When using a foreign table (e.g. a mysql table) clickhouse doesn't seem to properly send the result of functions if they are called inside `WHERE` statements.

Executing `SELECT foo FROM foreign_table WHERE a=10 AND b='2019-10-05'` will send a statement like:

```
SELECT `foo`, `a`, `b` FROM `db`.`foreign_table`  WHERE (`a` = 10) AND (`b` = '2019-10-05')
```

But executing `SELECT foo FROM foreign_table WHERE a=10 AND b=toDate('2019-10-05')` will send:

```
SELECT `foo`, `a`, `b` FROM `db`.`foreign_table`  WHERE (`a` = 10)
```

I assume that the function is executed afterwards and `b` is parsed clickhouse-side.

I'm not sure if there is an easy fix for this, but I assume this would help with the efficiency of mysql foreign tables and might be critical in some high-load scenarios where one wants to filter on indexed columns.

My specific issue here is different and relates to the clickhouse mindsdb integration, but I think this "bug" (if you'd call it such) is more generic.
ContextMutablePtr context;
    static const State & instance()
    {
        static State state;
        return state;
    }
    const NamesAndTypesList & getColumns() const
    {
        return tables[0].columns;
    }
    std::vector<TableWithColumnNamesAndTypes> getTables(size_t num = 0) const
    {
        std::vector<TableWithColumnNamesAndTypes> res;
        for (size_t i = 0; i < std::min(num, tables.size()); ++i)
            res.push_back(tables[i]);
        return res;
    }
private:
    static DatabaseAndTableWithAlias createDBAndTable(String table_name)