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Rename find_em_expr_usable_for_sorting_rel. 

I didn't particularly like this function name, as it fails to
express what's going on.  Also, returning the sort expression
alone isn't too helpful --- typically, a caller would also
need some other fields of the EquivalenceMember.  But the
sole caller really only needs a bool result, so let's make
it "bool relation_can_be_sorted_early()".

Discussion: https://postgr.es/m/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com
This commit is contained in:
Tom Lane 2021-04-20 11:37:36 -04:00
parent 3753982441
commit 7645376774
3 changed files with 28 additions and 27 deletions
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21
src/backend/optimizer/path/allpaths.c
View File

@ -2697,20 +2697,19 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel,
EquivalenceClass *pathkey_ec = pathkey->pk_eclass;
/*
* We can only build a sort for pathkeys which contain an EC
* member in the current relation's target, so ignore any suffix
* of the list as soon as we find a pathkey without an EC member
* in the relation.
* We can only build a sort for pathkeys that contain a
* safe-to-compute-early EC member computable from the current
* relation's reltarget, so ignore the remainder of the list as
* soon as we find a pathkey without such a member.
*
* By still returning the prefix of the pathkeys list that does
* meet criteria of EC membership in the current relation, we
* enable not just an incremental sort on the entirety of
* query_pathkeys but also incremental sort below a JOIN.
* It's still worthwhile to return any prefix of the pathkeys list
* that meets this requirement, as we may be able to do an
* incremental sort.
*
* If requested, ensure the expression is parallel safe too.
* If requested, ensure the sort expression is parallel-safe too.
*/
if (!find_em_expr_usable_for_sorting_rel(root, pathkey_ec, rel,
require_parallel_safe))
if (!relation_can_be_sorted_early(root, rel, pathkey_ec,
require_parallel_safe))
break;
npathkeys++;

27
src/backend/optimizer/path/equivclass.c
View File

@ -961,18 +961,21 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
}
/*
* Find an equivalence class member expression that can be used to build
* a sort node using the provided relation; return NULL if no candidate.
* relation_can_be_sorted_early
* Can this relation be sorted on this EC before the final output step?
*
* To succeed, we must find an EC member that prepare_sort_from_pathkeys knows
* how to sort on, given the rel's reltarget as input. There are also a few
* additional constraints based on the fact that the desired sort will be done
* within the scan/join part of the plan. Also, non-parallel-safe expressions
* are ignored if 'require_parallel_safe'.
* "early", within the scan/join part of the plan. Also, non-parallel-safe
* expressions are ignored if 'require_parallel_safe'.
*
* At some point we might want to return the identified EquivalenceMember,
* but for now, callers only want to know if there is one.
*/
Expr *
find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
RelOptInfo *rel, bool require_parallel_safe)
bool
relation_can_be_sorted_early(PlannerInfo *root, RelOptInfo *rel,
EquivalenceClass *ec, bool require_parallel_safe)
{
PathTarget *target = rel->reltarget;
EquivalenceMember *em;
@ -982,7 +985,7 @@ find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
* Reject volatile ECs immediately; such sorts must always be postponed.
*/
if (ec->ec_has_volatile)
return NULL;
return false;
/*
* Try to find an EM directly matching some reltarget member.
@ -1012,7 +1015,7 @@ find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
!is_parallel_safe(root, (Node *) em->em_expr))
continue;
return em->em_expr;
return true;
}
/*
@ -1021,7 +1024,7 @@ find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
em = find_computable_ec_member(root, ec, target->exprs, rel->relids,
require_parallel_safe);
if (!em)
return NULL;
return false;
/*
* Reject expressions involving set-returning functions, as those can't be
@ -1030,9 +1033,9 @@ find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
* belong to multi-member ECs.)
*/
if (IS_SRF_CALL((Node *) em->em_expr))
return NULL;
return false;
return em->em_expr;
return true;
}
/*

7
src/include/optimizer/paths.h
View File

@ -144,10 +144,9 @@ extern EquivalenceMember *find_computable_ec_member(PlannerInfo *root,
Relids relids,
bool require_parallel_safe);
extern Expr *find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel);
extern Expr *find_em_expr_usable_for_sorting_rel(PlannerInfo *root,
EquivalenceClass *ec,
RelOptInfo *rel,
bool require_parallel_safe);
extern bool relation_can_be_sorted_early(PlannerInfo *root, RelOptInfo *rel,
EquivalenceClass *ec,
bool require_parallel_safe);
extern void generate_base_implied_equalities(PlannerInfo *root);
extern List *generate_join_implied_equalities(PlannerInfo *root,
Relids join_relids,