chore: Add deprecation warning for PGVector (#169)

Co-authored-by: Averi Kitsch <[email protected]>

Author: dishaprakash

langchain_postgres/vectorstores.py

@@ -19,6 +19,7 @@
     Type,
     Union,
 )
+import warnings
 from typing import (
     cast as typing_cast,
 )
@@ -48,6 +49,8 @@
 
 from langchain_postgres._utils import maximal_marginal_relevance
 
+warnings.simplefilter("once", PendingDeprecationWarning)
+
 
 class DistanceStrategy(str, enum.Enum):
     """Enumerator of the Distance strategies."""
@@ -428,6 +431,13 @@ def __init__(
         self._async_engine: Optional[AsyncEngine] = None
         self._async_init = False
 
+        warnings.warn(
+            "PGVector is being deprecated and will be removed in the future. "
+            "Please migrate to PGVectorStore. "
+            "Refer to the migration guide at [https://github.com/langchain-ai/langchain-postgres/blob/main/examples/migrate_pgvector_to_pgvectorstore.md] for details.",
+            PendingDeprecationWarning,
+        )
+
         if isinstance(connection, str):
             if async_mode:
                 self._async_engine = create_async_engine(