diff --git a/doc/src/sgml/indexam.sgml b/doc/src/sgml/indexam.sgml
index e03ebc9..de5b825 100644
--- a/doc/src/sgml/indexam.sgml
+++ b/doc/src/sgml/indexam.sgml
@@ -1104,19 +1104,44 @@ amcostestimate (PlannerInfo *root,
 
    <step>
     <para>
-     Compute the index access cost.  A generic estimator might do this:
+     Compute the index access cost.  A generic estimator might compute this as
+     the disk access cost plus the CPU cost.
+     The disk access cost might be estimated:
 
 <programlisting>
 /*
- * Our generic assumption is that the index pages will be read
- * sequentially, so they cost seq_page_cost each, not random_page_cost.
- * Also, we charge for evaluation of the indexquals at each index row.
- * All the costs are assumed to be paid incrementally during the scan.
+ * For a single index scan, we just charge spc_random_page_cost per
+ * page touched.
  */
-cost_qual_eval(&amp;index_qual_cost, path-&gt;indexquals, root);
-*indexStartupCost = index_qual_cost.startup;
-*indexTotalCost = seq_page_cost * numIndexPages +
-    (cpu_index_tuple_cost + index_qual_cost.per_tuple) * numIndexTuples;
+*indexTotalCost = numIndexPages * spc_random_page_cost;
+</programlisting>
+
+     The CPU cost might be estimated:
+
+<programlisting>
+/*
+ * 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.	Because we have numIndexTuples as a per-scan
+ * number, we have to multiply by num_sa_scans to get the correct result
+ * for ScalarArrayOpExpr cases.  Similarly add in costs for any index
+ * ORDER BY expressions.
+ */
+cost_qual_eval(&amp;index_qual_cost, indexQuals, root);
+qual_arg_cost = index_qual_cost.startup + index_qual_cost.per_tuple;
+cost_qual_eval(&amp;index_qual_cost, indexOrderBys, root);
+qual_arg_cost += index_qual_cost.startup + index_qual_cost.per_tuple;
+qual_op_cost = cpu_operator_cost *
+    (list_length(indexQuals) + list_length(indexOrderBys));
+qual_arg_cost -= qual_op_cost;
+if (qual_arg_cost < 0)		/* just in case... */
+    qual_arg_cost = 0;
+
+*indexStartupCost = qual_arg_cost;
+*indexTotalCost += qual_arg_cost;
+*indexTotalCost += numIndexTuples * num_sa_scans * (cpu_index_tuple_cost + qual_op_cost);
 </programlisting>
 
      However, the above does not account for amortization of index reads