-
+
<application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language
When a cursor is positioned on a table row, that row can be updated
- or deleted using the cursor to identify the row. Note that this
- only works for simple (non-join, non-grouping) cursor queries.
- For additional information see the
+ or deleted using the cursor to identify the row. There are
+ restrictions on what the cursor's query can be (in particular,
+ no grouping) and it's best to use FOR UPDATE in the
+ cursor. For additional information see the
reference page.
specified, then backward fetches are disallowed in any case.
+ Backward fetches are also disallowed when the query
+ includes FOR UPDATE or FOR SHARE; therefore
+ SCROLL may not be specified in this case.
+
+
If the cursor's query includes FOR UPDATE or FOR
SHARE, then returned rows are locked at the time they are first
these options.
In addition, the returned rows will be the most up-to-date versions;
therefore these options provide the equivalent of what the SQL standard
- calls a sensitive cursor. It is often wise to use FOR
- UPDATE if the cursor is intended to be used with UPDATE
- ... WHERE CURRENT OF or DELETE ... WHERE CURRENT OF,
- since this will prevent other sessions from changing the rows between
- the time they are fetched and the time they are updated. Without
- FOR UPDATE, a subsequent WHERE CURRENT OF command
- will have no effect if the row was changed meanwhile.
+ calls a sensitive cursor. (Specifying INSENSITIVE
+ together with FOR UPDATE or FOR SHARE is an error.)
- SCROLL may not be specified when the query
- includes FOR UPDATE or FOR SHARE.
-
+
+ It is generally recommended to use FOR UPDATE if the cursor
+ is intended to be used with UPDATE ... WHERE CURRENT OF or
+ DELETE ... WHERE CURRENT OF. Using FOR UPDATE
+ prevents other sessions from changing the rows between the time they are
+ fetched and the time they are updated. Without FOR UPDATE,
+ a subsequent WHERE CURRENT OF command will have no effect if
+ the row was changed since the cursor was created.
+
+
+ Another reason to use FOR UPDATE is that without it, a
+ subsequent WHERE CURRENT OF might fail if the cursor query
+ does not meet the SQL standard's rules for being simply
+ updatable (in particular, the cursor must reference just one table
+ and not use grouping or ORDER BY). Cursors
+ that are not simply updatable might work, or might not, depending on plan
+ choice details; so in the worst case, an application might work in testing
+ and then fail in production.
+
+
+ The main reason not to use FOR UPDATE with WHERE
+ CURRENT OF is if you need the cursor to be scrollable, or to be
+ insensitive to the subsequent updates (that is, continue to show the old
+ data). If this is a requirement, pay close heed to the caveats shown
+ above.
+
+
The SQL standard only makes provisions for cursors in embedded
The name of the cursor to use in a WHERE CURRENT OF
condition. The row to be deleted is the one most recently fetched
- from this cursor. The cursor must be a simple (non-join, non-aggregate)
+ from this cursor. The cursor must be a non-grouping
query on the DELETE's target table.
Note that WHERE CURRENT OF cannot be
- specified together with a Boolean condition.
+ specified together with a Boolean condition. See
+
+ for more information about using cursors with
+ WHERE CURRENT OF.
Clear the table films:
DELETE FROM films;
-
+
Delete completed tasks, returning full details of the deleted rows:
DELETE FROM tasks WHERE status = 'DONE' RETURNING *;
-
+
c_tasks is currently positioned:
DELETE FROM tasks WHERE CURRENT OF c_tasks;
-
+
The name of the cursor to use in a WHERE CURRENT OF
condition. The row to be updated is the one most recently fetched
- from this cursor. The cursor must be a simple (non-join, non-aggregate)
+ from this cursor. The cursor must be a non-grouping
query on the UPDATE's target table.
Note that WHERE CURRENT OF cannot be
- specified together with a Boolean condition.
+ specified together with a Boolean condition. See
+
+ for more information about using cursors with
+ WHERE CURRENT OF.
c_films is currently positioned:
UPDATE films SET kind = 'Dramatic' WHERE CURRENT OF c_films;
-
+
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/backend/executor/execCurrent.c,v 1.7 2008/05/12 00:00:48 alvherre Exp $
+ * $PostgreSQL: pgsql/src/backend/executor/execCurrent.c,v 1.8 2008/11/16 17:34:28 tgl Exp $
*
*-------------------------------------------------------------------------
*/
char *table_name;
Portal portal;
QueryDesc *queryDesc;
- ScanState *scanstate;
- bool lisnull;
- Oid tuple_tableoid;
- ItemPointer tuple_tid;
/* Get the cursor name --- may have to look up a parameter reference */
if (cexpr->cursor_name)
errmsg("cursor \"%s\" is not a SELECT query",
cursor_name)));
queryDesc = PortalGetQueryDesc(portal);
- if (queryDesc == NULL)
+ if (queryDesc == NULL || queryDesc->estate == NULL)
ereport(ERROR,
(errcode(ERRCODE_INVALID_CURSOR_STATE),
errmsg("cursor \"%s\" is held from a previous transaction",
cursor_name)));
/*
- * Dig through the cursor's plan to find the scan node. Fail if it's not
- * there or buried underneath aggregation.
