Repair misestimation of indexscan CPU costs. When an indexqual contains

a run-time key (that is, a nonconstant expression compared to the index
variable), the key is evaluated just once per scan, but we were charging
costs as though it were evaluated once per visited index entry.

Author: tglsfdc

src/backend/utils/adt/selfuncs.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.154 2004/01/07 18:56:28 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.155 2004/01/17 20:09:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3897,6 +3897,8 @@ genericcostestimate(Query *root, RelOptInfo *rel,
 	double		numIndexTuples;
 	double		numIndexPages;
 	QualCost	index_qual_cost;
+	double		qual_op_cost;
+	double		qual_arg_cost;
 	List	   *selectivityQuals;
 
 	/*
@@ -3976,16 +3978,31 @@ genericcostestimate(Query *root, RelOptInfo *rel,
 	/*
 	 * Compute the index access cost.
 	 *
-	 * Our generic assumption is that the index pages will be read
-	 * sequentially, so they have cost 1.0 each, not random_page_cost.
-	 * Also, we charge for evaluation of the indexquals at each index
-	 * tuple. All the costs are assumed to be paid incrementally during
-	 * the scan.
+	 * Disk cost: our generic assumption is that the index pages will be
+	 * read sequentially, so they have cost 1.0 each, not random_page_cost.
+	 */
+	*indexTotalCost = numIndexPages;
+
+	/*
+	 * CPU cost: any complex expressions in the indexquals will need to
+	 * be evaluated once at the start of the scan to reduce them to runtime
+	 * keys to pass to the index AM (see nodeIndexscan.c).  We model the
+	 * per-tuple CPU costs as cpu_index_tuple_cost plus one cpu_operator_cost
+	 * per indexqual operator.
+	 *
+	 * Note: this neglects the possible costs of rechecking lossy operators
+	 * and OR-clause expressions.  Detecting that that might be needed seems
+	 * more expensive than it's worth, though, considering all the other
+	 * inaccuracies here ...
 	 */
 	cost_qual_eval(&index_qual_cost, indexQuals);
-	*indexStartupCost = index_qual_cost.startup;
-	*indexTotalCost = numIndexPages +
-		(cpu_index_tuple_cost + index_qual_cost.per_tuple) * numIndexTuples;
+	qual_op_cost = cpu_operator_cost * length(indexQuals);
+	qual_arg_cost = index_qual_cost.startup +
+		index_qual_cost.per_tuple - qual_op_cost;
+	if (qual_arg_cost < 0)		/* just in case... */
+		qual_arg_cost = 0;
+	*indexStartupCost = qual_arg_cost;
+	*indexTotalCost += numIndexTuples * (cpu_index_tuple_cost + qual_op_cost);
 
 	/*
 	 * Generic assumption about index correlation: there isn't any.