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Allow planner to use Merge Append to efficiently implement UNION

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Until now, UNION queries have often been suboptimal as the planner has
only ever considered using an Append node and making the results unique
by either using a Hash Aggregate, or by Sorting the entire Append result
and running it through the Unique operator.  Both of these methods
always require reading all rows from the union subqueries.

Here we adjust the union planner so that it can request that each subquery
produce results in target list order so that these can be Merge Appended
together and made unique with a Unique node.  This can improve performance
significantly as the union child can make use of the likes of btree
indexes and/or Merge Joins to provide the top-level UNION with presorted
input.  This is especially good if the top-level UNION contains a LIMIT
node that limits the output rows to a small subset of the unioned rows as
cheap startup plans can be used.

Author: David Rowley
Reviewed-by: Richard Guo, Andy Fan
Discussion: https://postgr.es/m/CAApHDvpb_63XQodmxKUF8vb9M7CxyUyT4sWvEgqeQU-GB7QFoQ@mail.gmail.com
This commit is contained in:
David Rowley
2024-03-25 14:31:14 +13:00
parent 47f99a407d
commit 66c0185a3d
15 changed files with 707 additions and 253 deletions
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61
src/backend/optimizer/path/equivclass.c
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@@ -2867,6 +2867,67 @@ add_child_join_rel_equivalences(PlannerInfo *root,
MemoryContextSwitchTo(oldcontext);
}
/*
* add_setop_child_rel_equivalences
* Add equivalence members for each non-resjunk target in 'child_tlist'
* to the EquivalenceClass in the corresponding setop_pathkey's pk_class.
*
* 'root' is the PlannerInfo belonging to the top-level set operation.
* 'child_rel' is the RelOptInfo of the child relation we're adding
* EquivalenceMembers for.
* 'child_tlist' is the target list for the setop child relation. The target
* list expressions are what we add as EquivalenceMembers.
* 'setop_pathkeys' is a list of PathKeys which must contain an entry for each
* non-resjunk target in 'child_tlist'.
*/
void
add_setop_child_rel_equivalences(PlannerInfo *root, RelOptInfo *child_rel,
List *child_tlist, List *setop_pathkeys)
{
ListCell *lc;
ListCell *lc2 = list_head(setop_pathkeys);
foreach(lc, child_tlist)
{
TargetEntry *tle = lfirst_node(TargetEntry, lc);
EquivalenceMember *parent_em;
PathKey *pk;
if (tle->resjunk)
continue;
if (lc2 == NULL)
elog(ERROR, "too few pathkeys for set operation");
pk = lfirst_node(PathKey, lc2);
parent_em = linitial(pk->pk_eclass->ec_members);
/*
* We can safely pass the parent member as the first member in the
* ec_members list as this is added first in generate_union_paths,
* likewise, the JoinDomain can be that of the initial member of the
* Pathkey's EquivalenceClass.
*/
add_eq_member(pk->pk_eclass,
tle->expr,
child_rel->relids,
parent_em->em_jdomain,
parent_em,
exprType((Node *) tle->expr));
lc2 = lnext(setop_pathkeys, lc2);
}
/*
* transformSetOperationStmt() ensures that the targetlist never contains
* any resjunk columns, so all eclasses that exist in 'root' must have
* received a new member in the loop above. Add them to the child_rel's
* eclass_indexes.
*/
child_rel->eclass_indexes = bms_add_range(child_rel->eclass_indexes, 0,
list_length(root->eq_classes) - 1);
}
/*
* generate_implied_equalities_for_column