53 多个索引的选择发生在哪个阶段？
客户端 → 连接器 → 分析器 Parser → 优化器 Optimizer ✅ → 执行器 Executor
| 多个索引选择在哪个阶段？ | ✅ 在优化器（Optimizer）阶段完成 |
|---|---|
| 优化器怎么决定选哪个索引？ | 根据统计信息、过滤效率、排序、是否覆盖索引、执行成本等因素分析 |
| 能否查看选择过程？ | ✅ 可以，通过 EXPLAIN / optimizer_trace 查看 |
54 MySQL 支持哪些存储引擎？默认使用哪个？
mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine | Support | Comment | Transactions | XA | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| ndbcluster | NO | Clustered, fault-tolerant(容错) tables | NULL | NULL | NULL |
| CSV | YES | CSV storage engine | NO | NO | NO |
| ARCHIVE | YES | Archive storage engine | NO | NO | NO |
| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |
| ndbinfo | NO | MySQL Cluster system information storage engine | NULL | NULL | NULL |
| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |
| FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL |
| MyISAM | YES | MyISAM storage engine | NO | NO | NO |
| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |
| InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |
| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
mysql> SHOW VARIABLES LIKE '%default_storage_engine%';
+------------------------+--------+
| Variable_name | Value |
+------------------------+--------+
| default_storage_engine | InnoDB |
+------------------------+--------+
Blackhole（黑洞）存储引擎
“写得进去，查不出来” —— 所有写入（INSERT）会被接受，但不进行物理存储，查询（SELECT）永远返回空集.
| ✅ 主从复制过滤器/中继 | 主库写入数据，Blackhole 不存数据但生成 binlog，供从库同步 |
|---|
