added documentation on retrieving a PG vectorstore (#4578)

This PR adds in documentation on querying an existing vectorstore in PG 

Fixes 3191 (issue)

docs/modules/indexes/vectorstores/examples/pgvector.ipynb

@@ -222,6 +222,63 @@
     "    print(doc.page_content)\n",
     "    print(\"-\" * 80)\n"
    ]
+  },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Working with vectorstore in PG"
+   ]
+  },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Uploading a vectorstore in PG "
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "db = PGVector.from_documents(\n",
+    "    documents=data,\n",
+    "    embedding=embeddings,\n",
+    "    collection_name=collection_name,\n",
+    "    connection_string=connection_string,\n",
+    "    distance_strategy=DistanceStrategy.COSINE,\n",
+    "    openai_api_key=api_key,\n",
+    "    pre_delete_collection=False \n",
+    ")"
+   ]
+  },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Retrieving a vectorstore in PG"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "store = PGVector(\n",
+    "    connection_string=connection_string, \n",
+    "    embedding_function=embedding, \n",
+    "    collection_name=collection_name,\n",
+    "    distance_strategy=DistanceStrategy.COSINE\n",
+    ")\n",
+    "\n",
+    "retriever = store.as_retriever()"
+   ]
   }
  ],
  "metadata": {