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Commit3b1a7eb

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Don't zero tuple_fraction when planning UNIONs with ORDER BYs
Since66c0185, the planner is able to use Merge Append -> Unique toimplement UNION queries and each subquery is prompted to produce Pathscorrectly sorted by the UNION's targetlist.Here we remove some now redundant code which was zeroing thetuple_fraction at the parent level. This will allow the planner toconsider cheap startup paths when planning the UNION's subqueries.EXCEPT and INTERSECT set operations still have the tuple_fraction zeroedin generate_nonunion_paths(). These operations currently always readall of their subqueries' tuples.Reported-by: Tom LaneDiscussion:https://postgr.es/m/3703023.1711654574@sss.pgh.pa.us
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‎src/backend/optimizer/plan/planner.c

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@@ -1338,17 +1338,6 @@ grouping_planner(PlannerInfo *root, double tuple_fraction,
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if (parse->setOperations)
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{
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/*
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* If there's a top-level ORDER BY, assume we have to fetch all the
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* tuples. This might be too simplistic given all the hackery below
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* to possibly avoid the sort; but the odds of accurate estimates here
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* are pretty low anyway. XXX try to get rid of this in favor of
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* letting plan_set_operations generate both fast-start and
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* cheapest-total paths.
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*/
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if (parse->sortClause)
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root->tuple_fraction=0.0;
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/*
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* Construct Paths for set operations. The results will not need any
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* work except perhaps a top-level sort and/or LIMIT. Note that any

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