A.5.6. Deleting Rows from Related Tables
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.
