Python 3.6
  AWS Lambda 
  PostgreSQL
  Psycopg2(Postgres connector)
  SSL

  DB is already created with some table and access on AWS cloud. 

  1- Install Python3.6 or 3.8
  2- Create a lambda project in python
  3- Add psycopg2 dependency in your project root. (Add Image) (Reference)
  3- Get SSL certificate from AWS (), add this SSL Certificate to your project directory. (Add Image)
  4- Access this certificate in your code and use it in your psycopg2.connect() as sslMode and ssl_ca.(Add Image)
  5- Zip it and upload it in AWS lambda (Note: Zip name and lambda name should be same, and lambda_funcation.py should be on root of zip)

1- Create Lambda furcation (Image) with same name as project zip.
2- Create Lambda a Role, which should have RDSAccess Permission. 
3- Lambda should be in same VPC and subnets as PostgreSQL
4- Lambda should have a Security Group, this security group should have permission in PostgreSQL. So there should be a inbound rule in PostgreSQL Security Group for Lambda Security Group