A common reason is using a partition key with excessive cardinality. On creating a table, users can optionally specify a column as a partition key by which data will be separated. A new file system directory will be created for every key value. This is typically a data management technique, allowing users to cleanly separate data logically in a table. Operations such as DROP PARTITION subsequently allow fast deletion of data subsets. This powerful feature can, however, easily be misused, with users interpreting it as a simple optimization technique for queries.
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