I am currently evaluating all open source databases and possibly my fresh
opinion will be of interest.
I went over documentation and setup of Firebird, MySQL and PostgreSql and
here is "perception"(to get better understanding one has to run thing for
quite a while):
As for "user friendly" image: Firebird and MySQL are ok, PostgreSQL's web
site really has some glitches. Example, a search on everything more then 1-2
words never completes, i.e. one immeadiately notices SQL datbase runing
behind the scene... :-(
As for SQL features, Firebird and PostgreSQL are close - both have a
"triplet": stored procedures, triggers, views.
MySQL however has plans for all that in the next version.
As for speed: MySQL claims supremacy, at least that's in the benchmarks which
innoDB and mysql sites provide.
As for architecture then multi-versioning is implemented the same way for
both Firebird and PostgreSQL and both have some troubles when collecting
garbage. innoDB of MySQL doesn't even hide they implemented things "like
Oracle"(i.e. rollback segments): this model is good for garbage collection
but can potentially terminate long running queries with kind of "ora-01555
snapshot to old" error. Personally I prefer rollback segments solution as old
versions are reclaimed behind the scene while "snapshot to old" error can be
delt with.
Otherwise Firebird seems to have plans for WAL and shared SQL buffer.
Therefor if all goes like planned then both internals(in terms of features
like WAL, shared SQL buffers and so on) and SQL features of all databases
will be extremely similar with some differences in multi-versioning handling.
MySQL may(just may) claim a better speed, Firebird and postgreSql will have
more standard("clean") SQL.
Here is this year's "database of the year" poll
http://www.linuxquestions.org/questions/showthread.php?s=&threadid=116360