From: Yugo Nagata <nagata@sraoss.co.jp>
To: Yugo Nagata <nagata@sraoss.co.jp>
Cc: pgsql-hackers@postgresql.org
Subject: Re: CURRENT OF causes an error when IndexOnlyScan is used
Date: Thu, 1 Feb 2018 01:44:44 +0900
Message-ID: <20180201014444.f78358df.nagata@sraoss.co.jp> (raw)
In-Reply-To: <20180201013349.937dfc5f.nagata@sraoss.co.jp>
References: <20180201013349.937dfc5f.nagata@sraoss.co.jp>
On Thu, 1 Feb 2018 01:33:49 +0900
Yugo Nagata <nagata@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 <nagata@sraoss.co.jp>
--
Yugo Nagata <nagata@sraoss.co.jp>
Attachments:current_of_index_only_scan_v2.patch (text/x-diff, 1.9K) downloaddiff --git a/src/backend/executor/execCurrent.c b/src/backend/executor/execCurrent.cindex 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;
}
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To: pgsql-hackers@postgresql.org
Cc: nagata@sraoss.co.jp
Subject: Re: CURRENT OF causes an error when IndexOnlyScan is used
In-Reply-To: <20180201014444.f78358df.nagata@sraoss.co.jp>
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