Mysql之 optimizer_trace 相关总结_optimizer trace在mysql中的用法

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

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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。

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2. 使用方法

1. 查看optimizer trace配置

show variables like '%optimizer_trace%';
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查询结果：

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

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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";
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3. 执行需要进行分析的SQL语句

select * from test0816 where name > 'a' order by remark;
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4. 使用optimizer trace查看优化器的选择过程

SELECT * FROM information_schema.OPTIMIZER_TRACE;
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查询结果：

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

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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": "<fixed_sort_key, packed_additional_fields>"
            } /* filesort_summary */
          }
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}
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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