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Fix IOS planning when only some index columns can return an attribute.

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Since 9.5, it's possible that some but not all columns of an index
support returning the indexed value for index-only scans.  If the
same indexed column appears in index columns that behave both ways,
check_index_only() supposed that it'd be OK to do an index-only scan
testing that column; but that fails if we have to recheck the indexed
condition on one of the columns that doesn't support this.

In principle we could make this work by remapping the recheck expressions
to pull the value from a column that does support returning the indexed
value.  But such cases are so weird and rare that, at least for now,
it doesn't seem worth the trouble.  Instead, just teach check_index_only
that a value is returnable only if all the index columns containing it
are returnable, rather than any of them.

Per report from David Pereiro Lagares.  Back-patch to 9.5 where the
possibility of this situation appeared.

Kyotaro Horiguchi

Discussion: https://postgr.es/m/1516210494.1798.16.camel@nlpgo.com
This commit is contained in:
Tom Lane 2018-03-01 15:35:03 -05:00
parent 6452b098c0
commit b5febc1d12
3 changed files with 71 additions and 1 deletions
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39
contrib/btree_gist/expected/inet.out
View File

@ -64,3 +64,42 @@ SELECT count(*) FROM inettmp WHERE a > '89.225.196.191'::inet;
386
(1 row)
VACUUM inettmp;
-- gist_inet_ops lacks a fetch function, so this should not be index-only scan
EXPLAIN (COSTS OFF)
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
QUERY PLAN
--------------------------------------------------------
Aggregate
-> Bitmap Heap Scan on inettmp
Recheck Cond: (a = '89.225.196.191'::inet)
-> Bitmap Index Scan on inetidx
Index Cond: (a = '89.225.196.191'::inet)
(5 rows)
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
count
-------
1
(1 row)
DROP INDEX inetidx;
CREATE INDEX ON inettmp USING gist (a gist_inet_ops, a inet_ops);
-- likewise here (checks for core planner bug)
EXPLAIN (COSTS OFF)
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
QUERY PLAN
--------------------------------------------------------
Aggregate
-> Bitmap Heap Scan on inettmp
Recheck Cond: (a = '89.225.196.191'::inet)
-> Bitmap Index Scan on inettmp_a_a1_idx
Index Cond: (a = '89.225.196.191'::inet)
(5 rows)
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
count
-------
1
(1 row)

18
contrib/btree_gist/sql/inet.sql
View File

@ -29,3 +29,21 @@ SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
SELECT count(*) FROM inettmp WHERE a >= '89.225.196.191'::inet;
SELECT count(*) FROM inettmp WHERE a > '89.225.196.191'::inet;
VACUUM inettmp;
-- gist_inet_ops lacks a fetch function, so this should not be index-only scan
EXPLAIN (COSTS OFF)
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
DROP INDEX inetidx;
CREATE INDEX ON inettmp USING gist (a gist_inet_ops, a inet_ops);
-- likewise here (checks for core planner bug)
EXPLAIN (COSTS OFF)
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;
SELECT count(*) FROM inettmp WHERE a = '89.225.196.191'::inet;

15
src/backend/optimizer/path/indxpath.c
View File

@ -1866,6 +1866,7 @@ check_index_only(RelOptInfo *rel, IndexOptInfo *index)
bool result;
Bitmapset *attrs_used = NULL;
Bitmapset *index_canreturn_attrs = NULL;
Bitmapset *index_cannotreturn_attrs = NULL;
ListCell *lc;
int i;
@ -1905,7 +1906,11 @@ check_index_only(RelOptInfo *rel, IndexOptInfo *index)
/*
* Construct a bitmapset of columns that the index can return back in an
* index-only scan.
* index-only scan. If there are multiple index columns containing the
* same attribute, all of them must be capable of returning the value,
* since we might recheck operators on any of them. (Potentially we could
* be smarter about that, but it's such a weird situation that it doesn't
* seem worth spending a lot of sweat on.)
*/
for (i = 0; i < index->ncolumns; i++)
{
@ -1922,13 +1927,21 @@ check_index_only(RelOptInfo *rel, IndexOptInfo *index)
index_canreturn_attrs =
bms_add_member(index_canreturn_attrs,
attno - FirstLowInvalidHeapAttributeNumber);
else
index_cannotreturn_attrs =
bms_add_member(index_cannotreturn_attrs,
attno - FirstLowInvalidHeapAttributeNumber);
}
index_canreturn_attrs = bms_del_members(index_canreturn_attrs,
index_cannotreturn_attrs);
/* Do we have all the necessary attributes? */
result = bms_is_subset(attrs_used, index_canreturn_attrs);
bms_free(attrs_used);
bms_free(index_canreturn_attrs);
bms_free(index_cannotreturn_attrs);
return result;
}