The Vertica Database Forums

Median and percentile functions are commonly used data statistic functions. They are also used in other sophisticated data analysis algorithms, such as the robust z-Score normalization function. Vertica has exact MEDIAN and PERCENTILE_CONT functions, but these functions do not scale well for extremely large data sets, because the functions must sort the entire data set to calculate exact values. Sometimes, approximations of the median and percentile are adequate. Vertica 8.1.0 introduced support for two approximate functions: APPROXIMATE_MEDIAN and APPROXIMATE_PERCENTILE