gregfelice opened a new pull request, #2347:
URL: https://github.com/apache/age/pull/2347
## Summary
Implements the openCypher-standard `ON CREATE SET` and `ON MATCH SET`
clauses for the MERGE statement, resolving #1619. This allows conditional
property updates depending on whether MERGE created a new path or matched an
existing one:
```cypher
MERGE (n:Person {name: 'Alice'})
ON CREATE SET n.created = timestamp()
ON MATCH SET n.updated = timestamp()
```
## Design
The implementation spans parser, planner, and executor:
**Parser** — New grammar rules (`merge_actions_opt`, `merge_actions`,
`merge_action`) in `cypher_gram.y`. The `ON` keyword is added to
`cypher_kwlist.h`.
**Nodes** — `on_match` / `on_create` lists on `cypher_merge`, corresponding
`on_match_set_info` / `on_create_set_info` on `cypher_merge_information`, and
`prop_expr` on `cypher_update_item`. All fields serialized through
copy/out/read funcs.
**Transform** — `cypher_clause.c` transforms ON SET items and stores
`prop_expr` for direct expression evaluation.
**Executor** — `apply_update_list()` is extracted from
`process_update_list()` in `cypher_set.c` as reusable SET logic.
`cypher_merge.c` calls it at all merge decision points:
- Simple merge (MERGE as only clause)
- Terminal non-first clause
- Non-terminal with eager buffering (compatible with #2344)
- First-clause-with-followers (Case 3 sub-cases)
### Why prop_expr?
The PostgreSQL planner strips target list entries for SET expressions that
the CustomScan doesn't reference. This makes `prop_position` references into
the scan tuple dangling. The solution: store the `Expr*` directly in
`cypher_update_item->prop_expr` and evaluate it with `ExecInitExpr` /
`ExecEvalExpr`, independent of scan tuple layout. This is only done for MERGE
ON SET items — regular SET continues to use `prop_position` unchanged.
## Files changed (12)
| File | Changes |
|------|---------|
| `src/include/parser/cypher_kwlist.h` | Added `ON` keyword |
| `src/backend/parser/cypher_gram.y` | Grammar rules for merge actions |
| `src/include/nodes/cypher_nodes.h` | Node struct fields for
on_match/on_create |
| `src/backend/nodes/cypher_copyfuncs.c` | Serialization for new fields |
| `src/backend/nodes/cypher_outfuncs.c` | Serialization for new fields |
| `src/backend/nodes/cypher_readfuncs.c` | Deserialization for new fields |
| `src/backend/parser/cypher_clause.c` | Transform ON MATCH/CREATE SET items
|
| `src/include/executor/cypher_utils.h` | State fields + `apply_update_list`
declaration |
| `src/backend/executor/cypher_set.c` | Extracted `apply_update_list()` from
`process_update_list()` |
| `src/backend/executor/cypher_merge.c` | Wired `apply_update_list` at all
merge decision points |
| `regress/sql/cypher_merge.sql` | Regression tests |
| `regress/expected/cypher_merge.out` | Expected output |
## Test plan
- [x] All 31 existing regression tests pass (no regressions)
- [x] New tests cover: basic ON CREATE SET, basic ON MATCH SET, combined ON
CREATE + ON MATCH, multiple SET items in a single clause, expression
evaluation, interaction with WITH clause, edge property updates
- [x] Verified across all merge execution paths including non-terminal eager
buffering (#2344)
- [x] Backward compatible — existing MERGE queries without ON SET clauses
are unaffected
Closes #1619
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