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Commitad1c36b

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Fix foreign-key selectivity estimation in the presence of constants.
get_foreign_key_join_selectivity() looks for join clauses that equatethe two sides of the FK constraint. However, if we have a query like"WHERE fktab.a = pktab.a and fktab.a = 1", it won't find any such joinclause, because equivclass.c replaces the given clauses with "fktab.a= 1 and pktab.a = 1", which can be enforced at the scan level, leavingnothing to be done for column "a" at the join level.We can fix that expectation without much trouble, but then a new problemarises: applying the foreign-key-based selectivity rule produces arowcount underestimate, because we're effectively double-counting theselectivity of the "fktab.a = 1" clause. So we have to cancel thatselectivity out of the estimate.To fix, refactor process_implied_equality() so that it can pass back thenew RestrictInfo to its callers in equivclass.c, allowing the generated"fktab.a = 1" clause to be saved in the EquivalenceClass's ec_deriveslist. Then it's not much trouble to dig out the relevant RestrictInfowhen we need to adjust an FK selectivity estimate. (While at it, wecan also remove the expensive use of initialize_mergeclause_eclasses()to set up the new RestrictInfo's left_ec and right_ec pointers.The equivclass.c code can set those basically for free.)This seems like clearly a bug fix, but I'm hesitant to back-patch it,first because there's some API/ABI risk for extensions and second becausewe're usually loath to destabilize plan choices in stable branches.Per report from Sigrid Ehrenreich.Discussion:https://postgr.es/m/1019549.1603770457@sss.pgh.pa.usDiscussion:https://postgr.es/m/AM6PR02MB5287A0ADD936C1FA80973E72AB190@AM6PR02MB5287.eurprd02.prod.outlook.com
1 parentce7f772 commitad1c36b

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10 files changed

+366
-103
lines changed

10 files changed

+366
-103
lines changed

‎src/backend/nodes/outfuncs.c‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2352,6 +2352,7 @@ _outForeignKeyOptInfo(StringInfo str, const ForeignKeyOptInfo *node)
23522352
WRITE_ATTRNUMBER_ARRAY(confkey,node->nkeys);
23532353
WRITE_OID_ARRAY(conpfeqop,node->nkeys);
23542354
WRITE_INT_FIELD(nmatched_ec);
2355+
WRITE_INT_FIELD(nconst_ec);
23552356
WRITE_INT_FIELD(nmatched_rcols);
23562357
WRITE_INT_FIELD(nmatched_ri);
23572358
/* for compactness, just print the number of matches per column: */

‎src/backend/optimizer/path/costsize.c‎

Lines changed: 52 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -5066,9 +5066,16 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
50665066
* remove back into the worklist.
50675067
*
50685068
* Since the matching clauses are known not outerjoin-delayed, they
5069-
* should certainly have appeared in the initial joinclause list. If
5070-
* we didn't find them, they must have been matched to, and removed
5071-
* by, some other FK in a previous iteration of this loop. (A likely
5069+
* would normally have appeared in the initial joinclause list. If we
5070+
* didn't find them, there are two possibilities:
5071+
*
5072+
* 1. If the FK match is based on an EC that is ec_has_const, it won't
5073+
* have generated any join clauses at all. We discount such ECs while
5074+
* checking to see if we have "all" the clauses. (Below, we'll adjust
5075+
* the selectivity estimate for this case.)
5076+
*
5077+
* 2. The clauses were matched to some other FK in a previous
5078+
* iteration of this loop, and thus removed from worklist. (A likely
50725079
* case is that two FKs are matched to the same EC; there will be only
50735080
* one EC-derived clause in the initial list, so the first FK will
50745081
* consume it.) Applying both FKs' selectivity independently risks
@@ -5078,8 +5085,9 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
50785085
* Later we might think of a reasonable way to combine the estimates,
50795086
* but for now, just punt, since this is a fairly uncommon situation.
50805087
*/
5081-
if (list_length(removedlist)!=
5082-
(fkinfo->nmatched_ec+fkinfo->nmatched_ri))
5088+
if (removedlist==NIL||
5089+
list_length(removedlist)!=
5090+
(fkinfo->nmatched_ec-fkinfo->nconst_ec+fkinfo->nmatched_ri))
50835091
{
50845092
worklist=list_concat(worklist,removedlist);
50855093
continue;
@@ -5138,9 +5146,48 @@ get_foreign_key_join_selectivity(PlannerInfo *root,
51385146

