SQL: ALTER TABLE ADD CONSTRAINT FK does not emit reference edge

[Widthdom]

Summary

SQL reference extraction is asymmetric: CREATE TABLE … REFERENCES other_table(col) correctly emits a reference edge from the column to the referenced table, but ALTER TABLE … ADD CONSTRAINT … FOREIGN KEY (…) REFERENCES other_table(col) does not. Both forms are common in migration scripts; an ALTER-style foreign key should be tracked the same way.

Evidence

• src/CodeIndex/Indexer/Extraction/LanguageReferenceExtraction.Sql.cs — the REFERENCES clause handler is wired into the CREATE TABLE parser but not the ALTER TABLE parser.

• Reproducer:

  CREATE TABLE orders (id int PRIMARY KEY);
  CREATE TABLE order_items (order_id int);
  ALTER TABLE order_items ADD CONSTRAINT fk_order
    FOREIGN KEY (order_id) REFERENCES orders(id);

  After indexing, references on the orders table will miss the ALTER-created edge.

Impact

• Schemas managed by migration tools (Flyway, Liquibase, golang-migrate) typically use ALTER TABLE ADD CONSTRAINT instead of inline REFERENCES. The asymmetry means impact_analysis on table dependencies is incomplete for those projects.

Proposed direction

• Lift the REFERENCES clause parser into a shared helper used by both CREATE TABLE and ALTER TABLE … ADD CONSTRAINT.
• Add a unit test covering both forms.

Repro env

• Branch: main @ 2ee912d (release v1.21.0)

[Widthdom]

Work started on branch codex/fix-issue1443.