Revise handling of dropped columns in JOIN alias lists to avoid a

performance problem pointed out by phil@vodafone: to wit, we were
spending O(N^2) time to check dropped-ness in an N-deep join tree,
even in the case where the tree was freshly constructed and couldn't
possibly mention any dropped columns.  Instead of recursing in
get_rte_attribute_is_dropped(), change the data structure definition:
the joinaliasvars list of a JOIN RTE must have a NULL Const instead
of a Var at any position that references a now-dropped column.  This
costs nothing during normal parse-rewrite-plan path, and instead we
have a linear-time update to make when loading a stored rule that
might contain now-dropped columns.  While at it, move the responsibility
for acquring locks on relations referenced by rules into this separate
function (which I therefore chose to call AcquireRewriteLocks).
This saves effort --- namely, duplicated lock grabs in parser and rewriter
--- in the normal path at a cost of one extra non-locked heap_open()
in the stored-rule path; seems a good tradeoff.  A fringe benefit is
that it is now *much* clearer that we acquire lock on relations referenced
in rules before we make any rewriter decisions based on their properties.
(I don't know of any bug of that ilk, but it wasn't exactly clear before.)

1 files changed, 9 insertions, 1 deletions

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index d3dfe9edf7e..af3b5fb0f5b 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.135 2005/04/25 01:30:12 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.136 2005/06/03 23:05:28 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -100,6 +100,12 @@ ExplainQuery(ExplainStmt *stmt, DestReceiver *dest)
 	}
 	else
 	{
+		/*
+		 * Must acquire locks in case we didn't come fresh from the parser.
+		 * XXX this also scribbles on query, another reason for copyObject
+		 */
+		AcquireRewriteLocks(query);
+
 		/* Rewrite through rule system */
 		rewritten = QueryRewrite(query);
 
@@ -166,6 +172,8 @@ ExplainOneQuery(Query *query, ExplainStmt *stmt, TupOutputState *tstate)
 			cursorOptions = dcstmt->options;
 			/* Still need to rewrite cursor command */
 			Assert(query->commandType == CMD_SELECT);
+			/* get locks (we assume ExplainQuery already copied tree) */
+			AcquireRewriteLocks(query);
 			rewritten = QueryRewrite(query);
 			if (list_length(rewritten) != 1)
 				elog(ERROR, "unexpected rewrite result");