APPX_COUNT_DISTINCT Query Option (Impala 2.0 or higher only)
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
Parent topic: Query Options for the SET Statement