Re: SELECT *

[KimSeong Loh]

The difference with the following is the sorting required with the ORDER BY
SELECT * and ORDER BY RAND()

This one reads all rows from the table, but does not need to sort
SELECT col1, col2, col3 (...) FROM big_table

If you need to sort lots of data, try to increase the sort_buffer_size
Take a look at the status variable sort_merge_passes to check if you should increase the sort_buffer_size further, try to get as small sort_merge_passes as possible, 0 is best if you have sufficient memory to do in-memory sorting.
Of course, if you select only needed columns, the sorting may require less memory.