|From:||Daniel Gustafsson <daniel(at)yesql(dot)se>|
|Subject:||Re: Avoid extra Sort nodes between WindowAggs when sorting can be reused|
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> On 30 May 2018, at 18:19, Daniel Gustafsson <daniel(at)yesql(dot)se> wrote:
> 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
Attached is a rebased v2 addressing off-list review comments and including a
test. Parking this in the commitfest.
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