feat(checkpoint-postgres): expose prepare_threshold in PostgresSavers to support connection poolers#6741
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Description
This PR exposes the prepare_threshold parameter in PostgresSaver, AsyncPostgresSaver, and their shallow versions within the checkpoint-postgres package.
The Problem: Currently, prepare_threshold=0 is hardcoded in the from_conn_string methods. In psycopg (v3), a threshold of 0 forces the immediate creation of prepared statements. When LangGraph is deployed in environments using connection poolers in transaction mode (such as PgBouncer or Google Cloud SQL connection pooler), physical connections are often swapped between statements. This leads to the following error:
prepared statement "_pg3_3" does not exist
The Solution: By exposing prepare_threshold as an optional parameter, users can now set it to None. This disables prepared statements, ensuring stable operation in Serverless or pooled database architectures without sacrificing the default performance for standard persistent connections.
Key Changes:
Added prepare_threshold: int | None = 0 to from_conn_string across all four Postgres saver classes.
Passed the parameter directly to the underlying psycopg.connect and AsyncConnection.connect calls.
Included non-intrusive unit tests using unittest.mock to verify parameter pass-through without requiring a live database or triggering Windows-specific driver issues.
Issue
Fixes #6705
Dependencies
None.
Twitter handle
@ji_weiqi29149