PREPARE and EXPLAIN need to copy the source query just like we recently

had to do in DECLARE CURSOR.  AFAICS these are all the places affected.
PREPARE case per example from Michael Fuhr, EXPLAIN case located by
grepping for planner calls ...

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 1.127 2004/09/30 17:42:42 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.128 2004/12/12 20:17:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -75,6 +75,16 @@ ExplainQuery(ExplainStmt *stmt, DestReceiver *dest)
 	List	   *rewritten;
 	ListCell   *l;
 
+	/*
+	 * Because the planner is not cool about not scribbling on its input,
+	 * we make a preliminary copy of the source querytree.  This prevents
+	 * problems in the case that the EXPLAIN is in a portal or plpgsql
+	 * function and is executed repeatedly.  (See also the same hack in
+	 * DECLARE CURSOR and PREPARE.)  XXX the planner really shouldn't
+	 * modify its input ... FIXME someday.
+	 */
+	query = copyObject(query);
+
 	/* prepare for projection of tuples */
 	tstate = begin_tup_output_tupdesc(dest, ExplainResultDesc(stmt));
 

src/backend/commands/prepare.c

@@ -10,7 +10,7 @@
  * Copyright (c) 2002-2004, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.34 2004/12/03 21:26:31 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.35 2004/12/12 20:17:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -48,6 +48,7 @@ void
 PrepareQuery(PrepareStmt *stmt)
 {
 	const char *commandTag;
+	Query	   *query;
 	List	   *query_list,
 			   *plan_list;
 
@@ -87,8 +88,18 @@ PrepareQuery(PrepareStmt *stmt)
 	 * the query.
 	 */
 
+	/*
+	 * Because the planner is not cool about not scribbling on its input,
+	 * we make a preliminary copy of the source querytree.  This prevents
+	 * problems in the case that the PREPARE is in a portal or plpgsql
+	 * function and is executed repeatedly.  (See also the same hack in
+	 * DECLARE CURSOR and EXPLAIN.)  XXX the planner really shouldn't
+	 * modify its input ... FIXME someday.
+	 */
+	query = copyObject(stmt->query);
+
 	/* Rewrite the query. The result could be 0, 1, or many queries. */
-	query_list = QueryRewrite(stmt->query);
+	query_list = QueryRewrite(query);
 
 	/* Generate plans for queries.	Snapshot is already set. */
 	plan_list = pg_plan_queries(query_list, NULL, false);