perf: pre-filter autocomplete candidates with regex prefix query

src/documents/search/_backend.py

@@ -605,11 +605,26 @@ class TantivyBackend:
         # Apply permission filter for non-superusers so autocomplete words
         # from invisible documents don't leak to other users.
         if user is not None and not user.is_superuser:
-            base_query = build_permission_filter(self._schema, user)
+            permission_query = build_permission_filter(self._schema, user)
         else:
-            base_query = tantivy.Query.all_query()
+            permission_query = tantivy.Query.all_query()
 
-        results = searcher.search(base_query, limit=10000)
+        # Narrow to documents that actually contain a word starting with the
+        # prefix before loading stored fields, avoiding a full collection scan.
+        prefix_filter = tantivy.Query.regex_query(
+            self._schema,
+            "autocomplete_word",
+            regex.escape(normalized_term) + ".*",
+        )
+        results = searcher.search(
+            tantivy.Query.boolean_query(
+                [
+                    (tantivy.Occur.Must, permission_query),
+                    (tantivy.Occur.Must, prefix_filter),
+                ],
+            ),
+            limit=10000,
+        )
 
         # Count how many visible documents each word appears in.
         # Using Counter (not set) preserves per-word document frequency so

src/documents/search/_schema.py

@@ -53,7 +53,7 @@ def build_schema() -> tantivy.Schema:
     # CJK support - not stored, indexed only
     sb.add_text_field("bigram_content", stored=False, tokenizer_name="bigram_analyzer")
 
-    # Autocomplete prefix scan - stored, not indexed
+    # Autocomplete prefix scan - stored, indexed by default
     sb.add_text_field("autocomplete_word", stored=True, tokenizer_name="raw")
 
     sb.add_text_field("tag", stored=True, tokenizer_name="paperless_text")