Fix(snowflake): Handle form of CONVERT_TIMEZONE with a source TZ (tob…

…ymao#1598)

* Fix(snowflake): Handle form of CONVERT_TIMEZONE with a source TZ

Snowflake's [CONVERT_TIMEZONE](https://docs.snowflake.com/en/sql-reference/functions/convert_timezone) allows optionally specifying a source timezone for columns without timezone information. This currently causes incorrect SQL to be generated when parsed and re-rendered.

* Address code review comments

sqlglot/dialects/snowflake.py

@@ -143,6 +143,12 @@ def _datatype_sql(self: generator.Generator, expression: exp.DataType) -> str:
     return self.datatype_sql(expression)
 
 
+def _parse_convert_timezone(args: t.Sequence) -> exp.Expression:
+    if len(args) == 3:
+        return exp.Anonymous(this="CONVERT_TIMEZONE", expressions=args)
+    return exp.AtTimeZone(this=seq_get(args, 1), zone=seq_get(args, 0))
+
+
 class Snowflake(Dialect):
     null_ordering = "nulls_are_large"
     time_format = "'yyyy-mm-dd hh24:mi:ss'"
@@ -184,10 +190,7 @@ class Parser(parser.Parser):
             "ARRAYAGG": exp.ArrayAgg.from_arg_list,
             "ARRAY_CONSTRUCT": exp.Array.from_arg_list,
             "ARRAY_TO_STRING": exp.ArrayJoin.from_arg_list,
-            "CONVERT_TIMEZONE": lambda args: exp.AtTimeZone(
-                this=seq_get(args, 1),
-                zone=seq_get(args, 0),
-            ),
+            "CONVERT_TIMEZONE": _parse_convert_timezone,
             "DATE_TRUNC": date_trunc_to_time,
             "DATEADD": lambda args: exp.DateAdd(
                 this=seq_get(args, 2),

tests/dialects/test_snowflake.py

@@ -25,6 +25,7 @@ def test_snowflake(self):
             'COPY INTO NEW_TABLE ("foo", "bar") FROM (SELECT $1, $2, $3, $4 FROM @%old_table)'
         )
         self.validate_identity("COMMENT IF EXISTS ON TABLE foo IS 'bar'")
+        self.validate_identity("SELECT CONVERT_TIMEZONE('UTC', 'America/Los_Angeles', col)")
 
         self.validate_all(
             "SELECT i, p, o FROM qt QUALIFY ROW_NUMBER() OVER (PARTITION BY p ORDER BY o) = 1",