B+ tree index use (8) sorting use and precautions (20)

In the last article, we introduced matching column prefixes , Because the index sort is compared by letter , If % The index cannot be triggered before , And range matching , Range query , The leftmost column can trigger the index , When there is an exact value in front , such as name = ‘’, The second range can also trigger the index , The latter cannot trigger the index .

B+ Tree index uses (7) Match column prefix , Matching value range （ nineteen )

Sort

We use it sql Have you used it before order by. Traditional sorting does not sort in memory or disk , Sorting in memory can use some sorting algorithms , Quick sort , Merge, sort, etc , After sorting , Then return the result to the client . stay mysql in , The method of sorting in disk or memory is uniformly called file sorting ( English name ：filesort). Generally, it is related to documents , It's going to be slow , Compare the speed of disk and memory , It's like a plane is better than a green train , Even the disk is slower than the green train . But if it's used order by sentence , Such as ：order by name,birthday,phone; At this time, the query result is first pressed name Sort ,name If the same, press birthday Sort ,birthday identical , Then press phone Sort . because b+ Tree index sort , It is sorted according to this , So it saves the cost of sorting in memory or disk , Get data directly from the database .

Joint index sorting considerations

Order by When sorting , Followed by index fields . When sorting by joint index, you need to pay attention to ：

1）、 When order by name,birthday,phone When , At this time, press name Sort , When the same , Will press birthday Sort , If it's the same, press phone.

2）、 When used order by phone,birthday,name When sorting , Index sorting will not be triggered , The principle is the same as before , Because you need to name identical , Will trigger the subsequent sorting .

Or take an exact value before where name = ‘A’ order by birthday,phone;

At this time, the subsequent sorting will also trigger the index , The reason is similar to the above , Said many times .

Several cases where index sorting cannot be used

ASC、DESC A mixture of

For the usage scenario of joint index , We require that the order be consistent , Or press ASC Sort , Or press DESC Sort ( When not writing , The default with ASC Ascending ). Let's first recall the sort structure of the index ：1） According to the first name Sort .2） If it is the same, press birthday Sort .3） How to press the same phone Sort . If the sorting is consistent, how to get the value ？

order by name,birthday ASC limit 10; At this time, you only need to count from the leftmost side of the index 10 individual .

order by name,birthday DESC limt 10; Just count from the far right of the index 10 individual .

But if we press name Ascending , Press birthday Descending ：

Order by name asc,birthday desc limit 10; In this case, using index lookup is very complicated ,mysql Designers feel that this is not as fast as sorting files , So the sorting must be consistent .

The rank contains non indexed columns

Order by name,country limit 10;name and country Does not belong to the union index , therefore country Definitely not sort by index .

Sorting uses complex expressions

such as order by upper(name) limit 10; Used upper Then it's not a separate column , You can't use search engines .