fix (bug #10303: Misleading Last_query_cost value).

sql/sql_cache.cc:
  fix (bug #10303: Misleading Last_query_cost value).
  set the last_query_cost to 0.0 in case we use the query cache.

mysql-test/r/query_cache.result

@@ -1127,3 +1127,19 @@ Qcache_hits	9
 drop procedure f1;
 drop table t1;
 set GLOBAL query_cache_size=0;
+SET GLOBAL query_cache_size=102400;
+create table t1(a int);
+insert into t1 values(0), (1), (4), (5);
+select * from t1 where a > 3;
+a
+4
+5
+select * from t1 where a > 3;
+a
+4
+5
+show status like 'last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+drop table t1;
+SET GLOBAL query_cache_size=0;

mysql-test/r/status.result

@@ -17,3 +17,9 @@ Variable_name	Value
 Table_locks_immediate	3
 Table_locks_waited	1
 drop table t1;
+select 1;
+1
+1
+show status like 'last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000

mysql-test/t/query_cache.test

@@ -816,3 +816,16 @@ drop table t1;
 set GLOBAL query_cache_size=0;
 
 # End of 4.1 tests
+
+#
+# Bug #10303: problem with last_query_cost
+#
+
+SET GLOBAL query_cache_size=102400;
+create table t1(a int);
+insert into t1 values(0), (1), (4), (5);
+select * from t1 where a > 3;
+select * from t1 where a > 3;
+show status like 'last_query_cost';
+drop table t1;
+SET GLOBAL query_cache_size=0;

mysql-test/t/status.test

@@ -37,3 +37,10 @@ show status like 'Table_lock%';
 drop table t1;
 
 # End of 4.1 tests
+
+#
+# lost_query_cost
+#
+
+select 1;
+show status like 'last_query_cost';

sql/sql_cache.cc

@@ -1178,6 +1178,7 @@ sql mode: 0x%lx, sort len: %lu, conncat len: %lu",
 #endif /*!EMBEDDED_LIBRARY*/
 
   thd->limit_found_rows = query->found_rows();
+  thd->status_var.last_query_cost= 0.0;
 
   BLOCK_UNLOCK_RD(query_block);
   DBUG_RETURN(1);				// Result sent to client