add notice about what cost means in explain output (#7631)

gpdb-doc/dita/admin_guide/query/topics/query-profiling.xml

@@ -59,7 +59,16 @@
           sequential disk page read. The first estimate is the start-up cost of getting the first
           row and the second is the total cost of cost of getting all rows. The total cost assumes
           all rows will be retrieved, which is not always true; for example, if the query uses
-            <codeph>LIMIT</codeph>, not all rows are retrieved.</li>
+            <codeph>LIMIT</codeph>, not all rows are retrieved.<note>The cost values generated by
+            the Pivotal Query Optimizer and the Postgres Planner are not directly comparable. The
+            two optimizers use different cost models, as well as different algorithms, to determine
+            the cost of an execution plan. Nothing can or should be inferred by comparing cost
+            values between the two optimizers.<p>In addition, the cost generated for any given
+              optimizer is valid only for comparing plan alternatives for a given single query and
+              set of statistics. Different queries can generate plans with different costs, even
+              when keeping the optimizer a constant.</p><p>To summarize, the cost is essentially an
+              internal number used by a given optimizer, and nothing should be inferred by examining
+              only the cost value displayed in the <codeph>EXPLAIN</codeph> plans.</p></note></li>
         <li id="in182483"><b>rows</b> —The total number of rows output by this plan node. This
           number is usually less than the number of rows processed or scanned by the plan node,
           reflecting the estimated selectivity of any <codeph>WHERE</codeph> clause conditions.