Fix WHERE CURRENT OF when the referenced cursor uses an index-only scan.

"UPDATE/DELETE WHERE CURRENT OF cursor_name" failed, with an error message
like "cannot extract system attribute from virtual tuple", if the cursor
was using a index-only scan for the target table.  Fix it by digging the
current TID out of the indexscan state.

It seems likely that the same failure could occur for CustomScan plans
and perhaps some FDW plan types, so that leaving this to be treated as an
internal error with an obscure message isn't as good an idea as it first
seemed.  Hence, add a bit of heaptuple.c infrastructure to let us deliver
a more on-topic message.  I chose to make the message match what you get
for the case where execCurrentOf can't identify the target scan node at
all, "cursor "foo" is not a simply updatable scan of table "bar"".
Perhaps it should be different, but we can always adjust that later.

In the future, it might be nice to provide hooks that would let custom
scan providers and/or FDWs deal with this in other ways; but that's
not a suitable topic for a back-patchable bug fix.

It's been like this all along, so back-patch to all supported branches.

Yugo Nagata and Tom Lane

Discussion: https://postgr.es/m/20180201013349.937dfc5f.nagata@sraoss.co.jp

2 files changed, 34 insertions, 0 deletions

diff --git a/src/test/regress/expected/portals.out b/src/test/regress/expected/portals.out
index 462ad231c36..850b23f150f 100644
--- a/src/test/regress/expected/portals.out
+++ b/src/test/regress/expected/portals.out
@@ -1246,6 +1246,30 @@ FETCH FROM c1;
 DELETE FROM ucview WHERE CURRENT OF c1; -- fail, views not supported
 ERROR:  WHERE CURRENT OF on a view is not implemented
 ROLLBACK;
+-- Check WHERE CURRENT OF with an index-only scan
+BEGIN;
+EXPLAIN (costs off)
+DECLARE c1 CURSOR FOR SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
+                 QUERY PLAN                  
+---------------------------------------------
+ Index Only Scan using onek_stringu1 on onek
+   Index Cond: (stringu1 = 'DZAAAA'::name)
+(2 rows)
+
+DECLARE c1 CURSOR FOR SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
+FETCH FROM c1;
+ stringu1 
+----------
+ DZAAAA
+(1 row)
+
+DELETE FROM onek WHERE CURRENT OF c1;
+SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
+ stringu1 
+----------
+(0 rows)
+
+ROLLBACK;
 -- Make sure snapshot management works okay, per bug report in
 -- 235395b90909301035v7228ce63q392931f15aa74b31@mail.gmail.com
 BEGIN;
diff --git a/src/test/regress/sql/portals.sql b/src/test/regress/sql/portals.sql
index 01c3b85da9a..57a8c9312f0 100644
--- a/src/test/regress/sql/portals.sql
+++ b/src/test/regress/sql/portals.sql
@@ -462,6 +462,16 @@ FETCH FROM c1;
 DELETE FROM ucview WHERE CURRENT OF c1; -- fail, views not supported
 ROLLBACK;
 
+-- Check WHERE CURRENT OF with an index-only scan
+BEGIN;
+EXPLAIN (costs off)
+DECLARE c1 CURSOR FOR SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
+DECLARE c1 CURSOR FOR SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
+FETCH FROM c1;
+DELETE FROM onek WHERE CURRENT OF c1;
+SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
+ROLLBACK;
+
 -- Make sure snapshot management works okay, per bug report in
 -- 235395b90909301035v7228ce63q392931f15aa74b31@mail.gmail.com
 BEGIN;