fix: reuse PostgreSQL database connections (#574)

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## Description
Fix PostgreSQL database connection problems

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Refactor**
- Improved the system’s database connection process to enhance
compatibility across multiple relational databases. The application now
dynamically selects the optimal connection method—reusing established
connections when possible—to ensure improved stability and performance
without affecting the public interface.
- Streamlined the creation of the embedding engine by removing it as a
parameter and generating it internally.
- Removed dependency on the embedding engine in the vector engine
retrieval process.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

cognee/infrastructure/databases/vector/create_vector_engine.py

@@ -1,14 +1,17 @@
+from .embeddings import get_embedding_engine
+
 from functools import lru_cache
 
 
 @lru_cache
 def create_vector_engine(
-    embedding_engine,
     vector_db_url: str,
     vector_db_port: str,
     vector_db_key: str,
     vector_db_provider: str,
 ):
+    embedding_engine = get_embedding_engine()
+
     if vector_db_provider == "weaviate":
         from .weaviate_db import WeaviateAdapter
 

cognee/infrastructure/databases/vector/get_vector_engine.py

@@ -1,7 +1,6 @@
 from .config import get_vectordb_config
-from .embeddings import get_embedding_engine
 from .create_vector_engine import create_vector_engine
 
 
 def get_vector_engine():
-    return create_vector_engine(get_embedding_engine(), **get_vectordb_config().to_dict())
+    return create_vector_engine(**get_vectordb_config().to_dict())

cognee/infrastructure/databases/vector/pgvector/PGVectorAdapter.py

@@ -10,6 +10,7 @@ from cognee.exceptions import InvalidValueError
 from cognee.infrastructure.databases.exceptions import EntityNotFoundError
 from cognee.infrastructure.engine import DataPoint
 from cognee.infrastructure.engine.utils import parse_id
+from cognee.infrastructure.databases.relational import get_relational_engine
 
 from ...relational.ModelBase import Base
 from ...relational.sqlalchemy.SqlAlchemyAdapter import SQLAlchemyAdapter
@@ -36,8 +37,17 @@ class PGVectorAdapter(SQLAlchemyAdapter, VectorDBInterface):
         self.api_key = api_key
         self.embedding_engine = embedding_engine
         self.db_uri: str = connection_string
-        self.engine = create_async_engine(self.db_uri)
-        self.sessionmaker = async_sessionmaker(bind=self.engine, expire_on_commit=False)
+
+        relational_db = get_relational_engine()
+
+        # If postgreSQL is used we must use the same engine and sessionmaker
+        if relational_db.engine.dialect.name == "postgresql":
+            self.engine = relational_db.engine
+            self.sessionmaker = relational_db.sessionmaker
+        else:
+            # If not create new instances of engine and sessionmaker
+            self.engine = create_async_engine(self.db_uri)
+            self.sessionmaker = async_sessionmaker(bind=self.engine, expire_on_commit=False)
 
         # Has to be imported at class level
         # Functions reading tables from database need to know what a Vector column type is