bug#4497 - Serious regression if disk based TMP table is used.

Solved performance problems by enabling write buffer.



sql/sql_select.cc:
  bug#4497 - Serious regression if disk based TMP table is used.
  Solved performance problems by enabling write buffer.
  Supplied no test case, as it required too much data to see the performance regression.

sql/sql_select.cc

@@ -4533,6 +4533,20 @@ bool create_myisam_from_heap(THD *thd, TABLE *table, TMP_TABLE_PARAM *param,
     new_table.no_rows=1;
   }
 
+#ifdef TO_BE_DONE_LATER_IN_4_1
+  /*
+    To use start_bulk_insert() (which is new in 4.1) we need to find
+    all places where a corresponding end_bulk_insert() should be put.
+  */
+  table->file->info(HA_STATUS_VARIABLE); /* update table->file->records */
+  new_table.file->start_bulk_insert(table->file->records);
+#else
+  /*
+    HA_EXTRA_WRITE_CACHE can stay until close, no need to disable it explicitly.
+  */
+  new_table.file->extra(HA_EXTRA_WRITE_CACHE);
+#endif
+
   /* copy all old rows */
   while (!table->file->rnd_next(new_table.record[1]))
   {