ORDER BY NULL slower than ORDER BY column

according to MySQL documentation, adding ORDER BY NULL after GROUP BY should "avoid the overhead of sorting the result". If so, why is following query aprox 5 times slower

SELECT COUNT(*) advert_count, category.name
FROM advert
LEFT JOIN category ON advert.category_id = category.category_id
WHERE (
advert.state_id = 2
)
GROUP BY advert.category_id
ORDER BY NULL
LIMIT 5 

than query with ORDER BY advert_count?

SELECT COUNT(*) advert_count, category.name
FROM advert
LEFT JOIN category ON advert.category_id = ca开发者_StackOverflow社区tegory.category_id
WHERE (
advert.state_id = 2
)
GROUP BY advert.category_id
ORDER BY advert_count DESC
LIMIT 5 

From phpMyAdmin profiling:

1st query:

Sorting for group   -
Sorting result      0.000002
Sending data        12.069774

2nd query:

Sorting for group   2.436986
Sorting result      0.000028
Sending data        0.000021

I am confused by this, could anyone explain me what is going on there?

ORDER BY NULL is not ordering by anything at all. Every record gets the same position.

So the first query is selecting the first 5 groups found in the data. But the second query is calculating the results for all groups in the data, and finding the top 5 based on the count.

This is born out by the first two overheads you have shown.

The 3rd overhead is the difference - Sending Data. For a non-database related reason, when you ran it it took a long time to transfer the data. This is likely due to server or network load at that moment in time.