MySQL优化器追踪(Optimizer Trace)的使用小结

-- 开启 Optimizer Trace
SET optimizer_trace="enabled=on";
 
-- 创建测试表
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    department_id INT,
    INDEX idx_dept(department_id)
);
 
INSERT INTO employees VALUES
(1,'Alice',1),
(2,'Bob',2),
(3,'Charlie',1);
 
-- 执行简单查询
SELECT * FROM employees WHERE department_id = 1;
 
-- 查看 Optimizer Trace
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE\G
 
-- 关闭 Optimizer Trace
SET optimizer_trace="enabled=off";

{
  "trace": [
    {
      "step": 1,
      "phase": "join_preparation",
      "table": "employees",
      "access_method": "ALL",
      "rows": 3
    },
    {
      "step": 2,
      "phase": "join_optimization",
      "join_order": ["employees"],
      "cost": 0.03,
      "chosen_access": "index(idx_dept)"
    },
    {
      "step": 3,
      "phase": "final_plan",
      "plan": [
        {
          "table": "employees",
          "access_method": "index",
          "key": "idx_dept",
          "rows_examined": 2
        }
      ]
    }
  ]
}

-- 开启 Trace
SET optimizer_trace='enabled=on';
 
-- 创建测试表
CREATE TABLE departments (
    id INT PRIMARY KEY,
    name VARCHAR(50)
);
 
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    department_id INT,
    INDEX idx_dept(department_id)
);
 
INSERT INTO departments VALUES (1,'HR'),(2,'IT');
INSERT INTO employees VALUES (1,'Alice',1),(2,'Bob',2),(3,'Charlie',1);
 
-- 执行多表 JOIN 查询
SELECT e.name, d.name
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.id = 1;
 
-- 查看 Trace
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE\G
 
-- 关闭 Trace
SET optimizer_trace='off';

-- 开启 Trace
SET optimizer_trace='enabled=on';
 
-- 关闭 Trace
SET optimizer_trace='enabled=off';

SET optimizer_trace='{
    "enabled": true,
    "trace_objects": "all",
    "max_elements": 1000
}';

SET optimizer_trace_features='all'; 

SET max_optimizer_trace_elements=2000; 

-- 创建测试表
CREATE TABLE test_employees (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    dept_id INT,
    INDEX idx_dept(dept_id)
);
 
INSERT INTO test_employees VALUES (1,'Alice',1),(2,'Bob',2),(3,'Charlie',1);
 
-- 开启 Trace 并设置输出全部详细信息
SET optimizer_trace='enabled=on';
SET optimizer_trace_features='all';
SET max_optimizer_trace_elements=5000;
 
-- 执行查询
SELECT * FROM test_employees WHERE dept_id=1;
 
-- 查看 Trace
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE\G
 
-- 调整参数只输出 basic
SET optimizer_trace_features='basic';
 
-- 再次执行查询查看差异
SELECT * FROM test_employees WHERE dept_id=1;
 
-- 关闭 Trace
SET optimizer_trace='enabled=off';

SET optimizer_trace='enabled=on';

SET optimizer_trace_features='all';  -- 输出所有信息，包括 I/O 和内存估算
SET max_optimizer_trace_elements=5000;  -- 避免复杂查询输出被截断

SET optimizer_trace='enabled=off';

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    department_id INT,
    INDEX idx_dept(department_id)
);
 
CREATE TABLE departments (
    id INT PRIMARY KEY,
    name VARCHAR(50)
);
 
INSERT INTO departments VALUES (1,'HR'),(2,'IT');
INSERT INTO employees VALUES (1,'Alice',1),(2,'Bob',2),(3,'Charlie',1);
 
-- 执行复杂 JOIN 查询
SELECT e.name, d.name
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.id = 1;

SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE\G; 

EXPLAIN SELECT e.name, d.name
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.id = 1\G;

-- 查询未使用索引
SELECT * FROM employees WHERE department_id=1;
 
-- Trace 显示 access_method=ALL（全表扫描）
-- 分析可能原因：
-- 1. 索引统计信息不准确
-- 2. 表行数过少，优化器认为全表扫描成本低
-- 调整统计信息
ANALYZE TABLE employees;
 
-- 再次执行查询，观察 Trace 输出是否选择 idx_dept 索引

SELECT * FROM employees WHERE department_id = 1; 

{
  "trace": [
    {
      "phase": "join_preparation",
      "table": "employees",
      "access_method": "ALL",
      "rows": 3
    },
    {
      "phase": "final_plan",
      "plan": [
        {
          "table": "employees",
          "access_method": "ALL",
          "rows_examined": 3
        }
      ]
    }
  ]
}

-- 更新表统计信息
ANALYZE TABLE employees;
 
-- 再次执行查询，观察 Trace 输出
SELECT * FROM employees WHERE department_id=1;
 
-- 预期 Trace 输出：
-- access_method=index
-- 使用 idx_dept 索引

SELECT * FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.id IN (1,2);

{
  "phase": "join_optimization",
  "join_order": ["departments","employees"],
  "cost": 0.05,
  "chosen_access": "index(idx_dept)"
}

-- 更新表统计信息
ANALYZE TABLE employees;
ANALYZE TABLE departments;
 
-- 调整查询逻辑或提示优化器
SELECT /*+ JOIN_ORDER(d,e) */ e.name, d.name
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.id IN (1,2);

SELECT name
FROM employees
WHERE department_id IN (
    SELECT id FROM departments WHERE name='HR'
);

{
  "phase": "join_optimization",
  "subquery": {
    "type": "IN",
    "rows": 1,
    "access_method": "ALL"
  },
  "final_plan": {
    "employees": {"access_method": "index", "rows_examined": 2}
  }
}

-- 转换为 JOIN
SELECT e.name
FROM employees e
JOIN departments d ON e.department_id = d.id
WHERE d.name='HR';

SELECT department_id, COUNT(*) 
FROM employees
GROUP BY department_id
ORDER BY COUNT(*) DESC;

{
  "phase": "final_plan",
  "plan": [
    {
      "table": "employees",
      "access_method": "index",
      "using_temporary": true,
      "using_filesort": true
    }
  ]
}

-- 使用覆盖索引或调整查询
CREATE INDEX idx_dept_count ON employees(department_id);
 
-- 执行查询并观察 Trace 输出
-- 预期不再使用文件排序，性能提升