Mysql之 optimizer_trace 相关总结

Mysql之 optimizer_trace 相关总结

MySQL官网介绍:https://dev.mysql.com/doc/dev/mysql-server/latest/PAGE_OPT_TRACE.html


1. 简介

MySQL优化器可以生成Explain执行计划,通过执行计划查看sql是否使用了索引,使用了哪种索;
但是有些时候,你会发现为什么没想按照我们所想的思路执行:
为什么会使用这个索引 ?!
为什么没有使用添加的索引 ?!

于是,MySQL5.6版本之后开始引入 optimizer trace(优化器追踪),它可以查看优化器生成执行计划的整个过程,以及做出的各种决策,包括访问表的方法、各种开销计算、各种转换等等,帮助我们更好的去优化sql。

另外,optimizer_trace的开关默认是关闭的 ,开启trace工具会影响mysql性能,所以只适合临时分析sql使用,用完之后最好及时关闭。


2. 使用方法

1. 查看optimizer trace配置

show variables like '%optimizer_trace%';

查询结果:

Mysql之 optimizer_trace 相关总结_第1张图片
查询结果字段说明:

  • optimizer_trace: 主配置,enabled的on表示开启,off表示关闭,one_line表示是否展示成一行
  • optimizer_trace_features: 表示优化器的可选特性,包括贪心搜索、范围优化等
  • optimizer_trace_limit: 表示优化器追踪最大显示数目,默认是1条
  • optimizer_trace_max_mem_size: 表示优化器追踪占用的最大容量
  • optimizer_trace_offset: 表示显示的第一个优化器追踪的偏移量

2. 开启/关闭 optimizer trace

#开启trace
set session optimizer_trace="enabled=on",end_markers_in_json=on;
#关闭trace
set session optimizer_trace="enabled=off";

3. 执行需要进行分析的SQL语句

select * from test0816 where name > 'a' order by remark;

4. 使用optimizer trace查看优化器的选择过程

SELECT * FROM information_schema.OPTIMIZER_TRACE;

查询结果:
在这里插入图片描述
查询结果对应字段说明:

  • QUERY: 表示我们执行的查询语句
  • TRACE: 优化器生成执行计划的过程(重点关注)
  • MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 优化过程其余的信息会被显示在这一列
  • INSUFFICIENT_PRIVILEGES: 表示是否有权限查看优化过程,0是,1否

5. 分析

trace的内容:

