This was true for RDS serverless v1 which scaled to 0 but is no longer offered. V2 requires a minimum 0.5 ACU hourly commit ($40+ /mo).

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cobolcomesback a day ago | parent [-]

V2 scales to zero as of last year.

https://aws.amazon.com/blogs/database/introducing-scaling-to...

It only scales down after a period of inactivity though - it’s not pay-per-request like other serverless offerings. DSQL looks to be more cost effective for small projects if you can deal with the deviations from Postgres.

	▲	jread a day ago \| parent [-]
		Ah, good to know, I hadn't seen that V2 update. Looks like a min 5m inactivity to auto-pause (i.e., scale to 0), and any connection attempt (valid or not) resumes the DB.