> For example, I have seen several cases where poorly designed systems that unexpectedly used too much memory
> using a managed database system like RDS vs self-managing the same RDBMS: using the managed version saves on labor
As a DBRE / SRE, I can confidently assert that belief in the latter is often directly responsible for the former. AWS is quite clear in their shared responsibility model [0] that you are still responsible for making sound decisions, tuning various configurations, etc. Having staff that knows how to do these things often prevents the poor decisions from being made in the first place.
Not a DB admin, but I do install and manage DBs for small clients.
My experience is that AWS makes the easy things easy and the difficult things difficult, and the knowledge is not transferable.
With a CLI or non-cloud management tools I can create, admin and upgrade a database (or anything else) exactly the same way, locally, on a local VM, and on a cloud VM from any provider (including AWS). Doing it with a managed database means learning how the provider does it - which takes longer and I personally find it more difficult (and stressful).
What I cannot do as well as a real DB admin could do is things like tuning. Its not really an issue for small clients (a few generic changes to scale settings to available resources is enough - and cheaper than paying someone to tune it). Come to think of it, I do not even know how to make those changes on AWS and just hope the defaults match the size of RDS you are paying for (and change when you scale up?).
having written the above I am now doubting whether I have done the right thing in the past.
> using a managed database system like RDS vs self-managing the same RDBMS: using the managed version saves on labor
As a DBRE / SRE, I can confidently assert that belief in the latter is often directly responsible for the former. AWS is quite clear in their shared responsibility model [0] that you are still responsible for making sound decisions, tuning various configurations, etc. Having staff that knows how to do these things often prevents the poor decisions from being made in the first place.
[0]: https://aws.amazon.com/compliance/shared-responsibility-mode...