABLE
db=# create table test_b (id int);
CREATE TABLE
db=# explain select test_a.* from test_a left join test_b using (id);
                              QUERY PLAN
----------------------------------------------------------------------
 Merge Left Join  (cost=359.57..860.00 rows=32512 width=4)
   Merge Cond: (test_a.id = test_b.id)
   ->  Sort  (cost=179.78..186.16 rows=2550 width=4)
         Sort Key: test_a.id
         ->  Seq Scan on test_a  (cost=0.00..35.50 rows=2550 width=4)
   ->  Sort  (cost=179.78..186.16 rows=2550 width=4)
         Sort Key: test_b.id
         ->  Seq Scan on test_b  (cost=0.00..35.50 rows=2550 width=4)
(8 rows)

Also, how would that work with duplicate rows in the referenced table?


-- 
Tomas Vondra                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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