51395147
fkselec *=1.0 /ref_tuples;
51405148
}
5149+
5150+
/*
5151+
* If any of the FK columns participated in ec_has_const ECs, then
5152+
* equivclass.c will have generated "var = const" restrictions for
5153+
* each side of the join, thus reducing the sizes of both input
5154+
* relations. Taking the fkselec at face value would amount to
5155+
* double-counting the selectivity of the constant restriction for the
5156+
* referencing Var. Hence, look for the restriction clause(s) that
5157+
* were applied to the referencing Var(s), and divide out their
5158+
* selectivity to correct for this.
5159+
*/
5160+
if (fkinfo->nconst_ec>0)
5161+
{
5162+
for (inti=0;i<fkinfo->nkeys;i++)
5163+
{
5164+
EquivalenceClass*ec=fkinfo->eclass[i];
5165+
5166+
if (ec&&ec->ec_has_const)
5167+
{
5168+
EquivalenceMember*em=fkinfo->fk_eclass_member[i];
5169+
RestrictInfo*rinfo=find_derived_clause_for_ec_member(ec,
5170+
em);
5171+
5172+
if (rinfo)
5173+
{
5174+
Selectivitys0;
5175+
5176+
s0=clause_selectivity(root,
5177+
(Node*)rinfo,
5178+
0,
5179+
jointype,
5180+
sjinfo);
5181+
if (s0>0)
5182+
fkselec /=s0;
5183+
}
5184+
}
5185+
}
5186+
}
51415187
}
51425188

51435189
*restrictlist=worklist;
5190+
CLAMP_PROBABILITY(fkselec);
51445191
returnfkselec;
51455192
}
51465193

‎src/backend/optimizer/path/equivclass.c‎

Lines changed: 96 additions & 25 deletions
Original file line numberDiff line numberDiff line change
@@ -840,10 +840,8 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
840840
* scanning of the quals and before Path construction begins.
841841
*
842842
* We make no attempt to avoid generating duplicate RestrictInfos here: we
843-
* don't search ec_sources for matches, nor put the created RestrictInfos
844-
* into ec_derives. Doing so would require some slightly ugly changes in
845-
* initsplan.c's API, and there's no real advantage, because the clauses
846-
* generated here can't duplicate anything we will generate for joins anyway.
843+
* don't search ec_sources or ec_derives for matches. It doesn't really
844+
* seem worth the trouble to do so.
847845
*/
848846
void
849847
generate_base_implied_equalities(PlannerInfo*root)
@@ -969,6 +967,7 @@ generate_base_implied_equalities_const(PlannerInfo *root,
969967
{
970968
EquivalenceMember*cur_em= (EquivalenceMember*)lfirst(lc);
971969
Oideq_op;
970+
RestrictInfo*rinfo;
972971

973972
Assert(!cur_em->em_is_child);/* no children yet */
974973
if (cur_em==const_em)
@@ -982,14 +981,31 @@ generate_base_implied_equalities_const(PlannerInfo *root,
982981
ec->ec_broken= true;
983982
break;
984983
}
985-
process_implied_equality(root,eq_op,ec->ec_collation,
986-
cur_em->em_expr,const_em->em_expr,
987-
bms_copy(ec->ec_relids),
988-
bms_union(cur_em->em_nullable_relids,
989-
const_em->em_nullable_relids),
990-
ec->ec_min_security,
991-
ec->ec_below_outer_join,
992-
cur_em->em_is_const);
984+
rinfo=process_implied_equality(root,eq_op,ec->ec_collation,
985+
cur_em->em_expr,const_em->em_expr,
986+
bms_copy(ec->ec_relids),
987+
bms_union(cur_em->em_nullable_relids,
988+
const_em->em_nullable_relids),
989+
ec->ec_min_security,
990+
ec->ec_below_outer_join,
991+
cur_em->em_is_const);
992+
993+
/*
994+
* If the clause didn't degenerate to a constant, fill in the correct
995+
* markings for a mergejoinable clause, and save it in ec_derives. (We
996+
* will not re-use such clauses directly, but selectivity estimation
997+
* may consult the list later. Note that this use of ec_derives does
998+
* not overlap with its use for join clauses, since we never generate
999+
* join clauses from an ec_has_const eclass.)
1000+
*/
1001+
if (rinfo&&rinfo->mergeopfamilies)
1002+
{
1003+
/* it's not redundant, so don't set parent_ec */
1004+
rinfo->left_ec=rinfo->right_ec=ec;
1005+
rinfo->left_em=cur_em;
1006+
rinfo->right_em=const_em;
1007+
ec->ec_derives=lappend(ec->ec_derives,rinfo);
1008+
}
9931009
}
9941010
}
9951011

@@ -1028,6 +1044,7 @@ generate_base_implied_equalities_no_const(PlannerInfo *root,
10281044
{
10291045
EquivalenceMember*prev_em=prev_ems[relid];
10301046
Oideq_op;
1047+
RestrictInfo*rinfo;
10311048

