Fix for BUG#20954: avg(keyval) retuns 0.38 but max(keyval) returns an empty set

The problem was in that opt_sum_query() replaced MIN/MAX functions
with the corresponding constant found in a key, but due to imprecise
representation of float numbers, when evaluating the where clause,
this comparison failed.

When MIN/MAX optimization detects that all tables can be removed,
also remove all conjuncts in a where clause that refer to these
tables. As a result of this fix, these conditions are not evaluated
twice, and in the case of float number comparisons we do not discard
result rows due to imprecise float representation.

As a side-effect this fix also corrects an unnoticed problem in
bug 12882.

sql/sql_select.cc

@@ -547,6 +547,24 @@ JOIN::optimize()
       }
       zero_result_cause= "Select tables optimized away";
       tables_list= 0;				// All tables resolved
+      /*
+        Extract all table-independent conditions and replace the WHERE
+        clause with them. All other conditions were computed by opt_sum_query
+        and the MIN/MAX/COUNT function(s) have been replaced by constants,
+        so there is no need to compute the whole WHERE clause again.
+        Notice that make_cond_for_table() will always succeed to remove all
+        computed conditions, because opt_sum_query() is applicable only to
+        conjunctions.
+      */
+      if (conds)
+      {
+        COND *table_independent_conds=
+          make_cond_for_table(conds, PSEUDO_TABLE_BITS, 0);
+        DBUG_EXECUTE("where",
+                     print_where(table_independent_conds,
+                                 "where after opt_sum_query()"););
+        conds= table_independent_conds;
+      }
     }
   }
   if (!tables_list)