Help me decide which ORDBMS should I use

Hi I’m working in this project by myself and I got stuck when I had to choose the ORDBMS. I have to develop an app for managing Requests for Payment Instrument (RPI), this doc is used to generate paychecks for paying bills in my company. The users that will use my app are stored in a LDAP server, so I only have to assign them roles in my database. I had planned to use MySQL as the ORDBMS but I found that in the community server, one can’t create roles. So I do some digging and found that PostgreSQL does have this feature.
Right now I can only choose between MySQL 5.5 y PostgreSQL 8.4.9, both services are running on Centos 5.8. Would you please help me decide what service should I use for my app? For now my app will be deployed for windows, XP mostly.
I had never used Postgre so I would have to start learning how to work with it, no problem but I will still appreciate any feedback since this is my first time developing a large project.
I apologize if my English writing isn’t the best. I’m from Cuba, so this is not my native language.

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Originally Posted by batman

I had planned to use MySQL as the ORDBMS but I found that in the community server, one can’t create roles. So I do some digging and found that PostgreSQL does have this feature.

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Both are capable RDBMS systems. Since your users are in an LDAP directory, can you use LDAP group membership to map to your user's role(s)? When the user logs in for example, you could determine his/her role based on LDAP group membership and then your app checks the users' role to hide or disable functionality that is not applicable for their role, etc?

It would be best IMHO to leave the user role out of the RDBMS...

PostgreSQL because it has a decent transaction system.

I did some digging and found I can use an LDAP authentication in PostgreSQL 8.4. To do so I only have to put the users that are allowed in my app to this ORDBMS. It all fits now, I will use PostgreSQL 8.4 as my database server.