# 常见 DB 基础细节

## Mongo

* cmds

  ```
    db.happygame_sdk.test.getIndexes();
    db.happygame_sdk.test.explain("executionStats").find({'playerid':'ad3e5eb8-a03c-5011-8f2d-ce32c2a7ab31'});
  ```

## Mysql

* 了解数据库的一些基本理论知识：
  * 数据的存储格式 (堆组织表 vs 聚簇索引表)
  * 并发控制协议 (MVCC vs Lock-Based CC)
  * Two-Phase Locking
  * 数据库的隔离级别定义 (Isolation Level)
* 了解SQL本身的执行计划
  * 主键扫描 vs 唯一键扫描 vs 范围扫描 vs 全表扫描
  * [innodb index types](https://dev.mysql.com/doc/refman/8.0/en/innodb-index-types.html)
* 了解数据库本身的一些实现细节
  * 过滤条件提取
    * Index Key/Index Filter/Table Filter
    * [where条件提取与应用浅析](http://hedengcheng.com/?p=577)
  * Index Condition Pushdown
  * Semi-Consistent Read
* 查看数据库空间占用：

  ```
    SELECT CONCAT(table_schema,'.',table_name) AS 'Table Name', 
        CONCAT(ROUND(table_rows/1000000,4),'M') AS 'Number of Rows', 
        CONCAT(ROUND(data_length/(1024*1024*1024),4),'G') AS 'Data Size', 
        CONCAT(ROUND(index_length/(1024*1024*1024),4),'G') AS 'Index Size',
        CONCAT(ROUND((data_length+index_length)/(1024*1024*1024),4),'G') AS'Total'
    FROM information_schema.TABLES WHERE table_schema LIKE 'weixin_game' order by Total desc;
  ```
* 锁
  * 乐观锁、悲观锁(读锁、互斥锁)
  * mysql 死锁分析
    * [Mysql查询语句使用select.. for update导致的数据库死锁分析](https://www.cnblogs.com/Lawson/p/5008741.html)


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