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

src/backend/access/common/heaptuple.c

@@ -1366,6 +1366,32 @@ slot_attisnull(TupleTableSlot *slot, int attnum)
 	return heap_attisnull(tuple, attnum);
 }
 
+/*
+ * slot_getsysattr
+ *		This function fetches a system attribute of the slot's current tuple.
+ *		Unlike slot_getattr, if the slot does not contain system attributes,
+ *		this will return false (with a NULL attribute value) instead of
+ *		throwing an error.
+ */
+bool
+slot_getsysattr(TupleTableSlot *slot, int attnum,
+				Datum *value, bool *isnull)
+{
+	HeapTuple	tuple = slot->tts_tuple;
+
+	Assert(attnum < 0);			/* else caller error */
+	if (tuple == NULL ||
+		tuple == &(slot->tts_minhdr))
+	{
+		/* No physical tuple, or minimal tuple, so fail */
+		*value = (Datum) 0;
+		*isnull = true;
+		return false;
+	}
+	*value = heap_getsysattr(tuple, attnum, slot->tts_tupleDescriptor, isnull);
+	return true;
+}
+
 /*
  * heap_freetuple
  */

src/backend/executor/execCurrent.c

@@ -12,6 +12,7 @@
  */
 #include "postgres.h"
 
+#include "access/relscan.h"
 #include "access/sysattr.h"
 #include "catalog/pg_type.h"
 #include "executor/executor.h"
@@ -149,16 +150,13 @@ execCurrentOf(CurrentOfExpr *cexpr,
 	}
 	else
 	{
-		ScanState  *scanstate;
-		bool		lisnull;
-		Oid			tuple_tableoid PG_USED_FOR_ASSERTS_ONLY;
-		ItemPointer tuple_tid;
-
 		/*
 		 * Without FOR UPDATE, we dig through the cursor's plan to find the
 		 * scan node.  Fail if it's not there or buried underneath
 		 * aggregation.
 		 */
+		ScanState  *scanstate;
+
 		scanstate = search_plan_tree(queryDesc->planstate, table_oid);
 		if (!scanstate)
 			ereport(ERROR,
@@ -183,21 +181,62 @@ execCurrentOf(CurrentOfExpr *cexpr,
 		if (TupIsNull(scanstate->ss_ScanTupleSlot))
 			return false;
 
-		/* Use slot_getattr to catch any possible mistakes */
-		tuple_tableoid =
-			DatumGetObjectId(slot_getattr(scanstate->ss_ScanTupleSlot,
-										  TableOidAttributeNumber,
-										  &lisnull));
-		Assert(!lisnull);
-		tuple_tid = (ItemPointer)
-			DatumGetPointer(slot_getattr(scanstate->ss_ScanTupleSlot,
-										 SelfItemPointerAttributeNumber,
-										 &lisnull));
-		Assert(!lisnull);
-
-		Assert(tuple_tableoid == table_oid);
-
-		*current_tid = *tuple_tid;
+		/*
+		 * Extract TID of the scan's current row.  The mechanism for this is
+		 * in principle scan-type-dependent, but for most scan types, we can
+		 * just dig the TID out of the physical scan tuple.
+		 */
+		if (IsA(scanstate, IndexOnlyScanState))
+		{
+			/*
+			 * For IndexOnlyScan, the tuple stored in ss_ScanTupleSlot may be
+			 * a virtual tuple that does not have the ctid column, so we have
+			 * to get the TID from xs_ctup.t_self.
+			 */
+			IndexScanDesc scan = ((IndexOnlyScanState *) scanstate)->ioss_ScanDesc;
+
+			*current_tid = scan->xs_ctup.t_self;
+		}
+		else
+		{
+			/*
+			 * Default case: try to fetch TID from the scan node's current
+			 * tuple.  As an extra cross-check, verify tableoid in the current
+			 * tuple.  If the scan hasn't provided a physical tuple, we have
+			 * to fail.
+			 */
+			Datum		ldatum;
+			bool		lisnull;
+			ItemPointer tuple_tid;
+
+#ifdef USE_ASSERT_CHECKING
+			if (!slot_getsysattr(scanstate->ss_ScanTupleSlot,
+								 TableOidAttributeNumber,
+								 &ldatum,
+								 &lisnull))
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_CURSOR_STATE),
+						 errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
+								cursor_name, table_name)));
+			Assert(!lisnull);
+			Assert(DatumGetObjectId(ldatum) == table_oid);
+#endif
+
+			if (!slot_getsysattr(scanstate->ss_ScanTupleSlot,
+								 SelfItemPointerAttributeNumber,
+								 &ldatum,
+								 &lisnull))
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_CURSOR_STATE),
+						 errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
+								cursor_name, table_name)));
+			Assert(!lisnull);
+			tuple_tid = (ItemPointer) DatumGetPointer(ldatum);
+
+			*current_tid = *tuple_tid;
+		}
+
+		Assert(ItemPointerIsValid(current_tid));
 
 		return true;
 	}

src/include/executor/tuptable.h

@@ -170,5 +170,7 @@ extern Datum slot_getattr(TupleTableSlot *slot, int attnum, bool *isnull);
 extern void slot_getallattrs(TupleTableSlot *slot);
 extern void slot_getsomeattrs(TupleTableSlot *slot, int attnum);
 extern bool slot_attisnull(TupleTableSlot *slot, int attnum);
+extern bool slot_getsysattr(TupleTableSlot *slot, int attnum,
+				Datum *value, bool *isnull);
 
 #endif							/* TUPTABLE_H */

src/test/regress/expected/portals.out

@@ -1245,6 +1245,30 @@ 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

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;