[main/v1] partiql-ast: modeling of `s_f_w.set_op` does not allow for left associative set operations

[alancai98]

Description

• Currently in the main and v1 branches, the ast's modeling of set_op.s_f_w does not allow for defining left associative set operations (UNION, EXCEPT, INTERSECT).

Current modeling on main/v1:

partiql-lang-kotlin/partiql-ast/src/main/resources/partiql_ast.ion

Lines 494 to 510 in edef7fb

	// The PartiQL `<sfw>` query expression, think SQL `<query specification>`
	s_f_w::{
	select: select, // oneof SELECT / SELECT VALUE / PIVOT
	exclude: optional::exclude,
	from: from,
	let: optional::let,
	where: optional::expr,
	group_by: optional::group_by,
	having: optional::expr,
	set_op: optional::{
	type: '.set_op',
	operand: '.expr.s_f_w',
	},
	order_by: optional::order_by,
	limit: optional::expr,
	offset: optional::expr,
	},

Assuming SFW_1, SFW_2, SFW_3 represent 3 different SFW queries. Then the following cannot be modeled in the current AST using set_op.s_f_w:

(SFW_1 EXCEPT SFW_2) EXCEPT SFW_3

Set operations are by default left-associative, so SFW_1 <set_op> SFW_2 <set_op> SFW_3 is also not representable.

To Reproduce

Steps to reproduce the behavior:

1. Attempt to create the above query using the AST

Expected Behavior

• Allow users + library owner to create a left associative set operation

Additional Context

• Java version: 17
• PartiQL version: 0.14.5 and later + v1 branch
• Add any other context about the problem here.

[alancai98]

From prior discussion, the modeling of SQL set ops needs to also support LIMIT, OFFSET, and ORDER BY, which is not currently supported in the parser.

For example the following does not currently parse:

PartiQL> (SELECT a FROM <<{'a': 1}>> LIMIT 1 UNION SELECT a FROM <<{'a': 2}>> LIMIT 1) LIMIT 1
   | !!
Parser Error: at line 1, column 79: unexpected token found, LIMIT : LIMIT
mismatched input 'LIMIT' expecting {<EOF>, ';'}

In the AST, we can consider a modeling more similar to what partiql-lang-rust uses:
https://github.com/partiql/partiql-lang-rust/blob/c91c5a4727f65523dea55fbdd9b9530d1df19d63/partiql-ast/src/ast.rs#L270-L278.

Alternatively, we can look at what another SQL parsing crate uses: https://docs.rs/sqlparser/0.48.0/sqlparser/ast/enum.SetExpr.html.

[alancai98]

Closed by #1538.