Feature: Rel tables with only forward directed storage

[ray6080]

Description

Currently, when we create rel tables, we always store rel tuples in a duplicated way that each tuple is stored in both forward and backward directed storage. This is to allow the flexibility of planner to pick plans that scan from either forward or backward directions. The downsides are: 1) storage space overheads; 2) copy/insert/update/delete overheads.

There are cases where only scan from forward or backward is needed, thus we don't need to keep the storage duplicated for both directions:

1. advanced users are aware that the rel tables can always be scanned in one direction;
2. full text search index always scan the rel tables from one direction.

Syntax changes

TBD

Planner and storage changes

TBD