jrgemignani opened a new pull request, #2448:
URL: https://github.com/apache/age/pull/2448
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 <[email protected]>
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
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