The external table file path cache may out of date, which will cause orc reader to visit non-exist files.
In this case, orc file reader is nullptr.
This pr is to check the reader before using it to avoid core dump of visiting nullptr.
Read predicate columns firstly, and use VExprContext(push-down predicates)
to generate the select vector, which is then applied to read the non-predicate columns.
The data in non-predicate columns may be skipped by select vector, so the value-decode-time can be reduced.
If a whole page can be skipped, the decompress-time can also be reduced.
# Proposed changes
Implement predicate pushdown in `OrcReader` by converting doris `ColumnValueRange` to orc `SearchArgument`.
## Remaining problems
1. Orc support `not in`, which may have effect on bloom filter. However, doris `ScanNode` has not push down `not in` to file scanner.
2. Orc support `is null`, and row range has `hasNull` identifier. However, `_contain_null` in `ColumnValueRange` is ambiguous. `_contain_null = true` only means that the value can be nullable, not equal to null.
3. `DateTimeV2` has lost microsecond precision in `ColumnValueRange`, which may cause filtering error when a min-max value equals to the predicate value.
4. `DateTimeV1` is not accurate enough, and only saved to seconds.
5. Orc support the predicate pushdown of `float&double` type, but doris has not push down `float&double` type for precision reason.
