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Commitfe66fc6

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Limit the number of index clauses considered in choose_bitmap_and().
classify_index_clause_usage() is O(N^2) in the number of distinct indexqual clauses it considers, because of its use of a simple search list tostore them. For nearly all queries, that's fine because only a few clauseswill be considered. But Alexander Kuzmenkov reported a machine-generatedquery with 80000 (!) index qual clauses, which caused this code to takeforever. Somewhat remarkably, this is the only O(N^2) behavior we nowhave for such a query, so let's fix it.We can get rid of the O(N^2) runtime for cases like this without muchdamage to the functionality of choose_bitmap_and() by separating outpaths with "too many" qual or pred clauses, and deeming them to alwaysbe nonredundant with other paths. Then their clauses needn't go intothe search list, so it doesn't get too long, but we don't lose theability to consider bitmap AND plans altogether. I set the thresholdfor "too many" to be 100 clauses per path, which should be plenty toensure no change in planning behavior for normal queries.There are other things we could do to make this go faster, but it's notclear that it's worth any additional effort. 80000 qual clauses requirea whole lot of work in many other places, too.The code's been like this for a long time, so back-patch to all supportedbranches. The troublesome query only works back to 9.5 (in 9.4 it failswith stack overflow in the parser); so I'm not sure that fixing this in9.4 has any real-world benefit, but perhaps it does.Discussion:https://postgr.es/m/90c5bdfa-d633-dabe-9889-3cf3e1acd443@postgrespro.ru
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‎src/backend/optimizer/path/indxpath.c

Lines changed: 31 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -69,6 +69,7 @@ typedef struct
6969
List*quals;/* the WHERE clauses it uses */
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List*preds;/* predicates of its partial index(es) */
7171
Bitmapset*clauseids;/* quals+preds represented as a bitmapset */
72+
boolunclassifiable;/* has too many quals+preds to process? */
7273
}PathClauseUsage;
7374

7475
/* Callback argument for ec_member_matches_indexcol */
@@ -1385,9 +1386,18 @@ choose_bitmap_and(PlannerInfo *root, RelOptInfo *rel, List *paths)
13851386
Path*ipath= (Path*)lfirst(l);
13861387

13871388
pathinfo=classify_index_clause_usage(ipath,&clauselist);
1389+
1390+
/* If it's unclassifiable, treat it as distinct from all others */
1391+
if (pathinfo->unclassifiable)
1392+
{
1393+
pathinfoarray[npaths++]=pathinfo;
1394+
continue;
1395+
}
1396+
13881397
for (i=0;i<npaths;i++)
13891398
{
1390-
if (bms_equal(pathinfo->clauseids,pathinfoarray[i]->clauseids))
1399+
if (!pathinfoarray[i]->unclassifiable&&
1400+
bms_equal(pathinfo->clauseids,pathinfoarray[i]->clauseids))
13911401
break;
13921402
}
13931403
if (i<npaths)
@@ -1422,6 +1432,10 @@ choose_bitmap_and(PlannerInfo *root, RelOptInfo *rel, List *paths)
14221432
* For each surviving index, consider it as an "AND group leader", and see
14231433
* whether adding on any of the later indexes results in an AND path with
14241434
* cheaper total cost than before. Then take the cheapest AND group.
1435+
*
1436+
* Note: paths that are either clauseless or unclassifiable will have
1437+
* empty clauseids, so that they will not be rejected by the clauseids
1438+
* filter here, nor will they cause later paths to be rejected by it.
14251439
*/
14261440
for (i=0;i<npaths;i++)
14271441
{
@@ -1638,6 +1652,21 @@ classify_index_clause_usage(Path *path, List **clauselist)
16381652
result->preds=NIL;
16391653
find_indexpath_quals(path,&result->quals,&result->preds);
16401654

1655+
/*
1656+
* Some machine-generated queries have outlandish numbers of qual clauses.
1657+
* To avoid getting into O(N^2) behavior even in this preliminary
1658+
* classification step, we want to limit the number of entries we can
1659+
* accumulate in *clauselist. Treat any path with more than 100 quals +
1660+
* preds as unclassifiable, which will cause calling code to consider it
1661+
* distinct from all other paths.
1662+
*/
1663+
if (list_length(result->quals)+list_length(result->preds)>100)
1664+
{
1665+
result->clauseids=NULL;
1666+
result->unclassifiable= true;
1667+
returnresult;
1668+
}
1669+
16411670
/* Build up a bitmapset representing the quals and preds */
16421671
clauseids=NULL;
16431672
foreach(lc,result->quals)
@@ -1655,6 +1684,7 @@ classify_index_clause_usage(Path *path, List **clauselist)
16551684
find_list_position(node,clauselist));
16561685
}
16571686
result->clauseids=clauseids;
1687+
result->unclassifiable= false;
16581688

16591689
returnresult;
16601690
}

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