This is something I have thought a lot recently since I recently saw a project that absolute didn’t care in the slightest about this and used many vendor specific features of MS SQL all over the place which had many advantages in terms of performance optimizations.
Basically everyone always advises you to write your backend so generically with technologies like ODBC, JDBC, Hibernate, … and never use anything vendor specific like stored procedures, vendor specific datatypes or meta queries with the argument being that you can later switch your DBMS without much hassle.
I really wonder if this actually happens in the real world with production Software or if this is just some advice that makes sense on surface level but in reality never pans out. I personally haven’t seen any large piece of Software switch to a different DBMS, even if there would be long term advantages of doing so, because the risk and work to retest everything would be far too great.
The only examples I know of (like SAP) were really part of a much larger rewrite or update rather than “just” switching DBMS.
I know of 2 projects that wanted to migrate from Oracle to Postgres, one of which was successful. Both migrations were driven by cost savings–Oracle can get exceedingly expensive.
In both cases there was up front analysis of Oracle specific features being used. A lot of that could be rewritten into standard SQL but some required code logic changes to compensate. Vendor lock-in is insidious and will show up in native queries, triggers, functions that use Oracle packages, etc.
Changing a project’s underlying database is rare, but not as rare as it used to be.