10个你必须掌握的顶级MySQL分析技巧

-- 1. 开启慢查询记录
SET GLOBAL slow_query_log = ON;
SET GLOBAL long_query_time = 1;  -- 超过1秒记录
SET GLOBAL log_queries_not_using_indexes = ON;

-- 2. 从performance_schema获取慢查询（MySQL 5.6+）
SELECT 
    DIGEST_TEXT AS query_sample,
    COUNT_STAR AS exec_count,
    SUM_TIMER_WAIT / 1000000000 AS total_secs,
    AVG_TIMER_WAIT / 1000000000 AS avg_secs,
    MAX_TIMER_WAIT / 1000000000 AS max_secs
FROM performance_schema.events_statements_summary_by_digest
ORDER BY SUM_TIMER_WAIT DESC
LIMIT 10;

-- 开启记录未使用索引的查询
SET GLOBAL log_queries_not_using_indexes = ON;

-- 通过performance_schema查找未使用索引的查询
SELECT 
    DIGEST_TEXT,
    COUNT_STAR,
    SUM_NO_INDEX_USED,
    SUM_NO_GOOD_INDEX_USED
FROM performance_schema.events_statements_summary_by_digest
WHERE SUM_NO_INDEX_USED > 0 OR SUM_NO_GOOD_INDEX_USED > 0
ORDER BY COUNT_STAR DESC;

-- 查看当前锁等待（MySQL 8.0）
SELECT 
    r.trx_id AS waiting_trx_id,
    r.trx_mysql_thread_id AS waiting_thread,
    r.trx_query AS waiting_query,
    b.trx_id AS blocking_trx_id,
    b.trx_mysql_thread_id AS blocking_thread,
    b.trx_query AS blocking_query
FROM information_schema.innodb_lock_waits w
JOIN information_schema.innodb_trx r ON w.requesting_trx_id = r.trx_id
JOIN information_schema.innodb_trx b ON w.blocking_trx_id = b.trx_id;

-- MySQL 8.0 推荐使用 performance_schema
SELECT 
    waiting_pid,
    waiting_query,
    blocking_pid,
    blocking_query
FROM sys.innodb_lock_waits;

-- 查看临时表使用统计
SELECT 
    DIGEST_TEXT,
    COUNT_STAR,
    SUM_CREATED_TMP_TABLES,
    SUM_CREATED_TMP_DISK_TABLES,
    ROUND(SUM_CREATED_TMP_DISK_TABLES / SUM_CREATED_TMP_TABLES * 100, 2) AS disk_tmp_ratio
FROM performance_schema.events_statements_summary_by_digest
WHERE SUM_CREATED_TMP_TABLES > 0
ORDER BY SUM_CREATED_TMP_DISK_TABLES DESC
LIMIT 10;

-- 查找未使用的索引（从sys库）
SELECT 
    table_schema,
    table_name,
    index_name,
    rows_selected,
    rows_inserted,
    rows_updated,
    rows_deleted
FROM sys.schema_unused_indexes;

-- 查找重复索引
SELECT 
    table_schema,
    table_name,
    redundant_index_name,
    dominant_index_name
FROM sys.schema_redundant_indexes;

-- 查看表碎片情况
SELECT 
    table_schema,
    table_name,
    ROUND(data_length / 1024 / 1024, 2) AS data_mb,
    ROUND(index_length / 1024 / 1024, 2) AS index_mb,
    ROUND(data_free / 1024 / 1024, 2) AS free_mb,
    ROUND(data_free / (data_length + index_length) * 100, 2) AS fragmentation_ratio
FROM information_schema.tables
WHERE table_schema NOT IN ('mysql', 'information_schema', 'performance_schema')
  AND data_free > 0
ORDER BY fragmentation_ratio DESC;

-- 查看Buffer Pool命中率
SELECT 
    (SELECT VARIABLE_VALUE 
     FROM performance_schema.global_status 
     WHERE VARIABLE_NAME = 'Innodb_buffer_pool_read_requests') AS read_requests,
    (SELECT VARIABLE_VALUE 
     FROM performance_schema.global_status 
     WHERE VARIABLE_NAME = 'Innodb_buffer_pool_reads') AS physical_reads,
    ROUND((1 - (SELECT VARIABLE_VALUE 
                 FROM performance_schema.global_status 
                 WHERE VARIABLE_NAME = 'Innodb_buffer_pool_reads') /
          (SELECT VARIABLE_VALUE 
           FROM performance_schema.global_status 
           WHERE VARIABLE_NAME = 'Innodb_buffer_pool_read_requests')) * 100, 2) AS hit_rate;

-- 查找高排序量的查询
SELECT 
    DIGEST_TEXT,
    COUNT_STAR AS exec_count,
    SUM_SORT_ROWS AS total_sorted_rows,
    AVG_SORT_ROWS AS avg_sorted_rows,
    SUM_SORT_MERGE_PASSES AS merge_passes
FROM performance_schema.events_statements_summary_by_digest
WHERE SUM_SORT_ROWS > 100000
ORDER BY SUM_SORT_ROWS DESC
LIMIT 10;

-- 查看当前连接状态
SELECT 
    command,
    COUNT(*) AS count,
    ROUND(COUNT(*) / (SELECT COUNT(*) FROM information_schema.processlist) * 100, 2) AS percentage
FROM information_schema.processlist
GROUP BY command;

-- 查看历史连接统计
SHOW STATUS LIKE 'Threads_%';
SHOW STATUS LIKE 'Max_used_connections';

-- 检查是否超过max_connections阈值
SELECT 
    @@max_connections AS max_conn,
    (SELECT VARIABLE_VALUE FROM performance_schema.global_status WHERE VARIABLE_NAME = 'Max_used_connections') AS max_used,
    ROUND((SELECT VARIABLE_VALUE FROM performance_schema.global_status WHERE VARIABLE_NAME = 'Max_used_connections') / @@max_connections * 100, 2) AS usage_pct;

-- 从历史统计中找高成本JOIN查询
SELECT 
    DIGEST_TEXT,
    COUNT_STAR,
    SUM_ROWS_EXAMINED AS total_rows_examined,
    SUM_ROWS_SENT AS total_rows_sent,
    ROUND(SUM_ROWS_EXAMINED / SUM_ROWS_SENT, 2) AS rows_examined_per_row_sent,
    SUM_TIMER_WAIT / 1000000000 AS total_secs
FROM performance_schema.events_statements_summary_by_digest
WHERE DIGEST_TEXT LIKE '%JOIN%'
  AND SUM_ROWS_EXAMINED / NULLIF(SUM_ROWS_SENT, 0) > 1000  -- 扫描行数/返回行数比例过高
ORDER BY rows_examined_per_row_sent DESC
LIMIT 10;

-- 创建每日性能检查的存储过程示例
DELIMITER $$
CREATE PROCEDURE daily_performance_check()
BEGIN
    -- 1. 检查top10慢查询
    SELECT 'Top 10 Slow Queries' AS check_item;
    CALL top_slow_queries();
    
    -- 2. 检查高碎片表
    SELECT 'High Fragmentation Tables' AS check_item;
    CALL check_fragmentation();
    
    -- 3. 检查未使用索引
    SELECT 'Unused Indexes' AS check_item;
    CALL check_unused_indexes();
    
    -- 4. 检查锁等待
    SELECT 'Lock Waits' AS check_item;
    CALL check_lock_waits();
END$$
DELIMITER ;

ANALYZE TABLE your_table;