Predicate pushdown support

Predicate structure

Currently, VAST Database allows pushing down filter predicate p of the following logical structure:

An AND between multiple column predicates: p = p1 & p2 & p3
Within each column predicate pi, an OR between range predicates: ci = r1 | r2 | r3 | r4
Each range predicate rj is one of the following operators (where "x" is a column name, and c is a constant):
- comparison operator:
  - t["x"] < c
  - t["x"] <= c
  - t["x"] == c
  - t["x"] > c
  - t["x"] >= c
  - t["x"] != c
- is_in operator:
  - t["x"].isin([c1, c2, c3, c4, c5])
- NULL checking:
  - t["x"].isnull()
  - ~t["x"].isnull()
- Prefix match:
  - t["x"].startswith("prefix")
- Substring match:
  - t["x"].contains("substr")

The following Arrow types support predicate pushdown: