Can somebody explain what is the meaning for HashAggregate?

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От	lin
Тема	Can somebody explain what is the meaning for HashAggregate?
Дата	20 сентября 2015 г. 13:56:55
Msg-id	60e52136.c16e.14feb0ad2db.Coremail.jluwln@163.com обсуждение
Ответы	Re: Can somebody explain what is the meaning for HashAggregate?
Список	pgsql-general

Can somebody explain what does the postgres done for the explain of sql shows HashAggregate( what is the meaning for HashAggregate )?

for example:

postgres=# explain verbose select oid,relname from pg_class group by oid,relname;

QUERY PLAN

-----------------------------------------------------------------------------

HashAggregate (cost=12.42..15.38 rows=295 width=68)

Output: oid, relname

Group Key: pg_class.oid, pg_class.relname

-> Seq Scan on pg_catalog.pg_class (cost=0.00..10.95 rows=295 width=68)

Output: oid, relname

(5 rows)

:: first, seq scan pg_class and return (oid,relname);

second, make group (oid,relname), is it fisrt sort by oid then in the oid group sort by relname?

Can somebody explain what does the database done for hashAggregate?

Thanks,

wanglin

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