Mysql如何选择合适的索引
    mysql> EXPLAIN select from employees where name > ‘a’;
    image.png
    如果用name索引需要遍历name字段联合索引树，然后还需要根据遍历出来的主键值去主键索引树里再去查出最终数据，成本比全表扫描还高，可以用覆盖索引优化，这样只需要遍历name字段的联合索引树就能拿到所有结果，如下：
    mysql> EXPLAIN select name,age,position from employees where name > ‘a’ ;
    image.png
    mysql> EXPLAIN select     from employees where name > ‘zzz’ ;
    image.png
    对于上面这两种 name>’a’ 和 name>’zzz’ 的执行结果，mysql最终是否选择走索引或者一张表涉及多个索引，mysql最终如何选择索引，我们可以用trace工具来一查究竟，开启trace工具会影响mysql性能，所以只能临时分析sql使用，用完之后立即关闭
    trace工具用法：
    mysql> set session optimizer_trace=”enabled=on”,end_markers_in_json=on; —开启trace
    mysql> select from employees where name > ‘a’ order by position;
    mysql> SELECT     FROM information_schema.OPTIMIZER_TRACE;

    查看trace字段：
    { “steps”: [
    {
    “join_preparation”: { —第一阶段：SQL准备阶段，格式化sql
    “select#”: 1,
    “steps”: [
    {
    “expanded_query”: “/ select#1 / select employees.id AS id,employees.name AS name,employees.age AS age,employees.position AS position,employees.hire_time AS hire_time from employees where (employees.name > ‘a’) order by employees.position“
    }
    ] / steps /
    } / join_preparation /
    },
    {
    “join_optimization”: { —第二阶段：SQL优化阶段
    “select#”: 1,
    “steps”: [
    {
    “condition_processing”: { —条件处理
    “condition”: “WHERE”,
    “original_condition”: “(employees.name > ‘a’)”,
    “steps”: [
    {
    “transformation”: “equality_propagation”,
    “resulting_condition”: “(employees.name > ‘a’)”
    },
    {
    “transformation”: “constant_propagation”,
    “resulting_condition”: “(employees.name > ‘a’)”
    },
    {
    “transformation”: “trivial_condition_removal”,
    “resulting_condition”: “(employees.name > ‘a’)”
    }
    ] / steps /
    } / condition_processing /
    },
    {
    “substitute_generated_columns”: {
    } / substitute_generated_columns /
    },
    {
    “table_dependencies”: [ —表依赖详情
    {
    “table”: “employees“,
    “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”: [ —预估表的访问成本
    {
    “table”: “employees“,
    “range_analysis”: {
    “table_scan”: { —全表扫描情况
    “rows”: 10123, —扫描行数
    “cost”: 2054.7 —查询成本
    } / table_scan /,
    “potential_range_indexes”: [ —查询可能使用的索引
    {
    “index”: “PRIMARY”, —主键索引
    “usable”: false,
    “cause”: “not_applicable”
    },
    {
    “index”: “idx_name_age_position”, —辅助索引
    “usable”: true,
    “key_parts”: [
    “name”,
    “age”,
    “position”,
    “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 /,
    “analyzing_range_alternatives”: { —分析各个索引使用成本
    “range_scan_alternatives”: [
    {
    “index”: “idx_name_age_position”,
    “ranges”: [
    “a < name” —索引使用范围
    ] / ranges /,
    “index_dives_for_eq_ranges”: true,
    “rowid_ordered”: false, —使用该索引获取的记录是否按照主键排序
    “using_mrr”: false,
    “index_only”: false, —是否使用覆盖索引
    “rows”: 5061, —索引扫描行数
    “cost”: 6074.2, —索引使用成本
    “chosen”: false, —是否选择该索引
    “cause”: “cost”
    }
    ] / range_scan_alternatives /,
    “analyzing_roworder_intersect”: {
    “usable”: false,
    “cause”: “too_few_roworder_scans”
    } / analyzing_roworder_intersect /
    } / analyzing_range_alternatives /
    } / range_analysis /
    }
    ] / rows_estimation /
    },
    {
    “considered_execution_plans”: [
    {
    “plan_prefix”: [
    ] / plan_prefix /,
    “table”: “employees“,
    “best_access_path”: { —最优访问路径
    “considered_access_paths”: [ —最终选择的访问路径
    {
    “rows_to_scan”: 10123,
    “access_type”: “scan”, —访问类型：为scan，全表扫描
    “resulting_rows”: 10123,
    “cost”: 2052.6,
    “chosen”: true, —确定选择
    “use_tmp_table”: true
    }
    ] / considered_access_paths /
    } / best_access_path /,
    “condition_filtering_pct”: 100,
    “rows_for_plan”: 10123,
    “cost_for_plan”: 2052.6,
    “sort_cost”: 10123,
    “new_cost_for_plan”: 12176,
    “chosen”: true
    }
    ] / considered_execution_plans /
    },
    {
    “attaching_conditions_to_tables”: {
    “original_condition”: “(employees.name > ‘a’)”,
    “attached_conditions_computation”: [
    ] / attached_conditions_computation /,
    “attached_conditions_summary”: [
    {
    “table”: “employees“,
    “attached”: “(employees.name > ‘a’)”
    }
    ] / attached_conditions_summary /
    } / attaching_conditions_to_tables /
    },
    {
    “clause_processing”: {
    “clause”: “ORDER BY”,
    “original_clause”: “employees.position“,
    “items”: [
    {
    “item”: “employees.position“
    }
    ] / items /,
    “resulting_clause_is_simple”: true,
    “resulting_clause”: “employees.position“
    } / clause_processing /
    },
    {
    “reconsidering_access_paths_for_index_ordering”: {
    “clause”: “ORDER BY”,
    “steps”: [
    ] / steps /,
    “index_order_summary”: {
    “table”: “employees“,
    “index_provides_order”: false,
    “order_direction”: “undefined”,
    “index”: “unknown”,
    “plan_changed”: false
    } / index_order_summary /
    } / reconsidering_access_paths_for_index_ordering /
    },
    {
    “refine_plan”: [
    {
    “table”: “employees“
    }
    ] / refine_plan /
    }
    ] / steps /
    } / join_optimization /
    },
    {
    “join_execution”: { —第三阶段：SQL执行阶段
    “select#”: 1,
    “steps”: [
    ] / steps /
    } / join_execution /
    }
    ] / steps /
    }

    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
    mysql> select from employees where name > ‘zzz’ order by position;
    mysql> SELECT     FROM information_schema.OPTIMIZER_TRACE;

    查看trace字段可知索引扫描的成本低于全表扫描，所以mysql最终选择索引扫描
    mysql> set session optimizer_trace=”enabled=off”; —关闭trace