Add MERGE ON CREATE SET / ON MATCH SET support (#1619)

[gregfelice]

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:

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 Fix chained MERGE not seeing sibling MERGE's changes (#1446) #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

• All 31 existing regression tests pass (no regressions)
• 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
• Verified across all merge execution paths including non-terminal eager buffering (Fix chained MERGE not seeing sibling MERGE's changes (#1446) #2344)
• Backward compatible — existing MERGE queries without ON SET clauses are unaffected

Closes #1619