Add enum representation notes

Author: pyramation

AST_RESEARCH.md

@@ -72,6 +72,18 @@ These changes indicate incremental evolution in the ASTs, with PG16 introducing
 | `varnosyn` & `varattnosyn` | `varnullingrels` | Var | PG16 |
 | `aggtranstype` | `aggtransno` | Aggref | PG16 |
 
+### Enum representation changes
+
+Historically libpg_query exposed enum fields in the JSON output as **numeric**
+codes. Starting with the PG15 bindings this switched to returning the **string**
+name of each enum value. The TypeScript type definitions reflect string literal
+unions across all versions, but the underlying JSON changed in PG15.
+
+| Version | Enum format |
+|---------|-------------|
+| 13–14   | integers    |
+| 15–17   | strings     |
+
 
 ## Version similarity
 

AST_TRANSLATION.md

@@ -38,6 +38,14 @@ Trade-offs:
 - Performance hit due to serializing and parsing again
 - Potential loss of fidelity if certain node properties are not round-trippable
 
+### Handling enums
+
+Older versions of `libpg_query` (PG13 and PG14) emitted numeric codes for enum
+fields. From PG15 onward the JSON output uses the enum **name** as a string.
+Translation code must therefore convert numeric enums to their string
+equivalents when upgrading from PG13/14. When moving between PG15–17 the
+representations already match.
+
 ## Translation step ordering
 
 ### Sequential upgrades