This PR introduce a new enum type `PushDownType`:
```
enum class PushDownType {
// The predicate can not be pushed down to data source
UNACCEPTABLE,
// The predicate can be pushed down to data source
// and the data source can fully evaludate it
ACCEPTABLE,
// The predicate can be pushed down to data source
// but the data source can not fully evaluate it.
PARTIAL_ACCEPTABLE
};
```
And derived class of VScanNode can override following method to determine whether to accept
a bianry/in/bloom filter/is null predicate:
```
PushDownType _should_push_down_binary_predicate();
PushDownType _should_push_down_in_predicate();
PushDownType _should_push_down_function_filter();
PushDownType _should_push_down_bloom_filter();
PushDownType _should_push_down_is_null_predicate();
```
8a274d7851