PlannerCompiler w/ SELECT DISTINCT and ORDER BY returns a bag

[alancai98]

Description

PlannerCompiler returns a bag rather than a list like the EvaluatingCompiler currently does.

To Reproduce

Steps to reproduce the behavior:

-- `PlanCompiler` returns a bag
PartiQL> SELECT DISTINCT a FROM <<{'a': 1}, {'a': 2}, {'a': 1}>> AS t ORDER BY t.a;
==='
<<
  {
    'a': 1
  },
  {
    'a': 2
  }
>>

-- `EvaluatingCompiler` returns a list
PartiQL> SELECT DISTINCT a FROM <<{'a': 1}, {'a': 2}, {'a': 1}>> AS t ORDER BY t.a;
==='
[
  {
    'a': 1
  },
  {
    'a': 2
  }
]

Expected Behavior

Expect both compilers to return the same output type (i.e. list).

Additional Context

• Java version: 11
• PartiQL version: 0.13.3

[alancai98]

Some investigation

The PlanCompiler performs the DISTINCT call on the output of the projection (source), which will call the builtin scalar function filter_distinct. Currently, filter_distinct will always return a bag: https://github.com/partiql/partiql-lang-kotlin/blob/main/partiql-lang/src/main/kotlin/org/partiql/lang/eval/builtins/ScalarBuiltinsExt.kt#L124-L138.

We could potentially modify filter_distinct to return a list if we can guarantee that the order of the input list can be preserved.