When the APPX_COUNT_DISTINCT query option is set to TRUE, Impala implicitly converts COUNT(DISTINCT) operations to the NDV() function calls. The resulting count is approximate rather than precise. Enable the query option when a tolerable amount of error is acceptable in order to obtain faster query results than with a COUNT (DISTINCT) queries.

Type: Boolean; recognized values are 1 and 0, or true and false; any other value interpreted as false

Default: false (shown as 0 in output of SET statement)

Related information:

COUNT Function, DISTINCT Operator, NDV Function