Rule instances in the Pega Platform database are heavily indexed to speed the searching operations used in rule resolution.
If you create a custom rule type you can also create a dedicated database table to hold the rules and the associated triggers that the table requires. This table eliminates the risk that future Pegasystems upgrades to the standard database tables listed above might interfere with the exposed columns and other details needed by the custom rule type. However, this also means that after an upgrade you might have to take extra steps to ensure that your rule table and its associated triggers are correct and up to date. In most cases, it is easier to use an existing rule table and, if necessary, optimize a few extra columns to expose the data your rule type needs.