[sqlalchemy-spanner]: _get_table_filter_query([]) generates invalid SQL '() AND ' when filter_names is empty #18838

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Summary of the issue

When filter_names is an empty collection ([] or ()), SpannerDialect._get_table_filter_query(filter_names, ...) returns "() AND ", causing get_multi_columns, get_multi_pk_constraint, get_multi_foreign_keys, and get_multi_indexes to execute invalid SQL (WHERE () AND ...).

Environment details

• OS: Linux
• Python version: 3.13
• sqlalchemy-spanner version: 1.17.2 (also reproduced on main / 1.20.1 at eb8a76628adf10093959600071a18f50285c2ecf)
• sqlalchemy version: 2.0.52

Problem description

In SpannerDialect._get_table_filter_query:

    def _get_table_filter_query(
        self, filter_names, info_schema_table, append_query=False
    ):
        table_filter_query = ""
        if filter_names is not None:
            for table_name in filter_names:
                query = f"{info_schema_table}.table_name = '{table_name}'"
                if table_filter_query != "":
                    table_filter_query = table_filter_query + " OR " + query
                else:
                    table_filter_query = query
            table_filter_query = "(" + table_filter_query + ") "
            if append_query:
                table_filter_query = table_filter_query + " AND "

        return table_filter_query

When filter_names is [] (not None, which means "reflect all tables", whereas [] means "reflect zero tables"), for table_name in filter_names: does not execute, and _get_table_filter_query([], "col", append_query=True) returns "() AND ".

In contrast, SQLAlchemy's get_multi_* contract returns {} when filter_names is an empty collection (e.g., when reflecting a schema with no matching tables).

Suggested fix

Either short-circuit and return {} in get_multi_* when filter_names is not None and not filter_names, or emit "FALSE " / "FALSE AND " in _get_table_filter_query when filter_names is empty (or bind @filter_names with IN UNNEST(@filter_names) as in #18205).

Reproduction steps: code

from google.cloud.sqlalchemy_spanner.sqlalchemy_spanner import SpannerDialect

dialect = SpannerDialect()
print(repr(dialect._get_table_filter_query([], "col", append_query=True)))

Actual output

'()  AND '

Expected output

Either get_multi_* short-circuits to {} on empty filter_names, or _get_table_filter_query([], ...) returns a valid SQL predicate such as 'FALSE AND '.