MySQL源码：Range访问方式相关的数据结构--续
前文着重介绍了MySQL的WHERE条件如何针对 单个 索引构造对应的SEL_ARG结构，本文是一个补充，将简单介绍多个索引对应的SEL_TREE结构。 对于一个完整的WHERE条件，MySQL会遍历所有可以使用的索引，逐一构造其对应的SEL_ARG结构，所有的SEL_ARG结构以指针数组
前文着重介绍了MySQL的WHERE条件如何针对单个索引构造对应的SEL_ARG结构，本文是一个补充，将简单介绍多个索引对应的SEL_TREE结构。
对于一个完整的WHERE条件，MySQL会遍历所有可以使用的索引，逐一构造其对应的SEL_ARG结构，所有的SEL_ARG结构以指针数组的形式存放在SEL_TREE->keys中。如果对应索引无法构造SEL_ARG，那么对应的指针为空。
class SEL_TREE :public Sql_alloc
{
...
SEL_ARG *keys[MAX_KEY];
...
};gdb打印对应的结构：
(gdb) p $1
$2 = (SEL_TREE *) 0x7f59c4038348
(gdb) p *$1
$3 = {
...
keys = {0x0, 0x7f59c4038598, 0x0 },
...
}SEL_TREE是一个数组，但如果像他的名字，他如果真是一棵树的话，那么将是如下结构：
[ key1 part1 ] [ key1 part2 ] [ key1 part3 ]
-\ /- $ $
- / SEL_ARG(-∞, 1) $ ===> SEL_ARG [5,5] ===> $ SEL_ARG [10,10]
|...... | |^ $ $ |^
| | next|| $ $ next||
......| | ||prev $ $ ||prev
0x0 | /--------->| || $ $ v
\ | | | || $ $ SEL_ARG [12,12]
\------| | | || $ $
[key4]| | | v| $ $
| | | SEL_ARG [2, 2] $=== next_key_part =====| $
| [link of SEL_ARG] | |^ $ | $
| / | next|| $ | $
|--------/ | ||prev $ |===>$ SEL_ARG[11,11]
| [key3] | v| $ | $ |^
| \ SEL_ARG [3, 3] $=== next_key_part =====| $ next||
| $ $ ||prev
| $ $ v|
| SEL_ARG[14,14]
[SEL_ARG] | *************************
\ | * structure of SEL_TREE *
\------| *************************
[key2]|
|
| [ key1 part1 ]
| / SEL_ARG (-∞,2] $ SEL_ARG [3,5)
| | |^ $ /\ Black
| | next|| $ / \
| [link of SEL_ARG] | ||prev $ / \
| / | | v| $ SEL_ARG (-∞,2] SEL_ARG (10,∞)
|--------/ | | SEL_ARG [3,5) $ Red Red
| [key1] |-------->| |^ $
| | next|| $
| | ||prev $
SEL_TREE | v| $
\ SEL_ARG (10,∞) $
$（大图）
That's all. 本文较为简单。
原文地址：MySQL源码：Range访问方式相关的数据结构--续, 感谢原作者分享。
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