Just my 2c here, which isn't worth much, but as an open source project MySQL help is easier to find than Postgress (even though it's open source too). The extra efficiency in running speed would require a sacrifice of development speed. Not a good tradeoff in my mind.
You'll get much faster results from improving algorithms than by switching to a different SQL engine, IMHO.
-Kelly
And switch to PostgreSQL. PostgreSQL obeys the SQL standard much better and thus it is more likely to attract SQL developers
I'd like to assume this statement is backed up with a plan to adapt MediaWiki to work with PostgreSQL and demonstrate superior performance with benchmarks. We're all in favor of choice here and would like MediaWiki to be more generalized, but PostgreSQL advocacy here has a history of being a lot of bluster followed by no action.