1. A method of evaluating a formula in a database system, the method comprising: receiving, at the database system, a formula defined to access data stored in one or more rows of a database table of the database system, wherein the formula is an expression that receives the accessed data as input and provides a result that is different than the input data; evaluating, with the database system, the formula to obtain a first result based on first values of the input data stored in the one or more rows of the database table; receiving, at the database system, a change in a first value of at least one of the input data stored in the one or more rows; obtaining, with the database system, an updated result of the formula by: determining whether the formula accesses input data from only one row of a the database table; and synchronously evaluating the formula to obtain the updated result when the formula accesses input data from only one row of the database table; and otherwise deciding whether to evaluate the formula synchronously or asynchronously to obtain the updated result when the formula accesses input data from at least two rows of the database table, wherein the formula accesses input data from multiple rows of the database table, further comprising synchronously evaluating the multiple-row formula to obtain the updated result when the updated result of the multiple-row formula is computable with a delta value combined with the first result of the multiple-row formula.