Hi
We have used many SQL databases with Ignition and I can suggest that go with MySQL.
It is most well documented and there is huge user group in internet if help needed.
We have projects where is over 40 000 I/O done with MySQL with no problems, also just engineering system where will be over 100 000 I/O and there also planning to use MySQL.
BR
Tommi Vahtera
THT Control Oy