1. A method, performed by a processor and memory of a computing device, of generating an outlier index for a database and a given workload, the workload comprising a set of previous queries directed to the database, wherein the queries in the workload have selection or group by conditions, the method comprising: identifying at least two sub-relations of tuples in the database induced by selection or group by conditions in queries in the workload, a tuple comprising a row of one or more values in the database, a condition comprising a portion of a query that determines which tuples do or do not satisfy the query, a sub-relation either comprising tuples that satisfy selection and group by conditions or comprising tuples that do not satisfy selection and group by conditions; generating, by statistical analysis of values of the tuples in each sub-relation, a variance for each respective sub-relation; selecting sub-relations based on the sub-relations having variances higher than a threshold; and generating outlier indices of tuples from the selected sub-relations having variances higher than the threshold, the outlier tuples comprising tuples having values that, relative to other tuples in the sub-relations, disproportionately contribute to an aggregation of the tuples in the sub-relations; and taking a union of the outlier indices generated from the sub-relations and forming the outlier index from the union, the outlier index comprising the union of the outlier indices to estimate results of a query to the database.