Syntax:
BENCHMARK(count,expr)

The BENCHMARK() function executes the expression expr repeatedly count
times. It may be used to time how quickly MySQL processes the
expression. The result value is 0, or NULL for inappropriate arguments
such as a NULL or negative repeat count.

The intended use is from within the mysql client, which reports query
execution times:

URL: https://dev.mysql.com/doc/refman/5.7/en/information-functions.html