+ * We have two different strategies depending on whether the cursor uses
+ * FOR UPDATE/SHARE or not. The reason for supporting both is that the
+ * FOR UPDATE code is able to identify a target table in many cases where
+ * the other code can't, while the non-FOR-UPDATE case allows use of WHERE
+ * CURRENT OF with an insensitive cursor.
*/
- scanstate = search_plan_tree(ExecGetActivePlanTree(queryDesc),
- table_oid);
- if (!scanstate)
- ereport(ERROR,
- (errcode(ERRCODE_INVALID_CURSOR_STATE),
- errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
- cursor_name, table_name)));
+ if (queryDesc->estate->es_rowMarks)
+ {
+ ExecRowMark *erm;
+ ListCell *lc;
- /*
- * The cursor must have a current result row: per the SQL spec, it's an
- * error if not. We test this at the top level, rather than at the scan
- * node level, because in inheritance cases any one table scan could
- * easily not be on a row. We want to return false, not raise error, if
- * the passed-in table OID is for one of the inactive scans.
- */
- if (portal->atStart || portal->atEnd)
- ereport(ERROR,
- (errcode(ERRCODE_INVALID_CURSOR_STATE),
- errmsg("cursor \"%s\" is not positioned on a row",
- cursor_name)));
+ /*
+ * Here, the query must have exactly one FOR UPDATE/SHARE reference to
+ * the target table, and we dig the ctid info out of that.
+ */
+ erm = NULL;
+ foreach(lc, queryDesc->estate->es_rowMarks)
+ {
+ ExecRowMark *thiserm = (ExecRowMark *) lfirst(lc);
+
+ if (RelationGetRelid(thiserm->relation) == table_oid)
+ {
+ if (erm)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_CURSOR_STATE),
+ errmsg("cursor \"%s\" has multiple FOR UPDATE/SHARE references to table \"%s\"",
+ cursor_name, table_name)));
+ erm = thiserm;
+ }
+ }
- /* Now OK to return false if we found an inactive scan */
- if (TupIsNull(scanstate->ss_ScanTupleSlot))
+ if (erm == NULL)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_CURSOR_STATE),
+ errmsg("cursor \"%s\" does not have a FOR UPDATE/SHARE reference to table \"%s\"",
+ cursor_name, table_name)));
+
+ /*
+ * The cursor must have a current result row: per the SQL spec, it's
+ * an error if not.
+ */
+ if (portal->atStart || portal->atEnd)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_CURSOR_STATE),
+ errmsg("cursor \"%s\" is not positioned on a row",
+ cursor_name)));
+
+ /* Return the currently scanned TID, if there is one */
+ if (ItemPointerIsValid(&(erm->curCtid)))
+ {
+ *current_tid = erm->curCtid;
+ return true;
+ }
+
+ /*
+ * This table didn't produce the cursor's current row; some other
+ * inheritance child of the same parent must have. Signal caller
+ * to do nothing on this table.
+ */
return false;
+ }
+ else
+ {
+ ScanState *scanstate;
+ bool lisnull;
+ Oid tuple_tableoid;
+ 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 = search_plan_tree(ExecGetActivePlanTree(queryDesc),
+ table_oid);
+ if (!scanstate)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_CURSOR_STATE),
+ errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
+ cursor_name, table_name)));
- /* 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);
+ /*
+ * The cursor must have a current result row: per the SQL spec, it's
+ * an error if not. We test this at the top level, rather than at the
+ * scan node level, because in inheritance cases any one table scan
+ * could easily not be on a row. We want to return false, not raise
+ * error, if the passed-in table OID is for one of the inactive scans.
