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• This code retrieves the DATABASE_URL from the environment.
• If DATABASE_URL starts with postgres://, it replaces it with postgresql+psycopg2://.
• The db instance is initialized with SQLAlchemy(app) for use with SQLAlchemy ORM.
• The replacement of "postgres://" with "postgresql+psycopg2://" is necessary because of a compatibility issue between the URI format provided by Heroku and the URI format expected by SQLAlchemy.

• When you provision a PostgreSQL database on Heroku, it sets an environment variable called DATABASE_URL with the connection string for your database.
• This URI uses the format postgres://.

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