Add commentary to show that even though ExecInitIndexScan()

contains much code that looks like it will handle indexquals with the index
key on either side of the operator, in fact indexquals must have the index
key on the left because of limitations of the ScanKey machinery.  Perhaps
someone will be motivated to fix that someday...

src/backend/executor/nodeIndexscan.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.41 1999/08/09 06:20:22 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.42 1999/08/12 00:42:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -741,7 +741,7 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 
 			/* ----------------
 			 *	Here we figure out the contents of the index qual.
-			 *	The usual case is (op var const) or (op const var)
+			 *	The usual case is (var op const) or (const op var)
 			 *	which means we form a scan key for the attribute
 			 *	listed in the var node and use the value of the const.
 			 *
@@ -759,6 +759,19 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 			 *	Hence, we set have_runtime_keys to true and then set
 			 *	the appropriate flag in run_keys to LEFT_OP or RIGHT_OP.
 			 *	The corresponding scan keys are recomputed at run time.
+			 *
+			 *	XXX Although this code *thinks* it can handle an indexqual
+			 *	with the indexkey on either side, in fact it cannot.
+			 *	Indexscans only work with quals that have the indexkey on
+			 *	the left (the planner/optimizer makes sure it never passes
+			 *	anything else).  The reason: the scankey machinery has no
+			 *	provision for distinguishing which side of the operator is
+			 *	the indexed attribute and which is the compared-to constant.
+			 *	It just assumes that the attribute is on the left :-(
+			 *
+			 *	I am leaving this code able to support both ways, even though
+			 *	half of it is dead code, on the off chance that someone will
+			 *	fix the scankey machinery someday --- tgl 8/11/99.
 			 * ----------------
 			 */
 
@@ -770,7 +783,9 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 			 */
 			leftop = (Node *) get_leftop(clause);
 
-			if (IsA(leftop, Var) &&var_is_rel((Var *) leftop))
+			Assert(leftop != NULL);
+
+			if (IsA(leftop, Var) && var_is_rel((Var *) leftop))
 			{
 				/* ----------------
 				 *	if the leftop is a "rel-var", then it means
@@ -781,6 +796,19 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 				varattno = ((Var *) leftop)->varattno;
 				scanvar = LEFT_OP;
 			}
+			else if (is_funcclause(leftop) &&
+					 var_is_rel(lfirst(((Expr *) leftop)->args)))
+			{
+				/* ----------------
+				 *	if the leftop is a func node then it means
+				 *	it identifies the value to place in our scan key.
+				 *	Since functional indices have only one attribute
+				 *	the attno must always be set to 1.
+				 * ----------------
+				 */
+				varattno = 1;
+				scanvar = LEFT_OP;
+			}
 			else if (IsA(leftop, Const))
 			{
 				/* ----------------
@@ -819,21 +847,6 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 					run_keys[j] = NO_OP;
 				}
 			}
-			else if (leftop != NULL &&
-					 is_funcclause(leftop) &&
-					 var_is_rel(lfirst(((Expr *) leftop)->args)))
-			{
-				/* ----------------
-				 *	if the leftop is a func node then it means
-				 *	it identifies the value to place in our scan key.
-				 *	Since functional indices have only one attribute
-				 *	the attno must always be set to 1.
-				 * ----------------
-				 */
-				varattno = 1;
-				scanvar = LEFT_OP;
-
-			}
 			else
 			{
 				/* ----------------
@@ -853,7 +866,9 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 			 */
 			rightop = (Node *) get_rightop(clause);
 
-			if (IsA(rightop, Var) &&var_is_rel((Var *) rightop))
+			Assert(rightop != NULL);
+
+			if (IsA(rightop, Var) && var_is_rel((Var *) rightop))
 			{
 				/* ----------------
 				 *	here we make sure only one op identifies the
@@ -872,12 +887,28 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 				 */
 				varattno = ((Var *) rightop)->varattno;
 				scanvar = RIGHT_OP;
+			}
+			else if (is_funcclause(rightop) &&
+					 var_is_rel(lfirst(((Expr *) rightop)->args)))
+			{
+				/* ----------------
+				 *	if the rightop is a func node then it means
+				 *	it identifies the value to place in our scan key.
+				 *	Since functional indices have only one attribute
+				 *	the attno must always be set to 1.
+				 * ----------------
+				 */
+				if (scanvar == LEFT_OP)
+					elog(ERROR, "ExecInitIndexScan: %s",
+						 "both left and right ops are rel-vars");
 
+				varattno = 1;
+				scanvar = RIGHT_OP;
 			}
 			else if (IsA(rightop, Const))
 			{
 				/* ----------------
-				 *	if the leftop is a const node then it means
+				 *	if the rightop is a const node then it means
 				 *	it identifies the value to place in our scan key.
 				 * ----------------
 				 */
@@ -912,29 +943,10 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 					run_keys[j] = NO_OP;
 				}
 			}
-			else if (rightop != NULL &&
-					 is_funcclause(rightop) &&
-					 var_is_rel(lfirst(((Expr *) rightop)->args)))
-			{
-				/* ----------------
-				 *	if the rightop is a func node then it means
-				 *	it identifies the value to place in our scan key.
-				 *	Since functional indices have only one attribute
-				 *	the attno must always be set to 1.
-				 * ----------------
-				 */
-				if (scanvar == LEFT_OP)
-					elog(ERROR, "ExecInitIndexScan: %s",
-						 "both left and right ops are rel-vars");
-
-				varattno = 1;
-				scanvar = RIGHT_OP;
-
-			}
 			else
 			{
 				/* ----------------
-				 *	otherwise, the leftop contains information usable
+				 *	otherwise, the rightop contains information usable
 				 *	at runtime to figure out the value to place in our
 				 *	scan key.
 				 * ----------------