Reading through this thread what scares me is it appears you also have some misconceptions about MySQL and yet you “show people to the door” who in this case seem to have a good grasp of the answer. Hope I don’t have to cross paths I wouldn’t want to waste my time.

I think I always had the general idea, but reading these posts gave an extra layer.

T

You will be amazed how many MySQL “experts” don’t read the manual. They either “know” or assume.

At the other had digging out such information is difficult. MySQL manual is not easy to navigate and it changes all the time. What’s in there now could be added just couple of hours ago.

So don’t blame people who didn’t know how to find such information.

As for such questions being bad for interviews it depends on what do you want to understand about the candidate and even more what job you are interviewing for.

If it’s about on-site consulting you definitely prefer one being able to answer everything on top of his head.

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Using filesort

MySQL must do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the join type and storing the sort key and pointer to the row for all rows that match the WHERE clause. The keys then are sorted and the rows are retrieved in sorted order.
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http://dev.mysql.com/doc/refman/5.0/en/using-explain.html

In fact, such quaestions are VERY bad on the interview. It is ok to test skills during the interview but it is bad to ask for very low details. Much better to test if the person can ~find~ the answer to the question. Someone who knows is not necessarily the best. Someone who does not know but can find the answer is typically better.

Thanks for posting!

it looks like i my comprehension is right ：）

and i had write a blog about “mysql order by” some month ago:

http://www.jianzhaoyang.com/database/mysql_order_by_implement