{
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `test0816`.`id` AS `id`,`test0816`.`name` AS `name`,`test0816`.`age` AS `age`,`test0816`.`remark` AS `remark`,`test0816`.`create_time` AS `create_time` from `test0816` where (`test0816`.`name` > 'a') order by `test0816`.`remark`"
          }
        ] /* steps */
      } /* join_preparation */
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "(`test0816`.`name` > 'a')",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(`test0816`.`name` > 'a')"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(`test0816`.`name` > 'a')"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(`test0816`.`name` > 'a')"
                }
              ] /* steps */
            } /* condition_processing */
          },
          {
            "substitute_generated_columns": {
            } /* substitute_generated_columns */
          },
          {
            "table_dependencies": [
              {
                "table": "`test0816`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ] /* depends_on_map_bits */
              }
            ] /* table_dependencies */
          },
          {
            "ref_optimizer_key_uses": [
            ] /* ref_optimizer_key_uses */
          },
          {
            "rows_estimation": [
              {
                "table": "`test0816`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 3,
                    "cost": 2.65
                  } /* table_scan */,
                  "potential_range_indexes": [
                    {
                      "index": "PRIMARY",
                      "usable": false,
                      "cause": "not_applicable"
                    },
                    {
                      "index": "idx_name_age_remark",
                      "usable": true,
                      "key_parts": [
                        "name",
                        "age",
                        "remark",
                        "id"
                      ] /* key_parts */
                    }
                  ] /* potential_range_indexes */,
                  "setup_range_conditions": [
                  ] /* setup_range_conditions */,
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  } /* group_index_range */,
                  "skip_scan_range": {
                    "potential_skip_scan_indexes": [
                      {
                        "index": "idx_name_age_remark",
                        "usable": false,
                        "cause": "query_references_nonkey_column"
                      }
                    ] /* potential_skip_scan_indexes */
                  } /* skip_scan_range */,
                  "analyzing_range_alternatives": {
                    "range_scan_alternatives": [
                      {
                        "index": "idx_name_age_remark",
                        "ranges": [
                          "a < name"
                        ] /* ranges */,
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": false,
                        "using_mrr": false,
                        "index_only": false,
                        "in_memory": 1,
                        "rows": 3,
                        "cost": 1.31,
                        "chosen": true
                      }
                    ] /* range_scan_alternatives */,
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    } /* analyzing_roworder_intersect */
                  } /* analyzing_range_alternatives */,
                  "chosen_range_access_summary": {
                    "range_access_plan": {
                      "type": "range_scan",
                      "index": "idx_name_age_remark",
                      "rows": 3,
                      "ranges": [
                        "a < name"
                      ] /* ranges */
                    } /* range_access_plan */,
                    "rows_for_plan": 3,
                    "cost_for_plan": 1.31,
                    "chosen": true
                  } /* chosen_range_access_summary */
                } /* range_analysis */
              }
            ] /* rows_estimation */
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ] /* plan_prefix */,
                "table": "`test0816`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "rows_to_scan": 3,
                      "access_type": "range",
                      "range_details": {
                        "used_index": "idx_name_age_remark"
                      } /* range_details */,
                      "resulting_rows": 3,
                      "cost": 1.61,
                      "chosen": true,
                      "use_tmp_table": true
                    }
                  ] /* considered_access_paths */
                } /* best_access_path */,
                "condition_filtering_pct": 100,
                "rows_for_plan": 3,
                "cost_for_plan": 1.61,
                "sort_cost": 3,
                "new_cost_for_plan": 4.61,
                "chosen": true
              }
            ] /* considered_execution_plans */
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`test0816`.`name` > 'a')",
              "attached_conditions_computation": [
              ] /* attached_conditions_computation */,
              "attached_conditions_summary": [
                {
                  "table": "`test0816`",
                  "attached": "(`test0816`.`name` > 'a')"
                }
              ] /* attached_conditions_summary */
            } /* attaching_conditions_to_tables */
          },
          {
            "optimizing_distinct_group_by_order_by": {
              "simplifying_order_by": {
                "original_clause": "`test0816`.`remark`",
                "items": [
                  {
                    "item": "`test0816`.`remark`"
                  }
                ] /* items */,
                "resulting_clause_is_simple": true,
                "resulting_clause": "`test0816`.`remark`"
              } /* simplifying_order_by */
            } /* optimizing_distinct_group_by_order_by */
          },
          {
            "reconsidering_access_paths_for_index_ordering": {
              "clause": "ORDER BY",
              "steps": [
              ] /* steps */,
              "index_order_summary": {
                "table": "`test0816`",
                "index_provides_order": false,
                "order_direction": "undefined",
                "index": "idx_name_age_remark",
                "plan_changed": false
              } /* index_order_summary */
            } /* reconsidering_access_paths_for_index_ordering */
          },
          {
            "finalizing_table_conditions": [
              {
                "table": "`test0816`",
                "original_table_condition": "(`test0816`.`name` > 'a')",
                "final_table_condition   ": "(`test0816`.`name` > 'a')"
              }
            ] /* finalizing_table_conditions */
          },
          {
            "refine_plan": [
              {
                "table": "`test0816`",
                "pushed_index_condition": "(`test0816`.`name` > 'a')",
                "table_condition_attached": null
              }
            ] /* refine_plan */
          },
          {
            "considering_tmp_tables": [
              {
                "adding_sort_to_table": "test0816"
              } /* filesort */
            ] /* considering_tmp_tables */
          }
        ] /* steps */
      } /* join_optimization */
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
          {
            "sorting_table": "test0816",
            "filesort_information": [
              {
                "direction": "asc",
                "expression": "`test0816`.`remark`"
              }
            ] /* filesort_information */,
            "filesort_priority_queue_optimization": {
              "usable": false,
              "cause": "not applicable (no LIMIT)"
            } /* filesort_priority_queue_optimization */,
            "filesort_execution": [
            ] /* filesort_execution */,
            "filesort_summary": {
              "memory_available": 262144,
              "key_size": 400,
              "row_size": 1091,
              "max_rows_per_buffer": 15,
              "num_rows_estimate": 15,
              "num_rows_found": 3,
              "num_initial_chunks_spilled_to_disk": 0,
              "peak_memory_used": 32800,
              "sort_algorithm": "std::sort",
              "sort_mode": ""
            } /* filesort_summary */
          }
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}

一共是3个阶段:

  • join_preparation:sql准备阶段,sql格式化;
  • join_optimization: sql分析优化阶段,是分析OPTIMIZER TRACE的重点。这段一般都比较长,分很多步,需要细看;
  • join_execution: sql执行阶段;

结论:全表扫描的成本低于索引扫描,所以MySQL最终选择全表扫描。

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trace结果及各关键字详解:https://zhuanlan.zhihu.com/p/475410214?utm_id=0

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