optional reusable connection (#12051)

My postgres out of connections after continuous PGVector usage, and the
reason because it constantly creates new connections, so adding a
reusable pre established connection seems like solves an issue

---------

Co-authored-by: Roman Vasilyev <rvasilyev@mozilla.com>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/langchain/langchain/vectorstores/pgvector.py

@@ -116,6 +116,7 @@ class PGVector(VectorStore):
         logger: Optional[logging.Logger] = None,
         relevance_score_fn: Optional[Callable[[float], float]] = None,
         *,
+        connection: Optional[sqlalchemy.engine.Connection] = None,
         engine_args: Optional[dict[str, Any]] = None,
     ) -> None:
         self.connection_string = connection_string
@@ -127,15 +128,13 @@ class PGVector(VectorStore):
         self.logger = logger or logging.getLogger(__name__)
         self.override_relevance_score_fn = relevance_score_fn
         self.engine_args = engine_args or {}
-        self.__post_init__()
+        # Create a connection if not provided, otherwise use the provided connection
+        self._conn = connection if connection else self.connect()
 
     def __post_init__(
         self,
     ) -> None:
-        """
-        Initialize the store.
-        """
-        self._conn = self.connect()
+        """Initialize the store."""
         self.create_vector_extension()
         from langchain.vectorstores._pgvector_data_models import (
             CollectionStore,