public static SqlOperator convert(final CombineType combineType) {
        Preconditions.checkState(REGISTRY.containsKey(combineType), "Unsupported combine type: `%s`", combineType);
        return REGISTRY.get(combineType);
    }
@Test
    void assertConvertSuccess() {
        assertThat(CombineOperatorConverter.convert(CombineType.UNION_ALL), is(SqlStdOperatorTable.UNION_ALL));
        assertThat(CombineOperatorConverter.convert(CombineType.UNION), is(SqlStdOperatorTable.UNION));
        assertThat(CombineOperatorConverter.convert(CombineType.INTERSECT_ALL), is(SqlStdOperatorTable.INTERSECT_ALL));
        assertThat(CombineOperatorConverter.convert(CombineType.INTERSECT), is(SqlStdOperatorTable.INTERSECT));
        assertThat(CombineOperatorConverter.convert(CombineType.EXCEPT_ALL), is(SqlStdOperatorTable.EXCEPT_ALL));
        assertThat(CombineOperatorConverter.convert(CombineType.EXCEPT), is(SqlStdOperatorTable.EXCEPT));
        assertThat(CombineOperatorConverter.convert(CombineType.MINUS_ALL), is(SqlStdOperatorTable.EXCEPT_ALL));
        assertThat(CombineOperatorConverter.convert(CombineType.MINUS), is(SqlStdOperatorTable.EXCEPT));
    }