[FEATURE] P0 Nested Function Support

[forestmvey]

Is your feature request related to a problem?
The nested query is supported in the legacy engine and not the V2 engine. As Legacy engine is becoming deprecated we need to support this functionality in the V2 engine.

Example:

SELECT nested(message.info) FROM nested_objects;
SELECT * FROM nested_objects WHERE nested(message.info)='b';
SELECT nested(message.info) FROM nested_objects ORDER BY nested(message.info);
SELECT nested(message.*) FROM nested_objects;

What solution would you like?
Port over functionality for the nested query to the V2 engine with the same functionality as in Legacy.

Release-Schedule

• P[0] SELECT clause support
  • P[0] DOT notation field support - Released: 1490
  • P[0] * support as inner field, Example: nestedField.innerFields.* - Released: 1773
• P[0] WHERE clause support; one syntax will be prioritized.
  • P[0] Syntax: nested(field | field, path) OPERATOR LITERAL - Released: 1657
• P[0] ORDER BY clause support - Released: 1789

What alternatives have you considered?
N/A

Do you have any additional context?
N/A

[MaxKsyunz]

Update 2023-04-25: Discussed this with Forest offline.

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@dai-chen I'd like to follow-up on this comment. As I understand it these two queries are to be equivalent

SELECT nested(message.info), name FROM table
SELECT m.info, t.name FROM table AS t, t.message AS m.

In particular, for table like

message	name
{info : 1 }	nameA
[{info : 2}, {info : 3}]	nameB

the result will be

message.info	name
1	nameA
2	nameB
3	nameB

What's the use case for the cartesian product behaviour? As an end-user, I'd expect SELECT <expression> FROM table to return SELECT COUNT(*) FROM table number of results. For UIs, complex relationships usually shown in a parent-child arrangement, so I'd expect to have deal with complex properties like message.info.

[forestmvey]

Demo video for nested function use in SELECT clause.

nested_select_clause_demo.mp4