This method overrides django.db.models.expressions.CombinedExpression.as_sql.
Different backends can provide their own implementation, by
providing an `as_{vendor}` method and patching the Expression:
```
def override_as_sql(self, compiler, connection):
# custom logic
return super().as_sql(compiler, connection)
setattr(Expression, 'as_' + connection.vendor, override_as_sql)
```
Arguments:
* compiler: the query compiler responsible for generating the query.
Must have a compile method, returning a (sql, [params]) tuple.
Calling compiler(value) will return a quoted `value`.
* connection: the database connection used for the current query.
Return: (sql, params)
Where `sql` is a string containing ordered sql parameters to be
replaced with the elements of the list `params`.