From: | Daniel Gustafsson <daniel(at)yesql(dot)se> |
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To: | pgsql-hackers(at)lists(dot)postgresql(dot)org |
Subject: | Avoid extra Sort nodes between WindowAggs when sorting can be reused |
Date: | 2018-05-30 16:19:48 |
Message-ID: | 124A7F69-84CD-435B-BA0E-2695BE21E5C2@yesql.se |
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Lists: | pgsql-hackers |
Currently, we can only reuse Sort nodes between WindowAgg nodes iff the
partitioning and ordering clauses are identical. If a window Sort node
sortorder is a prefix of another window, we could however reuse the Sort node
to hopefully produce a cheaper plan. In src/backend/optimizer/plan/planner.c
there is a comment alluding to this:
* ...
*
* There is room to be much smarter here, for example detecting whether
* one window's sort keys are a prefix of another's (so that sorting for
* the latter would do for the former), or putting windows first that
* match a sort order available for the underlying query. For the moment
* we are content with meeting the spec.
*/
The attached patch takes a stab at implementing the sorting on partitioning/
ordering prefix, inspired by a similar optimization in the Greenplum planner.
In testing the impact on planning time seems quite minimal, or within the error
margin.
cheers ./daniel
Attachment | Content-Type | Size |
---|---|---|
window_prefix_sort.patch | application/octet-stream | 3.9 KB |
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