A minor change to MySQL's hash where calculation of hash can be done...

A minor change to MySQL's hash where calculation of hash can be done separately to avoid doing it under contended mutex locks

include/hash.h

@@ -45,6 +45,7 @@ extern "C" {
 #define hash_element    my_hash_element
 #define hash_search     my_hash_search
 #define hash_first      my_hash_first
+#define hash_first_from_hash_value      my_hash_first_from_hash_value
 #define hash_next       my_hash_next
 #define hash_insert     my_hash_insert
 #define hash_delete     my_hash_delete
@@ -94,8 +95,16 @@ void my_hash_free(HASH *tree);
 void my_hash_reset(HASH *hash);
 uchar *my_hash_element(HASH *hash, ulong idx);
 uchar *my_hash_search(const HASH *info, const uchar *key, size_t length);
+uchar *my_hash_search_using_hash_value(const HASH *info, uint hash_value,
+                                       const uchar *key, size_t length);
+uint my_calc_hash(const HASH *info, const uchar *key, size_t length);
 uchar *my_hash_first(const HASH *info, const uchar *key, size_t length,
                      HASH_SEARCH_STATE *state);
+uchar *my_hash_first_from_hash_value(const HASH *info,
+                                     uint hash_value,
+                                     const uchar *key,
+                                     size_t length,
+                                     HASH_SEARCH_STATE *state);
 uchar *my_hash_next(const HASH *info, const uchar *key, size_t length,
                     HASH_SEARCH_STATE *state);
 my_bool my_hash_insert(HASH *info, const uchar *data);

mysys/hash.c

@@ -214,6 +214,20 @@ uchar* my_hash_search(const HASH *hash, const uchar *key, size_t length)
   return my_hash_first(hash, key, length, &state);
 }
 
+uchar* my_hash_search_using_hash_value(const HASH *hash, 
+                                       uint hash_value,
+                                       const uchar *key,
+                                       size_t length)
+{
+  HASH_SEARCH_STATE state;
+  return my_hash_first_from_hash_value(hash, hash_value,
+                                       key, length, &state);
+}
+
+uint my_calc_hash(const HASH *hash, const uchar *key, size_t length)
+{
+  return calc_hash(hash, key, length ? length : hash->key_length);
+}
 /*
   Search after a record based on a key
 
@@ -223,15 +237,26 @@ uchar* my_hash_search(const HASH *hash, const uchar *key, size_t length)
 
 uchar* my_hash_first(const HASH *hash, const uchar *key, size_t length,
                      HASH_SEARCH_STATE *current_record)
+{
+  return my_hash_first_from_hash_value(hash,
+                 calc_hash(hash, key, length ? length : hash->key_length),
+                 key, length, current_record);
+}
+                    
+uchar* my_hash_first_from_hash_value(const HASH *hash,
+                                     uint hash_value,
+                                     const uchar *key,
+                                     size_t length,
+                                     HASH_SEARCH_STATE *current_record)
 {
   HASH_LINK *pos;
   uint flag,idx;
-  DBUG_ENTER("my_hash_first");
+  DBUG_ENTER("my_hash_first_from_hash_value");
 
   flag=1;
   if (hash->records)
   {
-    idx= my_hash_mask(calc_hash(hash, key, length ? length : hash->key_length),
+    idx= my_hash_mask(hash_value,
                       hash->blength, hash->records);
     do
     {

sql/sql_base.cc

@@ -2513,6 +2513,7 @@ TABLE *open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
   char	key[MAX_DBKEY_LENGTH];
   uint	key_length;
   char	*alias= table_list->alias;
+  uint hash_value;
   HASH_SEARCH_STATE state;
   DBUG_ENTER("open_table");
 
@@ -2702,6 +2703,7 @@ TABLE *open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
     on disk.
   */
 
+  hash_value= my_calc_hash(&open_cache, (uchar*) key, key_length);
   VOID(pthread_mutex_lock(&LOCK_open));
 
   /*
@@ -2744,8 +2746,11 @@ TABLE *open_table(THD *thd, TABLE_LIST *table_list, MEM_ROOT *mem_root,
     an implicit "pending locks queue" - see
     wait_for_locked_table_names for details.
   */
-  for (table= (TABLE*) hash_first(&open_cache, (uchar*) key, key_length,
-                                  &state);
+  for (table= (TABLE*) hash_first_from_hash_value(&open_cache,
+                                                  hash_value,
+                                                  (uchar*) key,
+                                                  key_length,
+                                                  &state);
        table && table->in_use ;
        table= (TABLE*) hash_next(&open_cache, (uchar*) key, key_length,
                                  &state))