MySQL · 捉虫动态 · order by limit 造成优化器选择索引错误
最后更新于:2022-04-01 10:37:53
## 问题描述
bug 触发条件如下:
1. 优化器先选择了 where 条件中字段的索引,该索引过滤性较好;
2. SQL 中必须有 order by limit 从而引导优化器尝试使用 order by 字段上的索引进行优化,最终因代价问题没有成功。
### 复现case
表结构
~~~
create table t1(
id int auto_increment primary key,
a int, b int, c int,
key iabc (a, b, c),
key ic (c)
) engine = innodb;
~~~
构造数据
~~~
insert into t1 select null,null,null,null;
insert into t1 select null,null,null,null from t1;
insert into t1 select null,null,null,null from t1;
insert into t1 select null,null,null,null from t1;
insert into t1 select null,null,null,null from t1;
insert into t1 select null,null,null,null from t1;
update t1 set a = id / 2, b = id / 4, c = 6 - id / 8;
~~~
触发SQL
~~~
mysql> explain select id from t1 where a<3 and b in (1, 13) and c>=3 order by c limit 2\G
*************************** 1\. row ***************************
id: 1
select_type: SIMPLE
table: t1
type: index
possible_keys: iabc,ic
key: iabc
key_len: 15
ref: NULL
rows: 32
Extra: Using where; Using index; Using filesort
~~~
使用 force index 可以选择过滤性好的索引
~~~
mysql> explain select id from t1 force index(iabc) where a<3 and b in (1, 13) and c>=3 order by c limit 2\G
*************************** 1\. row ***************************
id: 1
select_type: SIMPLE
table: t1
type: range
possible_keys: iabc
key: iabc
key_len: 5
ref: NULL
rows: 3
Extra: Using where; Using index; Using filesort
~~~
## 问题分析
optimizer_trace 可以帮助分析这个问题。
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE\G
~~~
"range_scan_alternatives": [
{
"index": "iabc",
"ranges": [
"NULL < a < 3"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": false,
"using_mrr": false,
"index_only": true,
"rows": 3,
"cost": 1.6146,
"chosen": true
},
{
"index": "ic",
"ranges": [
"3 <= c"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": false,
"using_mrr": false,
"index_only": false,
"rows": 17,
"cost": 21.41,
"chosen": false,
"cause": "cost"
}
],
~~~
range_scan_alternatives 计算 range_scan,各个索引的开销,从上面的结果可以看出,联合索引 iabc 开销较小,应该选择 iabc。
~~~
"considered_execution_plans": [
{
"plan_prefix": [
],
"table": "`t1`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "range",
"rows": 3,
"cost": 2.2146,
"chosen": true
}
]
},
"cost_for_plan": 2.2146,
"rows_for_plan": 3,
"chosen": true
}
]
~~~
considered_execution_plans 表索引选择过程,access_type 是 range,rows_for_plan=3,到这里为止,执行计划还是符合预期的。
~~~
{
"clause_processing": {
"clause": "ORDER BY",
"original_clause": "`t1`.`c`",
"items": [
{
"item": "`t1`.`c`"
}
],
"resulting_clause_is_simple": true,
"resulting_clause": "`t1`.`c`"
}
},
{
"refine_plan": [
{
"table": "`t1`",
"access_type": "index_scan"
}
]
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "ORDER BY",
"index_order_summary": {
"table": "`t1`",
"index_provides_order": false,
"order_direction": "undefined",
"index": "unknown",
"plan_changed": false
}
}
}
~~~
clause_processing 用于简化 order by,经过 clause_processing access_type 变成 index_scan(全索引扫描,过滤性较range差),此时出现了和预期不符的结果。
因此可以推测优化器试图优化 order by 时出现了错误:
* 第一阶段,优化器选择了索引 iabc,采用 range 访问;
* 第二阶段,优化器试图进一步优化执行计划,使用 order by 的列访问,并清空了第一阶段的结果;
* 第三阶段,优化器发现使用 order by 的列访问,代价比第一阶段的结果更大,但是第一阶段结果已经被清空了,无法还原,于是选择了代价较大的访问方式(index_scan),触发了bug。
## 问题解决
1. 我们在索引优化函数`SQL_SELECT::test_quick_select` 最开始的时候保存访问计划变量(quick);
2. 在索引没变的时候,还原这个变量;
3. 在索引发生改变的时候,删除这个变量。
在不修改 mysql 源码的情况下,可以通过 force index 强制指定索引规避这个bug。
`SQL_SELECT::test_quick_select` 调用栈如下
~~~
#0 SQL_SELECT::test_quick_select
#1 make_join_select
#2 JOIN::optimize
#3 mysql_execute_select
#4 mysql_select
#5 mysql_explain_unit
#6 explain_query_expression
#7 execute_sqlcom_select
#8 mysql_execute_command
#9 mysql_parse
#10 dispatch_command
#11 do_command
~~~