10321049
eq_op=select_equality_operator(ec,
10331050
prev_em->em_datatype,
@@ -1038,14 +1055,29 @@ generate_base_implied_equalities_no_const(PlannerInfo *root,
10381055
ec->ec_broken= true;
10391056
break;
10401057
}
1041-
process_implied_equality(root,eq_op,ec->ec_collation,
1042-
prev_em->em_expr,cur_em->em_expr,
1043-
bms_copy(ec->ec_relids),
1044-
bms_union(prev_em->em_nullable_relids,
1045-
cur_em->em_nullable_relids),
1046-
ec->ec_min_security,
1047-
ec->ec_below_outer_join,
1048-
false);
1058+
rinfo=process_implied_equality(root,eq_op,ec->ec_collation,
1059+
prev_em->em_expr,cur_em->em_expr,
1060+
bms_copy(ec->ec_relids),
1061+
bms_union(prev_em->em_nullable_relids,
1062+
cur_em->em_nullable_relids),
1063+
ec->ec_min_security,
1064+
ec->ec_below_outer_join,
1065+
false);
1066+
1067+
/*
1068+
* If the clause didn't degenerate to a constant, fill in the
1069+
* correct markings for a mergejoinable clause. We don't put it
1070+
* in ec_derives however; we don't currently need to re-find such
1071+
* clauses, and we don't want to clutter that list with non-join
1072+
* clauses.
1073+
*/
1074+
if (rinfo&&rinfo->mergeopfamilies)
1075+
{
1076+
/* it's not redundant, so don't set parent_ec */
1077+
rinfo->left_ec=rinfo->right_ec=ec;
1078+
rinfo->left_em=prev_em;
1079+
rinfo->right_em=cur_em;
1080+
}
10491081
}
10501082
prev_ems[relid]=cur_em;
10511083
}
@@ -2151,6 +2183,10 @@ exprs_known_equal(PlannerInfo *root, Node *item1, Node *item2)
21512183
* we ignore that fine point here.) This is much like exprs_known_equal,
21522184
* except that we insist on the comparison operator matching the eclass, so
21532185
* that the result is definite not approximate.
2186+
*
2187+
* On success, we also set fkinfo->eclass[colno] to the matching eclass,
2188+
* and set fkinfo->fk_eclass_member[colno] to the eclass member for the
2189+
* referencing Var.
21542190
*/
21552191
EquivalenceClass*
21562192
match_eclasses_to_foreign_key_col(PlannerInfo*root,
@@ -2180,8 +2216,8 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
21802216
{
21812217
EquivalenceClass*ec= (EquivalenceClass*)list_nth(root->eq_classes,
21822218
i);
2183-
boolitem1member= false;
2184-
boolitem2member= false;
2219+
EquivalenceMember*item1_em=NULL;
2220+
EquivalenceMember*item2_em=NULL;
21852221
ListCell*lc2;
21862222

21872223
/* Never match to a volatile EC */
@@ -2206,12 +2242,12 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
22062242

22072243
/* Match? */
22082244
if (var->varno==var1varno&&var->varattno==var1attno)
2209-
item1member=true;
2245+
item1_em=em;
22102246
elseif (var->varno==var2varno&&var->varattno==var2attno)
2211-
item2member=true;
2247+
item2_em=em;
22122248

22132249
/* Have we found both PK and FK column in this EC? */
2214-
if (item1member&&item2member)
2250+
if (item1_em&&item2_em)
22152251
{
22162252
/*
22172253
* Succeed if eqop matches EC's opfamilies. We could test
@@ -2221,7 +2257,11 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
22212257
if (opfamilies==NIL)/* compute if we didn't already */
22222258
opfamilies=get_mergejoin_opfamilies(eqop);
22232259
if (equal(opfamilies,ec->ec_opfamilies))
2260+
{
2261+
fkinfo->eclass[colno]=ec;
2262+
fkinfo->fk_eclass_member[colno]=item2_em;
22242263
returnec;
2264+
}
22252265
/* Otherwise, done with this EC, move on to the next */
22262266
break;
22272267
}
@@ -2230,6 +2270,37 @@ match_eclasses_to_foreign_key_col(PlannerInfo *root,
22302270
returnNULL;
22312271
}
22322272

2273+
/*
2274+
* find_derived_clause_for_ec_member
2275+
* Search for a previously-derived clause mentioning the given EM.
2276+
*
2277+
* The eclass should be an ec_has_const EC, of which the EM is a non-const
2278+
* member. This should ensure there is just one derived clause mentioning
2279+
* the EM (and equating it to a constant).
2280+
* Returns NULL if no such clause can be found.
2281+
*/
2282+
RestrictInfo*
2283+
find_derived_clause_for_ec_member(EquivalenceClass*ec,
2284+
EquivalenceMember*em)
2285+
{
2286+
ListCell*lc;
2287+
2288+
Assert(ec->ec_has_const);
2289+
Assert(!em->em_is_const);
2290+
foreach(lc,ec->ec_derives)
2291+
{
2292+
RestrictInfo*rinfo= (RestrictInfo*)lfirst(lc);
2293+
2294+
/*
2295+
* generate_base_implied_equalities_const will have put non-const
2296+
* members on the left side of derived clauses.
2297+
*/
2298+
if (rinfo->left_em==em)
2299+
returnrinfo;
2300+
}
2301+
returnNULL;
2302+
}
2303+
22332304

22342305
/*
22352306
* add_child_rel_equivalences

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