Jorge Daniel <elga...@hotmail.com> writes:
> I have a problem with a query that grabs a bunch of rows and then does an
> aggreate operation, at that moment it gots killed by OOM-killer, I don't know
> why, the engine starts using tmpfiles as expected , and then tries to work
> in memory and gots killed.
> SELECT count(*)
> FROM "changelog_change_transaction"
> INNER JOIN "changelog_change_stats" ON (
> changelog_change_stats.changelog_change_transactionid =
> changelog_change_transaction.changelog_change_transactionid )
> LEFT OUTER JOIN "changelog_change_group" ON (
> changelog_change_transaction.changelog_change_groupid =
> changelog_change_group.changelog_change_groupid )
> WHERE ( changelog_change_group.companyid = 40 OR
> changelog_change_group.companyid = 1 OR changelog_change_group.companyid = 53
> OR changelog_change_group.companyid IS NULL )
> AND changelog_change_transaction.started_at > '2017-04-21'
> GROUP BY "changelog_change_transaction"."changelog_change_transactionid",
> "changelog_change_transaction"."epoch", "changelog_change_transaction"
> ."changelog_change_groupid", "changelog_change_transaction"."started_at",
> "changelog_change_transaction"."duration_microseconds",
> "changelog_change_transaction"."changed_items",
> "changelog_change_transaction"."xmin"
> ;
Why are you grouping on xmin?
> For sure if the GROUP BY the one that causes this OOM (when I removed it, the
> query finish ok ) , so I've change the query-plan to avoid the HashAggregate:
> But the explain still shows:
That's because type XID doesn't have sort support, only hash support,
so hash aggregation is the only way to do the query at all.
regards, tom lane
