Fix current_timestamp for Oracle

Author: erezsh

data_diff/sqeleton/databases/base.py

@@ -124,6 +124,8 @@ class BaseDialect(AbstractDialect):
     SUPPORTS_INDEXES = False
     TYPE_CLASSES: Dict[str, type] = {}
 
+    PLACEHOLDER_TABLE = None        # Used for Oracle
+
     def offset_limit(self, offset: Optional[int] = None, limit: Optional[int] = None):
         if offset:
             raise NotImplementedError("No support for OFFSET in query")

data_diff/sqeleton/databases/oracle.py

@@ -86,6 +86,7 @@ class Dialect(BaseDialect, Mixin_Schema):
         "VARCHAR2": Text,
     }
     ROUNDS_ON_PREC_LOSS = True
+    PLACEHOLDER_TABLE = "DUAL"
 
     def quote(self, s: str):
         return f'"{s}"'
@@ -152,6 +153,9 @@ def parse_type(
     def set_timezone_to_utc(self) -> str:
         return "ALTER SESSION SET TIME_ZONE = 'UTC'"
 
+    def current_timestamp(self) -> str:
+        return "LOCALTIMESTAMP"
+
 
 class Oracle(ThreadedDatabase):
     dialect = Dialect()

data_diff/sqeleton/queries/ast_classes.py

@@ -601,6 +601,9 @@ def compile(self, parent_c: Compiler) -> str:
 
         if self.table:
             select += " FROM " + c.compile(self.table)
+        elif c.dialect.PLACEHOLDER_TABLE:
+            select += f" FROM {c.dialect.PLACEHOLDER_TABLE}"
+
 
         if self.where_exprs:
             select += " WHERE " + " AND ".join(map(c.compile, self.where_exprs))