What is difference between session and connection in Postgres

Question

I use Postgres in Aws of db.m4.4xlarge instance which has threshold of 16 sessions. Does that mean Postgres only accepts 16 concurrent connections? Is my understanding correct? How can db.m4.4xlarge machine can accept only 16 connections? What exactly difference between session/connection are both are synonyms to each other.

• jjanes: "How can db.m4.4xlarge machine can accept only 16 connections?" Because that is how you, or someone, configured it. Surely that is not the maximum possible configuration.
• user1844634: threshold for db.m4.4xlarge postgress is 16 sessions in aws
• jjanes: I don't know what 'threshold' means here, but max_connections defaults to 5000 for that machine size with RDS.
• user1844634: I am taking about the current activity sessions in aws rds. beyonds 16 sessions the bar in the rds performance insights turning red
• jjanes: There usually isn't much advantage to having more active sessions than you have CPUs, but allowing it can make bookkeeping simpler. In any event, the color scheme chosen by Performance Insights surely doesn't disallow connections.

Answer 1

Database Connection: When client connect the server but not authenticated yet. A client backend (postgres sub process) starts in server.

Database Session: When client completes the authentication progress. Now the client can execute sql.

A database connection limit is database session limit. Without a connection client cannot have session and a connection have only one session in postgresql. So they are used as synonyms.

Answer 2

"Database session" and "database connection" are usually used as synonyms. I don't know about Amazon's services, and you didn't provide a link.

Answer 3

Sessions start after connections are pulled from the pool and end before they go back into the pool.