Yyunozor opened a new pull request, #2483:
URL: https://github.com/apache/age/pull/2483
A query that begins with `OPTIONAL MATCH` returns no rows when the pattern
matches nothing:
```
SELECT * FROM cypher('g', $$ OPTIONAL MATCH (n:NoSuchLabel) RETURN n $$) AS
(n agtype);
n
---
(0 rows)
```
Cypher gives every `OPTIONAL MATCH` left join semantics. With no clause to
its left it joins against the implicit single row that begins a query, so this
has to return one row with `n = NULL` — that is what separates `OPTIONAL MATCH`
from `MATCH`.
`transform_cypher_match_pattern` builds the null-preserving LATERAL LEFT
JOIN out of the previous clause, and falls through to the plain MATCH path when
there is none ("If there is no previous clause, transform to a general MATCH
clause"). A leading `OPTIONAL MATCH` has no previous clause, so it took the
plain path. Any preceding clause, however trivial, already produced the right
answer:
```
$$ WITH 1 AS x OPTIONAL MATCH (n:NoSuchLabel) RETURN n $$ -- 1 row, n =
NULL
$$ UNWIND [1] AS x OPTIONAL MATCH (n:NoSuchLabel) RETURN n $$ -- 1 row, n =
NULL
```
The join was already right; only its left side was missing. This gives a
leading `OPTIONAL MATCH` a previous clause that yields exactly one row, built
the way `transform_cypher_with` already builds a synthesized `cypher_return`.
Its lone column is named with the hidden variable prefix, so
`expand_pnsi_attrs` keeps it out of `RETURN *` expansion.
The clause is attached in `analyze_cypher` rather than in the transform. The
transform temporarily clears `prev` to recurse into the join's right side and
relies on `prev` being NULL there to take the plain MATCH path, so synthesizing
a `prev` inside it re-arms that recursion until the stack runs out.
Tests cover the no-match case for node and relationship patterns, a `WHERE`
that no binding survives, that a matching pattern gains no extra null row, that
`RETURN *` does not expose the internal column, that a leading `MATCH` still
returns no rows, and that `MATCH` following `OPTIONAL MATCH` still errors.
`make installcheck` passes 43/43 on this branch and on its parent; with only
the two test files applied to the parent, `cypher_match` fails.
Known limitation: a `UNION` branch beginning with `OPTIONAL MATCH` still
returns no rows. Branches are assembled through
`cypher_parse_sub_analyze_union`, not `analyze_cypher`, so they never reach
this path. Placing the same unit clause there would be a natural follow-up.
Closes #2473
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