数据库内核月报 - 2015 / 11

MySQL · 捉虫动态 · order by limit 造成优化器选择索引错误

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