Which database to use?

[crioto]

Hi everybody!
I have some questions related to database servers.
I writing in C++ under GNU/Linux, and i need to use database in my app.
Small description:
My app will use database very hard, so i need faster database.
And for now i need real advice. Which database to use? PostgreSQL? MySQL? Firebird? Any other?
I don't won't to use commercial databases, and i need it to have simple, well documented API.

[iamthwee]

MySQL is always a good option, never tried PostgreSQL.