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Re: UNION and bad performance

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: pasman pasmański <pasman(dot)p(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: UNION and bad performance
Date: 2010-12-20 18:57:09
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
2010/12/12 pasman pasmański <pasman(dot)p(at)gmail(dot)com>:
>> UNION will remove all duplicates, so that the result additionally requires to
>> be sorted.
>>Right, to avoid the SORT and UNIQUE - operation you can use UNION ALL
> by the way maybe apply hashing to calculate UNION be better ?

The planner already considers such plans.

rhaas=# explain select a from generate_series(1,100) a union select a
from generate_series(1,100) a;
                                      QUERY PLAN
 HashAggregate  (cost=45.00..65.00 rows=2000 width=4)
   ->  Append  (cost=0.00..40.00 rows=2000 width=4)
         ->  Function Scan on generate_series a  (cost=0.00..10.00
rows=1000 width=4)
         ->  Function Scan on generate_series a  (cost=0.00..10.00
rows=1000 width=4)
(4 rows)

Robert Haas
The Enterprise PostgreSQL Company

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