Re: UNION and bad performance

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



2010/12/12 pasman pasmaÅski <pasman.p@xxxxxxxxx>:
>> 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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

-- 
Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance



[Postgresql General]     [Postgresql PHP]     [PHP Users]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Yosemite]

  Powered by Linux