Re: parallel_force_local and parallel query
Date: Mon, 24 Feb 2020 23:15:30 +0000
Message-ID: <LNXP265MB1562737EE2453B8DFE3B7037A5EC0_at_LNXP265MB1562.GBRP265.PROD.OUTLOOK.COM>
The point of parallel_force_local is that the query coordinator and all its PX slave processes operate on the same node so that any PQ messages areinter-process on the node and no PQ messages (for the query) have to travel across the interconnect.
If you think about a parallel hash join using hash/hash distribution, for example; this will use two slave sets the first slave set scans the first table and distributes the interesting rows to the second slave set, then the first slave set scans the second table and distributes it's rows in the same way to the second slave set. If you don't set parallel_force_local to true then you could (in theory, though Oracle always makes some attempt to avoid it) end up with the first slave set on one node and the second slave set on another node, and all the data flying across the interconnect before any joining gets done - them maybe the entire result set could go flying back across the interconnect because the Query coordinator is on a third node. A single bad parallel query could flood the interconnect and basically kill the system - to the extent that nodes could get ejected because they couldn't get their heartbeat messages through fast enough.
Regards
Hopefully not a dumb question, By setting parallel force local to TRUE all the queries for a particular table and other objects happen in the local instance only. What happens if a different query is issued from a different node for the same table or object. Does the data get transferred over the interconnect, if data is still cached in the first node. Assume SELECTs only in both nodes.
Thanks
Jonathan Lewis
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Cee Pee <carlospena999_at_gmail.com>
Sent: 24 February 2020 20:38
To: Oracle-L Freelists
Subject: parallel_force_local and parallel query
CP
--
http://www.freelists.org/webpage/oracle-l
Received on Tue Feb 25 2020 - 00:15:30 CET