Support outer references in reduce() fold bodies

[jrgemignani]

Allow a reduce(acc = init, var IN list | body) fold body to reference loop-invariant values from the enclosing query -- outer-query variables and cypher() parameters -- in addition to the accumulator and element. These were previously rejected with ERRCODE_FEATURE_NOT_SUPPORTED.

How it works

The fold body is still compiled to a standalone expression evaluated by age_reduce_transfn, so an outer reference (which cannot be evaluated there) is captured at transform time and supplied as a value:

• After the accumulator and element are rewritten to PARAM_EXEC params 0 and 1, transform_cypher_reduce() walks the body and replaces each maximal agtype-typed, loop-invariant subtree -- one that references an outer Var or a cypher() $parameter but not the accumulator/element -- with a new PARAM_EXEC param 2, 3, ... in body order.
• The captured expressions are passed to the aggregate as a trailing agtype[] argument; age_reduce(agtype, text, agtype, agtype[]) and its transition function gain this argument.
• age_reduce_transfn sizes its param array to 2 + the number of captures and binds the captured values to params 2.. on every row. Because the captures are evaluated in the outer query context as ordinary aggregate arguments, a correlated capture is re-evaluated per group, so an outer value that varies per row (for example under UNWIND) is folded with the correct value.

This keeps the no-core-patch design: the body is still a serialized standalone expression, and the only new machinery is the captured-value plumbing.

Still rejected

Subqueries in the body (including a nested reduce()) and aggregate functions remain unsupported and raise a clean ERRCODE_FEATURE_NOT_SUPPORTED error: a subquery cannot be planned as a plain aggregate argument, and an aggregate in a per-element fold is undefined per the openCypher specification.

Tests

age_reduce gains an "Outer references in the fold body" section covering a plain outer variable, an outer variable used as a multiplier, two distinct outer variables, a property of an outer graph variable, the same outer variable referenced more than once, a property of an outer map, a subexpression that mixes an outer reference with the element (only the loop-invariant part is captured), an outer reference inside a CASE branch of the body, a NULL outer value propagating through the fold, multiple captures mixing a NULL and a non-NULL outer value, an outer variable that changes per row (captured per group), and a cypher() parameter supplied via a prepared statement. The previously-rejected outer-variable case is moved out of the not-supported section, which now covers a nested reduce() (any subquery in the body is unsupported) and an aggregate in the body.

The same change also broadens the base reduce() coverage with value-type folds (a float accumulator, negative numbers, a map accumulator passed through unchanged, and list elements indexed in the body), function calls in the fold body (a scalar function over the element and the list itself produced by a function), reduce() composed with surrounding expressions (consumed by another function and used in a comparison), and syntax-error checks for each required piece of the form -- the "= init", ", var IN list", and "| body" clauses, plus a rejected qualified iterator variable. 42/42 installcheck pass.

Co-authored-by: Copilot copilot@github.com

modified: age--1.7.0--y.y.y.sql
new file: regress/age_load/data/junk.csv
modified: regress/expected/age_reduce.out
modified: regress/sql/age_reduce.sql
modified: sql/age_aggregate.sql
modified: src/backend/parser/cypher_clause.c
modified: src/backend/utils/adt/agtype.c

[Copilot]

Pull request overview

This PR extends Cypher reduce(acc = init, var IN list | body) so the fold body can reference loop-invariant values from the enclosing query (outer-query variables and cypher() parameters) by capturing those outer references at transform time and plumbing them into the fold evaluator as additional PARAM_EXEC slots.

Changes:

• Capture loop-invariant outer-reference subtrees in transform_cypher_reduce() as PARAM_EXEC params 2..N and pass their values via a new trailing agtype[] “extras” aggregate argument.
• Update the age_reduce aggregate / age_reduce_transfn signature to accept the new agtype[] extras argument and bind captured values during transition evaluation.
• Expand regression coverage for outer references, value-type folds, function usage, composition, and syntax errors.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/backend/utils/adt/agtype.c	Extends age_reduce_transfn to size/bind additional PARAM_EXEC slots for captured outer values.
src/backend/parser/cypher_clause.c	Implements capture + rewrite of loop-invariant outer references into PARAM_EXEC params 2.. and builds the extras array argument.
sql/age_aggregate.sql	Updates extension SQL definitions for age_reduce_transfn and age_reduce to include agtype[] extras.
age--1.7.0--y.y.y.sql	Mirrors the aggregate/function signature changes in the extension install/upgrade SQL.
regress/sql/age_reduce.sql	Adds/updates regression queries covering outer references and additional reduce() behaviors.
regress/expected/age_reduce.out	Updates expected output to match the new regression cases and new supported behavior.
regress/age_load/data/junk.csv	Adds a new CSV fixture file (currently appears unrelated to the reduce() feature).

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[jrgemignani]

Resolved all 3 Copilot issues.