升序索引 mysql>explain select*from test order by c1;+---+---+---+---+---+---+---+---+---+---+---+---+|id|select_type|table|partitions|type|possible_keys|key|key_len|ref|rows|filtered|Extra|+---+---+---+---+---+---+---+--...
3 rows in set (0.00 sec) mysql> SELECT b FROM t1 FORCE INDEX(PRIMARY); +---+ | b | +---+ | 1 | | 2 | | 3 | +---+ 3 rows in set (0.00 sec) 如上例,可以看到指定不同的索引给出的结果顺序也是不一样的。 mysql> EXPLAIN SELECT * FROM t1 ORDER BY a DESC, b; +---+...
为了解决这个问题,从MySQL8.0版本开始支持在索引Key中倒序存储。你可以按照实际的sql负载来决定如何创建索引,例如你的查询中有Order by a desc, b asc,就可以创建索引key(a desc, b asc),而在8.0之前的版本中则可能需要代价比较大的filesort来进行, 此外逆序扫描Btree也有额外的开销,例如扫描时的page切换,page内...
In this blog, we’ll discuss descending indexes in MySQL 8.0. Summary The future MySQL 8.0 will (probably) have a great new feature: support for index sort order on disk (i.e., indexes can be physically sorted in descending order). In the MySQL 8.0 Labs release (new optimizer preview)...
对于MySQL-5.7,除了查询2和查询6之外,我们对所有查询都使用反向索引扫描或文件排序,因为这两个查询只需要升序。 Query 1: SELECT * FROM t1 ORDER BY a DESC; mysql8.0>explainSELECT*FROMt1ORDERBYaDESC;+---+---+---+---+---+---+---+---+---+---+---+---+|id|select_type|table|parti...
MySQL supports descending indexes: DESC in an index definition is no longer ignored but causes storage of key values in descending order. Previously, indexes could be scanned in reverse order but at a performance penalty. A descending index can be scanned in forward order, which is more efficien...
You can see in theExtracolumn of the output ofEXPLAINthat the optimizer is able to use a descending index, as shown here: mysql>CREATETABLEt1(->aINT,->bINT,->INDEXa_desc_b_asc(aDESC,bASC)->);mysql>EXPLAINSELECT*FROMt1ORDERBYaASC\G***1. row***id:1 select_type:SIMPLE table:t1 p...
(0.12sec)Records:25Duplicates:0Warnings:0mysql>descselect*fromtORDERBYc1ASC,c2ASC;+---+---+---+---+---+---+---+---+---+---+---+---+|id|select_type|table|partitions|type|possible_keys|key|key_len|ref|rows|filtered|Extra|+---+---+---+---+---+---+---+---...
As Ingo said: there's no need to support it from a performance standpoint, although MySQL does support the sort order option. http://dev.mysql.com/doc/mysql/en/create-index.html MySQL is clever enough to traverse B-tree indexes in any order, no matter how they are sorted. MySQL can ...
MySQL ORDER BY ASCending / DESCending Fastest Entity Framework Extensions Bulk Insert Bulk Delete Bulk Update Bulk Merge Example# ORDER BY x ASC -- same as default ORDER BY x DESC -- highest to lowest ORDER BY lastname, firstname -- typical name sorting; using two columns ORDER BY...