If the total length of the DELETE statement
for related_table is more than 1MB (the
default value of the max_allowed_packet
system variable), you should split it into smaller parts and
execute multiple DELETE statements. You
probably get the fastest DELETE by
specifying only 100 to 1,000 related_column
values per statement if the related_column
is indexed. If the related_column isn't
indexed, the speed is independent of the number of arguments
in the IN clause.
