always call start_bulk_insert, clarify this behaviour in comment block

change 10 to a #define'd constant

myisam/myisamdef.h

@@ -417,6 +417,7 @@ typedef struct st_mi_sort_param
 #define MI_MIN_SIZE_BULK_INSERT_TREE 16384             /* this is per key */
 #define MI_MIN_ROWS_TO_USE_BULK_INSERT 100
 #define MI_MIN_ROWS_TO_DISABLE_INDEXES 100
+#define MI_MIN_ROWS_TO_USE_WRITE_CACHE 10
 
 /* The UNIQUE check is done with a hashed long key */
 

sql/ha_myisam.cc

@@ -868,7 +868,7 @@ void ha_myisam::start_bulk_insert(ha_rows rows)
   ulong size= min(thd->variables.read_buff_size, table->avg_row_length*rows);
 
   /* don't enable row cache if too few rows */
-  if (!rows && rows > 10)
+  if (!rows && rows >  MI_MIN_ROWS_TO_USE_WRITE_CACHE)
     mi_extra(file, HA_EXTRA_WRITE_CACHE, (void*) &size);
 
   can_enable_indexes= (file->s->state.key_map ==

sql/sql_insert.cc

@@ -260,7 +260,15 @@ int mysql_insert(THD *thd,TABLE_LIST *table_list,
   thd->proc_info="update";
   if (duplic != DUP_ERROR)
     table->file->extra(HA_EXTRA_IGNORE_DUP_KEY);
-  if (lock_type != TL_WRITE_DELAYED && values_list.elements != 1);
+  /*
+    let's *try* to start bulk inserts. It won't necessary
+    start them as values_list.elements should be greater than
+    some - handler dependent - threshold.
+    So we call start_bulk_insert to perform nesessary checks on
+    values_list.elements, and - if nothing else - to initialize
+    the code to make the call of end_bulk_insert() below safe.
+  */
+  if (lock_type != TL_WRITE_DELAYED)
     table->file->start_bulk_insert(values_list.elements);
 
   while ((values= its++))