[深入理解MySQL系列] - sort_buffer

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

1.1 利用InnoDB的主键进行排序

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。 待续......

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