fix(snowflake): do not quote parameter names in ALTER SESSION

sqlmesh/core/dialect.py

@@ -569,6 +569,25 @@ def _parse_alter_table_alter(self: Parser) -> t.Optional[exp.Expr]:
     return alter_column
 
 
+# Snowflake session parameter names in ALTER SESSION SET / UNSET (e.g. TIMEZONE or QUERY_TAG) are
+# keywords, not identifiers, so Snowflake rejects them when quoted: ALTER SESSION SET "TIMEZONE" = 'UTC'
+# is a syntax error. sqlglot parses them as identifiers, which means they get normalized and quoted
+# along with the rest of the statement, so we turn them into vars instead. Vars are also left out of
+# case normalization, so a parameter written as query_tag stays lowercase; Snowflake treats parameter
+# names case-insensitively, so this is intentional.
+def _parse_alter_session(self: Snowflake.Parser) -> exp.AlterSession:
+    alter_session = self.__parse_alter_session()  # type: ignore
+
+    for item in alter_session.expressions:
+        name = item.this.this if isinstance(item.this, exp.EQ) else item.this
+        if isinstance(name, exp.Column) and not name.table:
+            name = name.this
+        if isinstance(name, exp.Identifier) and not name.quoted:
+            name.replace(exp.var(name.name))
+
+    return alter_session
+
+
 def altercolumn_sql(self: Generator, expression: exp.AlterColumn) -> str:
     sql = self._altercolumn_sql(expression)  # type: ignore
 
@@ -1430,6 +1449,7 @@ def extend_sqlglot() -> None:
     _override(Parser, _parse_interval_span)
     _override(Parser, _warn_unsupported)
     _override(Snowflake.Parser, _parse_table_parts)
+    _override(Snowflake.Parser, _parse_alter_session)
     _override(TSQL.Parser, _parse_alter_table_alter)
     _override(TSQL.Generator, altercolumn_sql)
 

tests/core/engine_adapter/test_snowflake.py

@@ -205,6 +205,18 @@ def test_session(
         assert to_sql_calls(adapter) == expected_exception_calls
 
 
+def test_alter_session_parameters_are_not_quoted(make_mocked_engine_adapter: t.Callable):
+    adapter = make_mocked_engine_adapter(SnowflakeEngineAdapter)
+
+    adapter.execute(parse_one("ALTER SESSION SET TIMEZONE = 'UTC'", read="snowflake"))
+    adapter.execute(parse_one("ALTER SESSION UNSET TIMEZONE", read="snowflake"))
+
+    assert to_sql_calls(adapter) == [
+        "ALTER SESSION SET TIMEZONE = 'UTC'",
+        "ALTER SESSION UNSET TIMEZONE",
+    ]
+
+
 def test_get_table_last_modified_ts_quoted_and_unquoted_catalogs(
     snowflake_mocked_engine_adapter: SnowflakeEngineAdapter,
 ) -> None:

tests/core/test_dialect.py

@@ -1395,6 +1395,41 @@ def test_tsql_alter_column_nullability():
     )
 
 
+def test_snowflake_alter_session_parameters_are_not_quoted():
+    # Issue #5666: session parameter names are not identifiers, so Snowflake rejects them when
+    # quoted, e.g. ALTER SESSION SET "TIMEZONE" = 'UTC' is a syntax error.
+    for sql, expected in [
+        (
+            "ALTER SESSION SET TIMEZONE = 'UTC', query_tag = @tag",
+            "ALTER SESSION SET TIMEZONE = 'UTC', query_tag = @tag",
+        ),
+        ("ALTER SESSION UNSET TIMEZONE, query_tag", "ALTER SESSION UNSET TIMEZONE, query_tag"),
+        # Explicit quoting is left as written
+        ("ALTER SESSION SET \"TIMEZONE\" = 'UTC'", "ALTER SESSION SET \"TIMEZONE\" = 'UTC'"),
+    ]:
+        assert parse_one(sql, read="snowflake").sql(dialect="snowflake", identify=True) == expected
+
+    model = load_sql_based_model(
+        parse(
+            """
+            MODEL (name db.t, kind FULL, dialect snowflake);
+
+            ALTER SESSION SET TIMEZONE = 'UTC';
+
+            SELECT 1 AS a;
+
+            ALTER SESSION UNSET TIMEZONE;
+            """
+        )
+    )
+    assert [s.sql(dialect="snowflake") for s in model.render_pre_statements()] == [
+        "ALTER SESSION SET TIMEZONE = 'UTC'"
+    ]
+    assert [s.sql(dialect="snowflake") for s in model.render_post_statements()] == [
+        "ALTER SESSION UNSET TIMEZONE"
+    ]
+
+
 def test_model_name_cannot_be_string():
     with pytest.raises(ParseError) as parse_error:
         parse(