On Thu, 1 Feb 2018 01:33:49 +0900
Yugo Nagata <nag...@sraoss.co.jp> wrote:
I'm sorry the patch attached in the previous mail is broken and
not raises a compile error. I attached the fixed patch.
Regards,
> Hi,
>
> I found that updating a cursor by using CURRENT OF causes the
> following error when the query is executed by IndexOnlyScan.
>
> ERROR: cannot extract system attribute from virtual tuple
>
> IndexOnlyScan returns a virtual tuple that doesn't have system
> column, so we can not get ctid in the same way of other plans.
> However, the error message is not convinient and users would
> not understand why the error occurs.
>
> Attached is a patch to fix this. By this fix, execCurrentOf
> get ctid from IndexScanDesc->xs_ctup.t_self when the plan is
> IndexOnlyScan, and it works sucessfully without errors.
>
>
> Here is the example of the error:
>
> =======
> postgres=# create table test (i int primary key);
> CREATE TABLE
> postgres=# insert into test values(1);
> INSERT 0 1
> postgres=# set enable_seqscan to off;
> SET
>
> postgres=# explain select * from test where i = 1;
> QUERY PLAN
> ---------------------------------------------------------------------------
> Index Only Scan using test_pkey on test (cost=0.15..8.17 rows=1 width=4)
> Index Cond: (i = 1)
> (2 rows)
>
> postgres=# begin;
> BEGIN
> postgres=# declare c cursor for select * from test where i = 1;
> DECLARE CURSOR
> postgres=# fetch from c;
> i
> ---
> 1
> (1 row)
>
> postgres=# update test set i=i+1 where current of c;
> ERROR: cannot extract system attribute from virtual tuple
> =======
>
> The patch fixes the error and allows this update successfully.
>
> Regards,
>
> --
> Yugo Nagata <nag...@sraoss.co.jp>
--
Yugo Nagata <nag...@sraoss.co.jp>
diff --git a/src/backend/executor/execCurrent.c b/src/backend/executor/execCurrent.c
index ce7d4ac..aa2da16 100644
--- a/src/backend/executor/execCurrent.c
+++ b/src/backend/executor/execCurrent.c
@@ -19,6 +19,7 @@
#include "utils/lsyscache.h"
#include "utils/portal.h"
#include "utils/rel.h"
+#include "access/relscan.h"
static char *fetch_cursor_param_value(ExprContext *econtext, int paramId);
@@ -183,21 +184,35 @@ 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;
+ /* In IndexOnlyScan case, the tuple stored in ss_ScanTupleSlot is a
+ * virtual tuple that does not have ctid column, so we have to get TID
+ * from xs_ctup.t_self. */
+ if (IsA(scanstate, IndexOnlyScanState))
+ {
+ IndexScanDesc scan = ((IndexOnlyScanState *)scanstate)->ioss_ScanDesc;
+
+ Assert(RelationGetRelid(scan.heapRelation) == table_oid);
+
+ *current_tid = scan->xs_ctup.t_self;
+ }
+ else
+ {
+ /* 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;
+ }
return true;
}
