MYSQL索引的3种主要用途

I often see people confuse different ways MySQL can use indexing, getting wrong ideas on what query performance they should expect. There are 3 main ways how MySQL can use the indexes for query execution, which are not mutually exclusive, in fact some queries will use indexes for all 3 purposes listed here.

Using index to find rows The main purpose of the index is to find rows quickly – without scanning whole data set.--最主要的功能,就是利用索引来查找数据 This is most typical reason index gets added on the first place. Most popular index type in MySQL – BTREE can speed up equality and prefix range matches. --相等条件的查询和部分前缀列的范围匹配 So if you have index on (A,B) This index can be used to lookup rows for WHERE clauses like A=5 ; A BETWEEN 5 AND 10 AND B BETWEEN 5 AND 10 it however will NOT be able to help lookup rows for B BETWEEN 5 AND 10. key_len column in explain plan to see how many index parts are actually used for row lookup. Very common problem I see is multi column indexes which are used but only to their short prefix which is not very selective. A lot of this mistakes come from missing one very important MySQL limitation – once MySQL runs into the interval range it will not use any further index parts. If you have A BETWEEN 5 AND 10 AND B=5 for the same index MySQL will use the index… but it will only use A prefix for row lookups and scan whole A BETWEEN 5 AND 10. A IN (5,6,7,8,9,10) AND B=5 you will quite likely see improved query performance. Beware however of large nested enumerated ranges they are very hard on the optimizer. This just describes how MySQL uses single index – there are more complex rules of how indexes will be used if you look at multiple indexes usage with “index merge” .--既然是这样,要么把B条件写在前面,在B上建立索引;要么采用in,充分利用索引,而不是只利用索引前缀

Using Index to Sort Data Another great benefit of BTREE index is – it allows to retrieve data in sorted form hence avoiding external sort process for executing of queries which require sorting.--利用索引排序 Using index for sorting often comes together with using index to find rows, however it can also be used just for sort. for example if you’re just using ORDER BY without any where clauses on the table. In such case you would see “Index” type in explain which correspond to scanning (potentially) complete table in the index order. --在排序的时候,是扫描索引来排序的,而不是整个表. It is very important to understand in which conditions index can be used to sort data together with restricting amount of rows. Looking at the same index (A,B) things like ORDER BY A ; ORDER BY A,B ; ORDER BY A DESC, B DESC will be able to use full index for sorting (note MySQL may not select to use index for sort if you sort full table without a limit). However ORDER BY B or ORDER BY A, B DESC will not be able to use index because requested order does not line up with the order of data in BTREE. --需要的排序规则与索引的物理存储规则不一致的时候,就不会利用索引来排序 If you have both restriction and sorting things like this would work A=5 ORDER BY B ; A=5 ORDER BY B DESC; A>5 ORDER BY A ; A>5 ORDER BY A,B ; A>5 ORDER BY A DESC which again can be easily visualized as scanning a range in BTREE. Things like this however would not work A>5 ORDER BY B , A>5 ORDER BY A,B DESC or A IN (3,4) ORDER BY B – in these cases getting data in sorting form would require a bit more than simple range scan in the BTREE and MySQL decides to pass it on. --这种情况下,用索引排序的代价比全表扫描排序代价大,所以不会使用索引排序

Using index to read data Some storage engines (MyISAM and Innodb included) can also use index to read the data, hence avoiding to read the row data itself. This is not simply savings of having 2 reads per index entry instead of one but it can save IO orders of magnitude in some cases – Indexes are sorted (at least on the page boundary) so doing index range scan you typically get many index entries from the same page but the rows itself can be scattered across many pages requiring potentially a lot of IOs. --索引本身是有序的,很多索引行可能在一个数据页,但是数据可能分布在多个页. On top of that if you just need access to couple of columns index can be simply much smaller than the data which is one of the reason covering indexes help to speed up queries even if data is in memory. If MySQL is only reading index and not accessing rows you will see “using index” in EXPLAIN output.--如果只是使用了索引,那么explain中将显示为using index.