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When FOR UPDATE/SHARE is used with LIMIT, put the LockRows plan node

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underneath the Limit node, not atop it.  This fixes the old problem that such
a query might unexpectedly return fewer rows than the LIMIT says, due to
LockRows discarding updated rows.

There is a related problem that LockRows might destroy the sort ordering
produced by earlier steps; but fixing that by pushing LockRows below Sort
would create serious performance problems that are unjustified in many
real-world applications, as well as potential deadlock problems from locking
many more rows than expected.  Instead, keep the present semantics of applying
FOR UPDATE after ORDER BY within a single query level; but allow the user to
specify the other way by writing FOR UPDATE in a sub-select.  To make that
work, track whether FOR UPDATE appeared explicitly in sub-selects or got
pushed down from the parent, and don't flatten a sub-select that contained an
explicit FOR UPDATE.
This commit is contained in:
Tom Lane
2009-10-28 14:55:47 +00:00
parent 44956c52c5
commit 46e3a16b05
13 changed files with 225 additions and 126 deletions
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35
src/backend/utils/adt/ruleutils.c
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@ -9,7 +9,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.310 2009/10/14 22:14:23 tgl Exp $
* $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.311 2009/10/28 14:55:44 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -2549,21 +2549,28 @@ get_select_query_def(Query *query, deparse_context *context,
}
/* Add FOR UPDATE/SHARE clauses if present */
foreach(l, query->rowMarks)
if (query->hasForUpdate)
{
RowMarkClause *rc = (RowMarkClause *) lfirst(l);
RangeTblEntry *rte = rt_fetch(rc->rti, query->rtable);
foreach(l, query->rowMarks)
{
RowMarkClause *rc = (RowMarkClause *) lfirst(l);
RangeTblEntry *rte = rt_fetch(rc->rti, query->rtable);
if (rc->forUpdate)
appendContextKeyword(context, " FOR UPDATE",
-PRETTYINDENT_STD, PRETTYINDENT_STD, 0);
else
appendContextKeyword(context, " FOR SHARE",
-PRETTYINDENT_STD, PRETTYINDENT_STD, 0);
appendStringInfo(buf, " OF %s",
quote_identifier(rte->eref->aliasname));
if (rc->noWait)
appendStringInfo(buf, " NOWAIT");
/* don't print implicit clauses */
if (rc->pushedDown)
continue;
if (rc->forUpdate)
appendContextKeyword(context, " FOR UPDATE",
-PRETTYINDENT_STD, PRETTYINDENT_STD, 0);
else
appendContextKeyword(context, " FOR SHARE",
-PRETTYINDENT_STD, PRETTYINDENT_STD, 0);
appendStringInfo(buf, " OF %s",
quote_identifier(rte->eref->aliasname));
if (rc->noWait)
appendStringInfo(buf, " NOWAIT");
}
}
context->windowClause = save_windowclause;