If you're asking about which database to use, it really depends on personal preference and what you want out of it. As I'm only familiar with MySQL, I'll answer the other part of the question assuming MySQL:
you will want to use
INNODB because your table is going to be write-intensive and for large tables, the row-locking of INNODB will be a life-saver over
As far as table design, it seems you only really need one table:
CREATE TABLE `wordpress`.`<table_name>` (
`id` smallint(4) NOT NULL AUTO_INCREMENT UNSIGNED,
`user` varchar(30) NOT NULL,
`filename` varchar(255) NOT NULL,
`date_insert` datetime NOT NULL,
PRIMARY KEY (`id`),
UNIQUE `userFile`(user, filename)
I went with an arbitrary 'id' column set to
AUTO_INCREMENT because the primary key is replicated in every entry of every index. So, doing a primary key of (user, filename) could cause performance issues if your filenames are extremely long.
The size of your 'id' column depends on how big your table is going to grow. Unsigned Smallint will give you 65k rows.
User and filenames are varchar, because they'll vary on length drastically I assume.
date_insert is just a way to order your results based on when it was inserted (helpful for your POP)