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It's interesting that nobody mentioned MySQL.
Hmmm, sounds like good news. *SCNR*
No, really. MySQL has some strengths, especially its raw read performance is mentioned repeatedly. But it has a lot of gotchas due to over-optimizing for some rare use cases.
Oracle also wasn't mentioned here. There are some free versions of Oracle, I don't know how they are called today, I think something like "Personal Edition" or "XE". I've worked with Oracle in at least three different projects, and I like it, except for some minor annoyances. Empty strings sometimes mutate into NULL, you can't (always) use column aliases in WHERE or HAVING conditions, and so on. And Oracle is *FAT*. Why does a relational database have to install its own versions of Apache, Perl, Java, and tons of other software?
I really like PostgreSQL. Nice, clean, fast and lean. Free as in beer and free as in speech. The pgAdminIII GUI often comes bundled, or you can download it as source. Compiling PostgreSQL is just fun, pgAdminIII has some dependencies that have to be resolved. And newer versions of PostgreSQL have the Auto-Vaccum feature, so you don't have to run VACUUM manually any more. Alas, there are some minor gotchas, too.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)