+ */
+ if (portal->atStart || portal->atEnd)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_CURSOR_STATE),
+ errmsg("cursor \"%s\" is not positioned on a row",
+ cursor_name)));
- Assert(tuple_tableoid == table_oid);
+ /* Now OK to return false if we found an inactive scan */
+ if (TupIsNull(scanstate->ss_ScanTupleSlot))
+ return false;
- *current_tid = *tuple_tid;
+ /* 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);
- return true;
+ Assert(tuple_tableoid == table_oid);
+
+ *current_tid = *tuple_tid;
+
+ return true;
+ }
}
/*
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.316 2008/11/15 19:43:45 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.317 2008/11/16 17:34:28 tgl Exp $
*
*-------------------------------------------------------------------------
*/
/* We'll locate the junk attrs below */
erm->ctidAttNo = InvalidAttrNumber;
erm->toidAttNo = InvalidAttrNumber;
+ ItemPointerSetInvalid(&(erm->curCtid));
estate->es_rowMarks = lappend(estate->es_rowMarks, erm);
}
if (tableoid != RelationGetRelid(erm->relation))
{
/* this child is inactive right now */
+ ItemPointerSetInvalid(&(erm->curCtid));
continue;
}
}
elog(ERROR, "unrecognized heap_lock_tuple status: %u",
test);
}
+
+ /* Remember tuple TID for WHERE CURRENT OF */
+ erm->curCtid = tuple.t_self;
}
}
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.195 2008/11/15 19:43:46 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.196 2008/11/16 17:34:28 tgl Exp $
*
*-------------------------------------------------------------------------
*/
bool noWait; /* NOWAIT option */
AttrNumber ctidAttNo; /* resno of its ctid junk attribute */
AttrNumber toidAttNo; /* resno of tableoid junk attribute, if any */
+ ItemPointerData curCtid; /* ctid of currently locked tuple, if any */
} ExecRowMark;
110 | hundred
(3 rows)
+-- Can update from a self-join, but only if FOR UPDATE says which to use
+BEGIN;
+DECLARE c1 CURSOR FOR SELECT * FROM uctest a, uctest b WHERE a.f1 = b.f1 + 5;
+FETCH 1 FROM c1;
+ f1 | f2 | f1 | f2
+----+-----+----+-------
+ 18 | one | 13 | three
+(1 row)
+
+UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1; -- fail
+ERROR: cursor "c1" is not a simply updatable scan of table "uctest"
+ROLLBACK;
+BEGIN;
+DECLARE c1 CURSOR FOR SELECT * FROM uctest a, uctest b WHERE a.f1 = b.f1 + 5 FOR UPDATE;
+FETCH 1 FROM c1;
+ f1 | f2 | f1 | f2
+----+-----+----+-------
+ 18 | one | 13 | three
+(1 row)
+
+UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1; -- fail
+ERROR: cursor "c1" has multiple FOR UPDATE/SHARE references to table "uctest"
+ROLLBACK;
+BEGIN;
+DECLARE c1 CURSOR FOR SELECT * FROM uctest a, uctest b WHERE a.f1 = b.f1 + 5 FOR SHARE OF a;
+FETCH 1 FROM c1;
+ f1 | f2 | f1 | f2
+----+-----+----+-------
+ 18 | one | 13 | three
+(1 row)
+
+UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1;
+SELECT * FROM uctest;
+ f1 | f2
+-----+---------
+ 13 | three
+ 28 | one
+ 110 | hundred
+(3 rows)
+
+ROLLBACK;
-- Check various error cases
DELETE FROM uctest WHERE CURRENT OF c1; -- fail, no such cursor
ERROR: cursor "c1" does not exist
ERROR: cursor "c" is not a simply updatable scan of table "uctest"
ROLLBACK;
BEGIN;
+DECLARE c CURSOR FOR SELECT * FROM tenk2 FOR SHARE;
+DELETE FROM uctest WHERE CURRENT OF c; -- fail, cursor on wrong table
+ERROR: cursor "c" does not have a FOR UPDATE/SHARE reference to table "uctest"
+ROLLBACK;
+BEGIN;
DECLARE c CURSOR FOR SELECT * FROM tenk1 JOIN tenk2 USING (unique1);
DELETE FROM tenk1 WHERE CURRENT OF c; -- fail, cursor is on a join
ERROR: cursor "c" is not a simply updatable scan of table "tenk1"
COMMIT;
SELECT * FROM uctest;
+-- Can update from a self-join, but only if FOR UPDATE says which to use
+BEGIN;
+DECLARE c1 CURSOR FOR SELECT * FROM uctest a, uctest b WHERE a.f1 = b.f1 + 5;
+FETCH 1 FROM c1;
+UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1; -- fail
+ROLLBACK;
+BEGIN;
+DECLARE c1 CURSOR FOR SELECT * FROM uctest a, uctest b WHERE a.f1 = b.f1 + 5 FOR UPDATE;
+FETCH 1 FROM c1;
+UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1; -- fail
+ROLLBACK;
+BEGIN;
+DECLARE c1 CURSOR FOR SELECT * FROM uctest a, uctest b WHERE a.f1 = b.f1 + 5 FOR SHARE OF a;
+FETCH 1 FROM c1;
+UPDATE uctest SET f1 = f1 + 10 WHERE CURRENT OF c1;
+SELECT * FROM uctest;
+ROLLBACK;
+
-- Check various error cases
DELETE FROM uctest WHERE CURRENT OF c1; -- fail, no such cursor
DELETE FROM uctest WHERE CURRENT OF c; -- fail, cursor on wrong table
ROLLBACK;
BEGIN;
+DECLARE c CURSOR FOR SELECT * FROM tenk2 FOR SHARE;
+DELETE FROM uctest WHERE CURRENT OF c; -- fail, cursor on wrong table
+ROLLBACK;
+BEGIN;
DECLARE c CURSOR FOR SELECT * FROM tenk1 JOIN tenk2 USING (unique1);
DELETE FROM tenk1 WHERE CURRENT OF c; -- fail, cursor is on a join
ROLLBACK;
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