[深入理解MySQL系列]

注：本系列文章主要探讨 MySQL 内存利用以及执行计划相关的一些知识点，从而为 MySQL 优化打下更好的基础。
作/译者：叶金荣（Email: ），来源：http://imysql.cn，转载请注明作/译者和出处，并且不能用于商业用途，违者必究。
注：本系列文章主要探讨 MySQL 内存利用以及执行计划相关的一些知识点，从而为 MySQL 优化打下更好的基础。
环境说明OS: AS4U6, 2.6.9-67.0.15.ELsmp, 16G Ram, MD3000阵列, xfs文件系统
MySQL 5.1.26 - percona(innodb plugin, innodb stat, user stat, msl, show patch, acc-pslist 补丁)
MySQL 主要配置参数
default_table_type = innodb
log_slow_queries
long_query_time = 0.001
log_slow_verbosity=query_plan,innodb
innodb_data_file_path = ibdata1:1024M:autoextend
innodb_log_file_size = 400M
innodb_log_files_in_group = 3
innodb_file_per_table
innodb_file_format="Barracuda"
其他参数均为默认值，因此其他几个内存相关参数值如下：
innodb_buffer_pool_size = 8388608
join_buffer_size = 131072
key_buffer_size = 8388600
max_heap_table_size = 16777216
query_cache_size = 0
read_buffer_size = 131072
read_rnd_buffer_size = 262144
sort_buffer_size = 2097144
tmp_table_size = 16777216
以后的所有例子中，如果没有特地注明，则测试相关的表都使用 InnoDB 引擎。
1、 排序缓冲相关参数：sort_buffer_size, read_rnd_buffer_size
EXPLAIN SELECT SQL_NO_CACHE * FROM T1 WHERE ID<10000 ORDER BY ID DESC;
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
| 1 | SIMPLE | T1 | range | PRIMARY | PRIMARY | 8 | NULL | 14872 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
# Query_time: 0.207893 Lock_time: 0.000056 Rows_sent: 9999 Rows_examined: 9999
# QC_Hit: No Full_scan: No Full_join: No Tmp_table: No Tmp_table_on_disk: No
# Filesort: No Filesort_on_disk: No Merge_passes: 0
# InnoDB_IO_r_ops: 91 InnoDB_IO_r_bytes: 1490944 InnoDB_IO_r_wait: 0.083391
# InnoDB_rec_lock_wait: 0.000000 InnoDB_queue_wait: 0.000000
# InnoDB_pages_distinct: 93
SELECT SQL_NO_CACHE * FROM T1 WHERE ID<10000 ORDER BY ID DESC;
由于是针对主键/索引进行排序，因此无需使用临时表
1.2 利用 InnoDB 使用非索引字段排序EXPLAIN SELECT SQL_NO_CACHE * FROM T1 WHERE ID<10000 ORDER BY C1 DESC;
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-----------------------------+
| 1 | SIMPLE | T1 | range | PRIMARY | PRIMARY | 8 | NULL | 14872 | Using where; Using filesort |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-----------------------------+
# Query_time: 0.120879 Lock_time: 0.000023 Rows_sent: 9999 Rows_examined: 19998
# QC_Hit: No Full_scan: No Full_join: No Tmp_table: No Tmp_table_on_disk: No
# Filesort: Yes Filesort_on_disk: Yes Merge_passes: 1
# InnoDB_IO_r_ops: 0 InnoDB_IO_r_bytes: 0 InnoDB_IO_r_wait: 0.000000
# InnoDB_rec_lock_wait: 0.000000 InnoDB_queue_wait: 0.000000
# InnoDB_pages_distinct: 93
SELECT SQL_NO_CACHE * FROM T1 WHERE ID<10000 ORDER BY C1 DESC;
由于 C1 不是索引字段，因此需要额外排序，并且由于 sort_buffer 和 read_rnd_buffer 不够大，也用到了磁盘文件。
加大 sort_buffer_size，再看看
set session sort_buffer_size = 1024 * 1024 * 5;
再次执行刚才的测试，结果发生了变化。
# Query_time: 0.080727 Lock_time: 0.000030 Rows_sent: 9999 Rows_examined: 19998
# QC_Hit: No Full_scan: No Full_join: No Tmp_table: No Tmp_table_on_disk: No
# Filesort: Yes Filesort_on_disk: No Merge_passes: 0
# InnoDB_IO_r_ops: 0 InnoDB_IO_r_bytes: 0 InnoDB_IO_r_wait: 0.000000
# InnoDB_rec_lock_wait: 0.000000 InnoDB_queue_wait: 0.000000
# InnoDB_pages_distinct: 93
SELECT SQL_NO_CACHE * FROM T1 WHERE ID<10000 ORDER BY C1 DESC;
可以看到，Filesort_on_disk 变成了 No, Merge_passes 也变成了 0，表示无需使用磁盘文件，而直接在内存里排序。
1.3 加大 read_rnd_buffer_size 看看对 filesort 是否有影响EXPLAIN SELECT SQL_NO_CACHE * FROM T1 AS T1 WHERE ID<10000 ORDER BY C1 DESC;
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-----------------------------+
| 1 | SIMPLE | T1 | range | PRIMARY | PRIMARY | 8 | NULL | 14872 | Using where; Using filesort |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-----------------------------+
# Query_time: 0.103654 Lock_time: 0.000045 Rows_sent: 9999 Rows_examined: 19998
# QC_Hit: No Full_scan: No Full_join: No Tmp_table: No Tmp_table_on_disk: No
# Filesort: Yes Filesort_on_disk: Yes Merge_passes: 1
# InnoDB_IO_r_ops: 0 InnoDB_IO_r_bytes: 0 InnoDB_IO_r_wait: 0.000000
# InnoDB_rec_lock_wait: 0.000000 InnoDB_queue_wait: 0.000000
# InnoDB_pages_distinct: 93
SELECT SQL_NO_CACHE * FROM T1 AS T1 WHERE ID<10000 ORDER BY C1 DESC;
具体过程不再每次重复贴了，结果是从 1M 到 512M，发现一直没什么变化，对 filesort 没什么帮助