Support UPDATE/DELETE WHERE CURRENT OF cursor_name, per SQL standard.

Along the way, allow FOR UPDATE in non-WITH-HOLD cursors; there may once
have been a reason to disallow that, but it seems to work now, and it's
really rather necessary if you want to select a row via a cursor and then
update it in a concurrent-safe fashion.

Original patch by Arul Shaji, rather heavily editorialized by Tom Lane.

src/backend/optimizer/path/clausesel.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.85 2007/04/21 21:01:44 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.86 2007/06/11 01:16:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -18,6 +18,7 @@
 #include "nodes/makefuncs.h"
 #include "optimizer/clauses.h"
 #include "optimizer/cost.h"
+#include "optimizer/pathnode.h"
 #include "optimizer/plancat.h"
 #include "parser/parsetree.h"
 #include "utils/fmgroids.h"
@@ -712,6 +713,15 @@ clause_selectivity(PlannerInfo *root,
 						 varRelid,
 						 jointype);
 	}
+	else if (IsA(clause, CurrentOfExpr))
+	{
+		/* CURRENT OF selects at most one row of its table */
+		CurrentOfExpr *cexpr = (CurrentOfExpr *) clause;
+		RelOptInfo *crel = find_base_rel(root, cexpr->cvarno);
+
+		if (crel->tuples > 0)
+			s1 = 1.0 / crel->tuples;
+	}
 	else if (IsA(clause, RelabelType))
 	{
 		/* Not sure this case is needed, but it can't hurt */

src/backend/optimizer/path/costsize.c

@@ -54,7 +54,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.184 2007/06/05 21:31:05 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.185 2007/06/11 01:16:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -770,6 +770,7 @@ cost_tidscan(Path *path, PlannerInfo *root,
 	Cost		startup_cost = 0;
 	Cost		run_cost = 0;
 	Cost		cpu_per_tuple;
+	QualCost	tid_qual_cost;
 	int			ntuples;
 	ListCell   *l;
 
@@ -799,12 +800,20 @@ cost_tidscan(Path *path, PlannerInfo *root,
 		}
 	}
 
+	/*
+	 * The TID qual expressions will be computed once, any other baserestrict
+	 * quals once per retrived tuple.
+	 */
+	cost_qual_eval(&tid_qual_cost, tidquals, root);
+
 	/* disk costs --- assume each tuple on a different page */
 	run_cost += random_page_cost * ntuples;
 
 	/* CPU costs */
-	startup_cost += baserel->baserestrictcost.startup;
-	cpu_per_tuple = cpu_tuple_cost + baserel->baserestrictcost.per_tuple;
+	startup_cost += baserel->baserestrictcost.startup +
+		tid_qual_cost.per_tuple;
+	cpu_per_tuple = cpu_tuple_cost + baserel->baserestrictcost.per_tuple -
+		tid_qual_cost.per_tuple;
 	run_cost += cpu_per_tuple * ntuples;
 
 	path->startup_cost = startup_cost;
@@ -1991,6 +2000,11 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 				cpu_operator_cost;
 		}
 	}
+	else if (IsA(node, CurrentOfExpr))
+	{
+		/* This is noticeably more expensive than a typical operator */
+		context->total.per_tuple += 100 * cpu_operator_cost;
+	}
 	else if (IsA(node, SubLink))
 	{
 		/* This routine should not be applied to un-planned expressions */

src/backend/optimizer/path/tidpath.c

@@ -12,6 +12,12 @@
  * this allows
  *		WHERE ctid IN (tid1, tid2, ...)
  *
+ * We also support "WHERE CURRENT OF cursor" conditions (CurrentOfExpr),
+ * which amount to "CTID = run-time-determined-TID".  These could in
+ * theory be translated to a simple comparison of CTID to the result of
+ * a function, but in practice it works better to keep the special node
+ * representation all the way through to execution.
+ *
  * There is currently no special support for joins involving CTID; in
  * particular nothing corresponding to best_inner_indexscan().	Since it's
  * not very useful to store TIDs of one table in another table, there
@@ -24,7 +30,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/tidpath.c,v 1.29 2007/01/05 22:19:31 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/tidpath.c,v 1.30 2007/06/11 01:16:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -174,6 +180,12 @@ TidQualFromExpr(Node *expr, int varno)
 		if (IsTidEqualAnyClause((ScalarArrayOpExpr *) expr, varno))
 			rlst = list_make1(expr);
 	}
+	else if (expr && IsA(expr, CurrentOfExpr))
+	{
+		/* another base case: check for CURRENT OF on this rel */
+		if (((CurrentOfExpr *) expr)->cvarno == varno)
+			rlst = list_make1(expr);
+	}
 	else if (and_clause(expr))
 	{
 		foreach(l, ((BoolExpr *) expr)->args)