closes #5691, add comment

git-svn-id: file:///svn/mysql/tokudb-engine/tokudb-engine@50057 c7de825b-a66e-492c-adef-691d508d4ae1

storage/tokudb/ha_tokudb.cc

@@ -5859,6 +5859,13 @@ int ha_tokudb::create_txn(THD* thd, tokudb_trx_data* trx) {
     uint32_t txn_begin_flags;
     if (trx->all == NULL) {
         txn_begin_flags = toku_iso_to_txn_flag(toku_iso_level);
+        //
+        // if the isolation level that the user has set is serializable,
+        // but autocommit is on and this is just a select,
+        // then we can go ahead and set the isolation level to
+        // be a snapshot read, because we can serialize
+        // the transaction to be the point in time at which the snapshot began.
+        // 
         if (txn_begin_flags == 0 && is_autocommit && thd_sql_command(thd) == SQLCOM_SELECT) {
             txn_begin_flags = DB_TXN_SNAPSHOT;
         }