1. A non-transitory computer-readable medium storing computer-executable instructions that when executed by a computer cause the computer to perform a method, the method comprising: detecting a first access statement that seeks access to a first row in a database table for which row level security is active; detecting a second access statement that seeks access to a second row in the database table for which row level security is active; identifying a first access control expression stored in the first row, where the first access control expression is a first predicate embodied as a first instance of an expression data type that conforms to a vocabulary; evaluating the identified first access control expression; identifying a second access control expression stored in the second row, where the second access control expression is a second predicate embodied as a second instance of the expression data type that conforms to the vocabulary; evaluating the identified second access control expression; and selectively executing the first and second access statements based, at least in part, on a result of the evaluating of the first and second access control expressions.