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When you have a large table and do a simple select, mitosis kicks in and splits up the select in pieces, just to combine the (basically unaltered) pieces again before returning them.
Try this:
create table t (i) as (select * from sys.generate_series(cast(0 as integer), 10000000, 1));
explain select * from t limit 1000;
explain select * from t;
Indeed, there is not much value in parallelizing query parts that do not (significantly) reduce the data volume (or do not have worse than linear complexity, or do not have significant CPU cost). We need to find a robust way to detect such situations easily and generically ...
Date: 2017-11-16 10:34:03 +0100
From: @sjoerdmullender
To: SQL devs <>
Version: 11.27.9 (Jul2017-SP2)
CC: @njnes, @drstmane
Last updated: 2017-12-14 14:46:07 +0100
Comment 25887
Date: 2017-11-16 10:34:03 +0100
From: @sjoerdmullender
When you have a large table and do a simple select, mitosis kicks in and splits up the select in pieces, just to combine the (basically unaltered) pieces again before returning them.
Try this:
create table t (i) as (select * from sys.generate_series(cast(0 as integer), 10000000, 1));
explain select * from t limit 1000;
explain select * from t;
Both selects use mitosis.
This bug is derived from bug #6462.
Comment 25890
Date: 2017-11-16 12:54:56 +0100
From: @drstmane
Indeed, there is not much value in parallelizing query parts that do not (significantly) reduce the data volume (or do not have worse than linear complexity, or do not have significant CPU cost). We need to find a robust way to detect such situations easily and generically ...
Comment 25916
Date: 2017-11-26 18:35:43 +0100
From: @njnes
make sure a simple topn(project(basetable)) (or smaller) query isn't parallelised (as its only output production)
Comment 25918
Date: 2017-11-26 20:46:37 +0100
From: MonetDB Mercurial Repository <>
Changeset ffab2b52402d made by Niels Nes niels@cwi.nl in the MonetDB repo, refers to this bug.
For complete details, see https//devmonetdborg/hg/MonetDB?cmd=changeset;node=ffab2b52402d
Changeset description:
The text was updated successfully, but these errors were encountered: