bzr commit into mysql-5.1 branch (kristofer.pettersson:2768) Bug#38551

[Kristofer Pettersson]

#At file:///home/thek/Development/cpp/mysqlbzr/mysql-5.1-bugteam/

 2768 Kristofer Pettersson	2008-10-09
      Bug#38551 query cache can still consume [very little] cpu time even when it is off.
                  
      When the query cache size is zero, all query cache functionallity should be
      skipped.
            
      By checking query_cache_size before attempting to invalidate a table or
      db we avoid grabbing the query cache mutex. If the rare cases the query cache
      size would change so we get a false read the cache has either been cleared or
modified:
  sql/sql_cache.cc

per-file messages:
  sql/sql_cache.cc
    * Added checks to see if the query cache is enabled before attempting to
    take the query cache mutex.
=== modified file 'sql/sql_cache.cc'
--- a/sql/sql_cache.cc	2008-09-30 13:47:01 +0000
+++ b/sql/sql_cache.cc	2008-10-09 11:10:30 +0000
@@ -327,6 +327,7 @@ TODO list:
 */
 
 #include "mysql_priv.h"
+
 #ifdef HAVE_QUERY_CACHE
 #include <m_ctype.h>
 #include <my_dir.h>
@@ -1638,6 +1639,9 @@ void Query_cache::invalidate(char *db)
   bool restart= FALSE;
   DBUG_ENTER("Query_cache::invalidate (db)");
 
+  if (query_cache_size == 0)
+    DBUG_VOID_RETURN;
+ 
   STRUCT_LOCK(&structure_guard_mutex);
   bool interrupt;
   wait_while_table_flush_is_in_progress(&interrupt);
@@ -2528,6 +2532,9 @@ void Query_cache::invalidate_table(THD *
 
 void Query_cache::invalidate_table(THD *thd, uchar * key, uint32  key_length)
 {
+  if (query_cache_size == 0)
+    return;
+ 
   bool interrupt;
   STRUCT_LOCK(&structure_guard_mutex);
   wait_while_table_flush_is_in_progress(&interrupt);