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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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11
src/include/nodes/parsenodes.h
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@@ -13,7 +13,7 @@
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/nodes/parsenodes.h,v 1.411 2009/10/26 02:26:41 tgl Exp $
* $PostgreSQL: pgsql/src/include/nodes/parsenodes.h,v 1.412 2009/10/28 14:55:46 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -118,6 +118,7 @@ typedef struct Query
bool hasSubLinks; /* has subquery SubLink */
bool hasDistinctOn; /* distinctClause is from DISTINCT ON */
bool hasRecursive; /* WITH RECURSIVE was specified */
bool hasForUpdate; /* FOR UPDATE or FOR SHARE was specified */
List *cteList; /* WITH list (of CommonTableExpr's) */
@@ -803,7 +804,12 @@ typedef struct WindowClause
* parser output representation of FOR UPDATE/SHARE clauses
*
* Query.rowMarks contains a separate RowMarkClause node for each relation
* identified as a FOR UPDATE/SHARE target.
* identified as a FOR UPDATE/SHARE target. If FOR UPDATE/SHARE is applied
* to a subquery, we generate RowMarkClauses for all normal and subquery rels
* in the subquery, but they are marked pushedDown = true to distinguish them
* from clauses that were explicitly written at this query level. Also,
* Query.hasForUpdate tells whether there were explicit FOR UPDATE/SHARE
* clauses in the current query level.
*/
typedef struct RowMarkClause
{
@@ -811,6 +817,7 @@ typedef struct RowMarkClause
Index rti; /* range table index of target relation */
bool forUpdate; /* true = FOR UPDATE, false = FOR SHARE */
bool noWait; /* NOWAIT option */
bool pushedDown; /* pushed down from higher query level? */
} RowMarkClause;
/*