1. A computer-implemented method of partition pruning, comprising: receiving a query that requests retrieval of data items from a partitioned object; wherein the partitioned object is partitioned based on a composite key; wherein the query includes a predicate that specifies a condition involving a constituent key of said composite key; wherein the constituent key does not constitute a prefix of the composite key; for each partition of a plurality of partitions of said partitioned object, performing the steps of (a) determining a constituent-key-range for said constituent key; (b) based on the constituent-key-range, determining whether it is possible for the partition to include key values that satisfy the condition; and (c) if it is not possible for the partition to include key values that satisfy the condition, then pruning said partition to exclude said partition from consideration during execution of the query.