The SHUFFLE_DISTINCT_EXPRS query option controls the
shuffling behavior when a query has both grouping and distinct expressions.
Impala can optionally include the distinct expressions in the hash exchange
to spread the data among more nodes. However, this plan requires one more
hash exchange phase.
It is recommended that you turn off this option if the NDVs of the grouping expressions are high.
Type: Boolean; recognized values are 1 and 0, or true and
false; any other value interpreted as false
Default: false