Add secondary indexes to accelerate cache lookups

README.md

@@ -218,6 +218,26 @@ engine = CachingEngine(
 engine.refresh()   # also reloads any expired TTL tables on demand
 ```
 
+## Secondary indexes
+
+To accelerate lookups, you can declare **secondary indexes** per table — they are created on the in-memory SQLite copy so `WHERE`/`JOIN` filters on those columns run as indexed searches instead of full scans:
+
+```python
+engine = CachingEngine(
+    base_engine,
+    indexes={
+        "VW_P_PRATVALUES": ["PRODUCT_PRODUCTNR"],   # single-column index
+        "VW_ELEMENTS": [["ELEMENT_ID", "ELEMENTVARIANT_ID"], "ELEMENTVARIANT_NAME"],
+    },
+)
+```
+
+Each value is a list of index definitions: a string is a single-column index, a nested list is a composite index.
+
+- **Index columns are pulled into the cache automatically** (like delta key columns), so the index exists from the first load even if your queries don't select those columns.
+- Indexes are **recreated after every (re)load** — full loads, TTL reloads, and `invalidate()` + re-fetch all rebuild them — so they're always present, and they persist in `cache.db` across restarts.
+- Delta-tracked tables already get a unique index on their key columns; secondary indexes are independent and can be combined with `delta` or `ttl`.
+
 ## Persistence
 
 The in-memory cache is persisted to `cache.db` on disk: