Histogram construction and selectivity estimation for string and substring match queries in databases of data having strings associated with attributes. The histogram construction counts string-attribute pairs in the documents, and outputs string-attribute-count triples sorted by count. The collection is partitioned into buckets. A synopsis is generated for the partition, having an average selectivity or count of the string-attribute-count triples in the partition and summary information representing the set of string-attribute pairs belonging to the bucket. Subsequent queries, both for exact and substring matches, use the synopsis to estimate the selectivity of buckets.