On: Google puts MySQL in App Engine Cloud with Google ...

[alvivar]

Why MySQL over PostgreSQL? I would like to know how they choose one over the other...

[axiak → alvivar]

I don't know their official reason, but they seem to have a lot of in-house knowledge of mysql internals [1].

1: http://code.google.com/p/google-mysql-tools/

[stonemetal → alvivar]

From what I understand they use or at least used to use Mysql internally. So they have experience scaling Mysql.

[bigwally → alvivar]

Maybe the app engine team just chose the one they had more experience in hacking.