Harrison/sqlalchemy cache store (#536)

Co-authored-by: Jason Gill <jasongill@gmail.com>

docs/modules/llms/examples/llm_caching.ipynb

@@ -276,6 +276,52 @@
     "# langchain.llm_cache = SQLAlchemyCache(engine)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "source": [
+    "### Custom SQLAlchemy Schemas"
+   ],
+   "metadata": {
+    "collapsed": false
+   }
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "outputs": [],
+   "source": [
+    "# You can define your own declarative SQLAlchemyCache child class to customize the schema used for caching. For example, to support high-speed fulltext prompt indexing with Postgres, use:\n",
+    "\n",
+    "from sqlalchemy import Column, Integer, String, Computed, Index, Sequence\n",
+    "from sqlalchemy import create_engine\n",
+    "from sqlalchemy.ext.declarative import declarative_base\n",
+    "from sqlalchemy_utils import TSVectorType\n",
+    "from langchain.cache import SQLAlchemyCache\n",
+    "\n",
+    "Base = declarative_base()\n",
+    "\n",
+    "\n",
+    "class FulltextLLMCache(Base):  # type: ignore\n",
+    "    \"\"\"Postgres table for fulltext-indexed LLM Cache\"\"\"\n",
+    "\n",
+    "    __tablename__ = \"llm_cache_fulltext\"\n",
+    "    id = Column(Integer, Sequence('cache_id'), primary_key=True)\n",
+    "    prompt = Column(String, nullable=False)\n",
+    "    llm = Column(String, nullable=False)\n",
+    "    idx = Column(Integer)\n",
+    "    response = Column(String)\n",
+    "    prompt_tsv = Column(TSVectorType(), Computed(\"to_tsvector('english', llm || ' ' || prompt)\", persisted=True))\n",
+    "    __table_args__ = (\n",
+    "        Index(\"idx_fulltext_prompt_tsv\", prompt_tsv, postgresql_using=\"gin\"),\n",
+    "    )\n",
+    "\n",
+    "engine = create_engine(\"postgresql://postgres:postgres@localhost:5432/postgres\")\n",
+    "langchain.llm_cache = SQLAlchemyCache(engine, FulltextLLMCache)"
+   ],
+   "metadata": {
+    "collapsed": false
+   }
+  },
   {
    "cell_type": "markdown",
    "id": "0c69d84d",