Re: BUG #16303: A condtion whether an index-only scan is possibleincludes a wrong

I send a patch for this problem.

thanks!

From: PG Bug reporting form <noreply@postgresql.org>
Subject: BUG #16303: A condtion whether an index-only scan is possible includes a wrong
Date: Mon, 16 Mar 2020 06:02:25 +0000

> The following bug has been logged on the website:
> 
> Bug reference:      16303
> Logged by:          Horimoto Yasuhiro
> Email address:      horimoto@clear-code.com
> PostgreSQL version: 12.2
> Operating system:   Debian 10.3
> Description:        
> 
> Hello, developers.
> 
> I think that the condition of whether an index-only scan is possible
> includes a wrong.
> 
> For example, in the following case, the index has no data to return. Because
> the query doesn't use specify columns.
> However, the query planner choice index-only scan.
> 
> create table gist_count_tbl (tsv tsvector);
> insert into gist_count_tbl values (null);
> create index gist_count_tbl_index on gist_count_tbl using gist (tsv);
> 
> vacuum analyze gist_count_tbl;
> 
> set enable_seqscan=off;
> set enable_bitmapscan=off;
> set enable_indexonlyscan=on;
> 
> explain (costs off)
> select count(*) from gist_count_tbl;
>                              QUERY PLAN                             
> --------------------------------------------------------------------
>  Aggregate
>    ->  Index Only Scan using gist_count_tbl_index on gist_count_tbl
> (2 rows)
> 
> In my opinion, we expected that the query planner doesn't choose an
> index-only scan in the above case. 
> 
> In fact, index_canreturn_attrs of
> https://github.com/postgres/postgres/blob/master/src/backend/optimizer/path/indxpath.c#L1951
> is NULL in the above case.
> 
> thanks!
>
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 2a50272da6..8fd3df8d69 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -1960,8 +1960,12 @@ check_index_only(RelOptInfo *rel, IndexOptInfo *index)
     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);
+    if (index_canreturn_attrs == NULL)
+        /* We don't have indexes that can return attributes. */
+        result = false;
+    else
+        /* 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);
diff --git a/src/test/regress/expected/gist.out b/src/test/regress/expected/gist.out
index 90edb4061d..a1b9fea2c2 100644
--- a/src/test/regress/expected/gist.out
+++ b/src/test/regress/expected/gist.out
@@ -317,3 +317,30 @@ reset enable_seqscan;
 reset enable_bitmapscan;
 reset enable_indexonlyscan;
 drop table gist_tbl;
+--
+-- Test Index-only plans on GiST indexes when columns needless query.
+--
+create table gist_count_tbl (tsv tsvector);
+insert into gist_count_tbl values (null);
+create index gist_count_tbl_index on gist_count_tbl using gist (tsv);
+vacuum analyze gist_count_tbl;
+set enable_seqscan=off;
+set enable_bitmapscan=off;
+set enable_indexonlyscan=on;
+-- Test that an index-only scan is not chosen,
+-- when the query doesn't use specify columns.
+-- Because the index has not data to return in this case.
+explain (costs off)
+select count(*) from gist_count_tbl;
+            QUERY PLAN            
+----------------------------------
+ Aggregate
+   ->  Seq Scan on gist_count_tbl
+(2 rows)
+
+-- Clean up
+reset enable_seqscan;
+reset enable_bitmapscan;
+reset enable_indexonlyscan;
+drop index gist_count_tbl_index;
+drop table gist_count_tbl;
diff --git a/src/test/regress/sql/gist.sql b/src/test/regress/sql/gist.sql
index b9d398ea94..09d4372a7f 100644
--- a/src/test/regress/sql/gist.sql
+++ b/src/test/regress/sql/gist.sql
@@ -148,3 +148,30 @@ reset enable_bitmapscan;
 reset enable_indexonlyscan;
 
 drop table gist_tbl;
+
+--
+-- Test Index-only plans on GiST indexes when columns needless query.
+--
+create table gist_count_tbl (tsv tsvector);
+insert into gist_count_tbl values (null);
+create index gist_count_tbl_index on gist_count_tbl using gist (tsv);
+
+vacuum analyze gist_count_tbl;
+
+set enable_seqscan=off;
+set enable_bitmapscan=off;
+set enable_indexonlyscan=on;
+
+-- Test that an index-only scan is not chosen,
+-- when the query doesn't use specify columns.
+-- Because the index has not data to return in this case.
+explain (costs off)
+select count(*) from gist_count_tbl;
+
+-- Clean up
+reset enable_seqscan;
+reset enable_bitmapscan;
+reset enable_indexonlyscan;
+
+drop index gist_count_tbl_index;
+drop table gist_count_tbl;