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Commita43d7a8

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Compare collations before merging UNION operations.
In the dim past we figured it was okay to ignore collationswhen combining UNION set-operation nodes into a single N-wayUNION operation. I believe that was fine at the time, butit stopped being fine when we added nondeterministic collations:the semantics of distinct-ness are affected by those. v17 madeit even less fine by allowing per-child sorting operations tobe merged via MergeAppend, although I think we accidentallyavoided any live bug from that.Add a check that collations match before deciding that twoUNION nodes are equivalent. I also failed to resist thetemptation to comment plan_union_children() a little better.Back-patch to all supported branches (v13 now), since theyall have nondeterministic collations.Discussion:https://postgr.es/m/3605568.1731970579@sss.pgh.pa.us
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‎src/backend/optimizer/prep/prepunion.c

Lines changed: 12 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -719,9 +719,9 @@ generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
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/*
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* If any of my children are identical UNION nodes (same op, all-flag, and
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* colTypes) then they can be merged into this node so that we generate
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* only one Append/MergeAppend and unique-ification for the lot. Recurse
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* to find such nodes.
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* colTypes/colCollations) then they can be merged into this node so that
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*we generateonly one Append/MergeAppend and unique-ification for the
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*lot. Recurseto find such nodes.
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*/
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rellist=plan_union_children(root,
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op,
@@ -1193,17 +1193,16 @@ generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
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}
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/*
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* Pull up children of a UNION node that are identically-propertied UNIONs.
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* Pull up children of a UNION node that are identically-propertied UNIONs,
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* and perform planning of the queries underneath the N-way UNION.
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*
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* The result is a list of RelOptInfos containing Paths for sub-nodes, with
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* one entry for each descendant that is a leaf query or non-identical setop.
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* We also return parallel lists of the childrens' targetlists and
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* is-trivial-tlist flags.
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*
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* NOTE: we can also pull a UNION ALL up into a UNION, since the distinct
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* output rows will be lost anyway.
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*
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* NOTE: currently, we ignore collations while determining if a child has
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* the same properties. This is semantically sound only so long as all
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* collations have the same notion of equality. It is valid from an
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* implementation standpoint because we don't care about the ordering of
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* a UNION child's result: UNION ALL results are always unordered, and
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* generate_union_paths will force a fresh sort if the top level is a UNION.
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*/
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staticList*
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plan_union_children(PlannerInfo*root,
@@ -1232,7 +1231,8 @@ plan_union_children(PlannerInfo *root,
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if (op->op==top_union->op&&
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(op->all==top_union->all||op->all)&&
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equal(op->colTypes,top_union->colTypes))
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equal(op->colTypes,top_union->colTypes)&&
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equal(op->colCollations,top_union->colCollations))
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{
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/* Same UNION, so fold children into parent */
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pending_rels=lcons(op->rarg,pending_rels);

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