General Properties

join-distribution-type

• Type: string

• Allowed values: AUTOMATIC, PARTITIONED, BROADCAST

• Default value: AUTOMATIC

The type of distributed join to use. When set to PARTITIONED, Presto uses hash distributed joins. When set to BROADCAST, it broadcasts the right table to all nodes in the cluster that have data from the left table. Partitioned joins require redistributing both tables using a hash of the join key. This can be slower, sometimes substantially, than broadcast joins, but allows much larger joins. In particular broadcast joins are faster, if the right table is much smaller than the left. However, broadcast joins require that the tables on the right side of the join after filtering fit in memory on each node, whereas distributed joins only need to fit in distributed memory across all nodes. When set to AUTOMATIC, Presto makes a cost based decision as to which distribution type is optimal. It considers switching the left and right inputs to the join. In AUTOMATIC mode, Presto defaults to hash distributed joins if no cost could be computed, such as if the tables do not have statistics. This can be specified on a per-query basis using the join_distribution_type session property.

redistribute-writes

• Type: boolean

• Default value: true

This property enables redistribution of data before writing. This can eliminate the performance impact of data skew when writing by hashing it across nodes in the cluster. It can be disabled, when it is known that the output data set is not skewed, in order to avoid the overhead of hashing and redistributing all the data across the network. This can be specified on a per-query basis using the redistribute_writes session property.