mysql: do not use unsafe TRUNCATE statement

TRUNCATE was chosen for performance reasons, but it's usually done on small
tables, and not for performance-critical operations. TRUNCATE commits
implicitely, so for pt/ttrans in particular, it's certainly slower due to extra
fsyncs to disk.

On the other side, committing too early can corrupt the database if the storage
node is stopped just after. For example, a failure in changePartitionTable()
can cause 'pt' to remain empty.

neo/storage/database/mysqldb.py

@@ -136,7 +136,7 @@ class MySQLDatabaseManager(DatabaseManager):
         if r in ("INSERT", "REPLACE", "DELETE", "UPDATE"):
             self._active = 1
         else:
-            assert r in ("ALTER", "CREATE", "DROP", "TRUNCATE"), query
+            assert r in ("ALTER", "CREATE", "DROP"), query
 
     @property
     def escape(self):
@@ -364,7 +364,7 @@ class MySQLDatabaseManager(DatabaseManager):
         offset_list = []
         q = self.query
         if reset:
-            q("TRUNCATE pt")
+            q("DELETE FROM pt")
         for offset, nid, state in cell_list:
             # TODO: this logic should move out of database manager
             # add 'dropCells(cell_list)' to API and use one query
@@ -414,8 +414,8 @@ class MySQLDatabaseManager(DatabaseManager):
     def dropUnfinishedData(self):
         q = self.query
         data_id_list = [x for x, in q("SELECT data_id FROM tobj") if x]
-        q("TRUNCATE tobj")
-        q("TRUNCATE ttrans")
+        q("DELETE FROM tobj")
+        q("DELETE FROM ttrans")
         self.releaseData(data_id_list, True)
 
     def storeTransaction(self, tid, object_list, transaction, temporary = True):