feat: acquire advisory lock before creating extension in pgvector (#12935)

- **Description:** Acquire advisory lock before attempting to create
extension on postgres server, preventing errors in concurrent
executions.
  - **Issue:** #12933
  - **Dependencies:** None

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/langchain/langchain/vectorstores/pgvector.py

@@ -159,7 +159,17 @@ class PGVector(VectorStore):
     def create_vector_extension(self) -> None:
         try:
             with Session(self._conn) as session:
-                statement = sqlalchemy.text("CREATE EXTENSION IF NOT EXISTS vector")
+                # The advisor lock fixes issue arising from concurrent
+                # creation of the vector extension.
+                # https://github.com/langchain-ai/langchain/issues/12933
+                # For more information see:
+                # https://www.postgresql.org/docs/16/explicit-locking.html#ADVISORY-LOCKS
+                statement = sqlalchemy.text(
+                    "BEGIN;"
+                    "SELECT pg_advisory_xact_lock(1573678846307946496);"
+                    "CREATE EXTENSION IF NOT EXISTS vector;"
+                    "COMMIT;"
+                )
                 session.execute(statement)
                 session.commit()
         except Exception as e: