Add secondary indexes to accelerate cache lookups

src/sqlmem/executor.py

@@ -18,6 +18,7 @@ class QueryExecutor:
         stats: StatsCollector,
         delta: dict[str, ResolvedDelta] | None = None,
         ttl: dict[str, int] | None = None,
+        index_columns: dict[str, list[str]] | None = None,
     ) -> None:
         self._cache = cache
         self._registry = registry
@@ -25,6 +26,7 @@ class QueryExecutor:
         self._stats = stats
         self._delta = delta or {}
         self._ttl = ttl or {}
+        self._index_columns = index_columns or {}
 
     def _ttl_expired(self, table: str) -> bool:
         """True if *table* has a TTL and its cached copy is older than that TTL."""
@@ -96,12 +98,15 @@ class QueryExecutor:
         self._load(table, all_columns, full=full)
 
     def _load(self, table: str, columns: list[str], full: bool) -> None:
-        """Fetch *table* into cache, adding delta key/timestamp columns when tracked."""
+        """Fetch *table* into cache, adding delta key/timestamp and index columns."""
         cfg = self._delta.get(table)
+        extra = list(self._index_columns.get(table, []))
         if cfg:
             # The cache must always hold the key (to upsert) and the change column
             # (to compute the watermark), even if no query referenced them.
-            columns = list(dict.fromkeys([*columns, *cfg.key_columns, cfg.change_column]))
+            extra += [*cfg.key_columns, cfg.change_column]
+        if extra:
+            columns = list(dict.fromkeys([*columns, *extra]))
 
         self._cache.load_table(table, columns, self._source_conn, full=full)
         self._registry.update(table, columns)