Enrich README.md for postgres usage, make some change to cater python version<12

examples/copy_postgres_llm_cache_to_json.py

@@ -0,0 +1,66 @@
+import asyncio
+import logging
+import os
+from dotenv import load_dotenv
+
+from lightrag.kg.postgres_impl import PostgreSQLDB, PGKVStorage
+from lightrag.storage import JsonKVStorage
+
+load_dotenv()
+ROOT_DIR = os.environ.get("ROOT_DIR")
+WORKING_DIR = f"{ROOT_DIR}/dickens-pg"
+
+logging.basicConfig(format="%(levelname)s:%(message)s", level=logging.INFO)
+
+if not os.path.exists(WORKING_DIR):
+    os.mkdir(WORKING_DIR)
+
+# AGE
+os.environ["AGE_GRAPH_NAME"] = "chinese"
+
+postgres_db = PostgreSQLDB(
+    config={
+        "host": "localhost",
+        "port": 15432,
+        "user": "rag",
+        "password": "rag",
+        "database": "r1",
+    }
+)
+
+
+async def main():
+    await postgres_db.initdb()
+
+    from_llm_response_cache = PGKVStorage(
+        namespace="llm_response_cache",
+        global_config={"embedding_batch_num": 6},
+        embedding_func=None,
+        db=postgres_db,
+    )
+
+    to_llm_response_cache = JsonKVStorage(
+        namespace="llm_response_cache",
+        global_config={"working_dir": WORKING_DIR},
+        embedding_func=None,
+    )
+
+    kv = {}
+    for c_id in await from_llm_response_cache.all_keys():
+        print(f"Copying {c_id}")
+        workspace = c_id["workspace"]
+        mode = c_id["mode"]
+        _id = c_id["id"]
+        postgres_db.workspace = workspace
+        obj = await from_llm_response_cache.get_by_mode_and_id(mode, _id)
+        if mode not in kv:
+            kv[mode] = {}
+        kv[mode][_id] = obj[_id]
+        print(f"Object {obj}")
+    await to_llm_response_cache.upsert(kv)
+    await to_llm_response_cache.index_done_callback()
+    print("Mission accomplished!")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())