Support for Distinct clause

[xinyee20]

Changes in this PR

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change of an existing feature
• [ ] Refactor/Performance enhancements

Overview

• Allow query to take in distinct keyword
• Add support for distinct clause (DistinctPlan and DistinctScan)
• Fixes #18

[xinyee20]

Test queries 1) select did from dept, course Projection result:

 did
----
 10
 10
 10
 10
 10
 10
 20
 20
 20
 20
 20
 20
 30
 30
 30
 30
 30
 30

Distinct result:

 did
----
 10
 20
 30

2) select distinct courseid from section Distinct result:

 courseid
---------
      12
      32
      62

3) select distinct sectid,courseid from section Distinct result:

 sectid courseid
----------------
    13      12
    23      12
    33      32
    43      32
    53      62