fix(db2): _truncate_table — use DELETE FROM instead of TRUNCATE IMMEDIATE

TRUNCATE TABLE ... IMMEDIATE cannot run inside an open unit of work
(SQL0428N). ibm_db_dbi forces AUTOCOMMIT_OFF on all connections, which
means _prepare_helper() inside execute() implicitly opens a unit of
work before the statement runs — making the IMMEDIATE constraint
impossible to satisfy in practice.

DELETE FROM has no such restriction and is rollback-safe, matching
the established pattern in trino.py and risingwave.py.

Author: iawanish

sqlmesh/core/engine_adapter/db2.py

@@ -733,19 +733,13 @@ def _create_table_like(
         )
 
     def _truncate_table(self, table_name: TableName) -> None:
-        # Db2's TRUNCATE TABLE ... IMMEDIATE commits instantly and cannot be
-        # rolled back (SQL0428N if inside an open transaction). When a
-        # transaction is already active, use DELETE which participates in the
-        # transaction normally and can be rolled back. When no transaction is
-        # active, use TRUNCATE TABLE ... IMMEDIATE — the IMMEDIATE keyword is
-        # mandatory in Db2 syntax (SQL0104N without it).
-        if self._connection_pool.is_transaction_active:
-            self.execute(exp.Delete(this=exp.to_table(table_name)))
-        else:
-            table = exp.to_table(table_name)
-            self.execute(
-                f"TRUNCATE TABLE {table.sql(dialect=self.dialect, identify=True)} IMMEDIATE"
-            )
+        # Db2's TRUNCATE TABLE ... IMMEDIATE requires being the first statement
+        # in a unit of work (SQL0428N). ibm_db_dbi forces AUTOCOMMIT_OFF on all
+        # connections, so _prepare_helper() inside execute() implicitly opens a
+        # unit of work before TRUNCATE runs — making it impossible to satisfy
+        # that constraint. DELETE FROM has no such restriction and is
+        # rollback-safe, matching the pattern used by trino.py and risingwave.py.
+        self.execute(exp.Delete(this=exp.to_table(table_name)))
 
     def _convert_df_datetime(self, df: DF, columns_to_types: t.Dict[str, exp.DataType]) -> None:
         """