mirror/postgresql
2
0
Fork 0
mirror of https://git.postgresql.org/git/postgresql.git synced 2024-11-21 03:13:05 +08:00
Code Issues Packages Projects Releases Wiki Activity

Compare collations before merging UNION operations.

Browse Source 
In the dim past we figured it was okay to ignore collations
when combining UNION set-operation nodes into a single N-way
UNION operation.  I believe that was fine at the time, but
it stopped being fine when we added nondeterministic collations:
the semantics of distinct-ness are affected by those.  v17 made
it even less fine by allowing per-child sorting operations to
be merged via MergeAppend, although I think we accidentally
avoided any live bug from that.

Add a check that collations match before deciding that two
UNION nodes are equivalent.  I also failed to resist the
temptation to comment plan_union_children() a little better.

Back-patch to all supported branches (v13 now), since they
all have nondeterministic collations.

Discussion: https://postgr.es/m/3605568.1731970579@sss.pgh.pa.us
This commit is contained in:
Tom Lane 2024-11-19 18:26:19 -05:00
parent c166454496
commit a43d7a8c7c
1 changed files with 12 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
src/backend/optimizer/prep/prepunion.c
View File

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