1. A computer-implemented method of partition pruning, comprising: receiving a query that requests retrieval of data items from a partitioned object; wherein said 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 said constituent key does not constitute a prefix of said 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 the partition to exclude the partition from consideration during execution of the query; and wherein each partition of said plurality of partitions does not include any defined subpartitions; wherein the method is performed by one or more computing devices.