Improve indexing performance for Postgres (remote database) for refresh (#14126)

**Description:** By combining the document timestamp refresh within a
single call to update(), this enables batching of multiple documents in
a single SQL statement. This is important for non-local databases where
tens of milliseconds has a huge impact on performance when doing
document-by-document SQL statements.
**Issue:** #11935 
**Dependencies:** None
**Tag maintainer:** @eyurtsev

libs/langchain/langchain/indexes/_api.py

@@ -303,15 +303,19 @@ def index(
         # Filter out documents that already exist in the record store.
         uids = []
         docs_to_index = []
+        docs_to_update = []
         for hashed_doc, doc_exists in zip(hashed_docs, exists_batch):
             if doc_exists:
-                # Must be updated to refresh timestamp.
-                record_manager.update([hashed_doc.uid], time_at_least=index_start_dt)
-                num_skipped += 1
+                docs_to_update.append(hashed_doc.uid)
                 continue
             uids.append(hashed_doc.uid)
             docs_to_index.append(hashed_doc.to_document())
 
+        # Update refresh timestamp
+        if docs_to_update:
+            record_manager.update(docs_to_update, time_at_least=index_start_dt)
+            num_skipped += len(docs_to_update)
+
         # Be pessimistic and assume that all vector store write will fail.
         # First write to vector store
         if docs_to_index: