使用子查询在mysql基础上删除非常慢

Question

这个mysql查询运行了大约10个小时,但还没有完成.有些事情是可怕的错误.

这里有两个表(文本和垃圾邮件).垃圾邮件将垃圾邮件的ID存储在我要删除的文本中.

DELETE FROM tname.text WHERE old_id IN (SELECT textid FROM spam);

垃圾邮件只有2列,都是整数.800K条目的文件大小为几Mbs.两个int都是主键.

文本有3列.id(prim键),文本,标志.大约1200K条目,大约2.1千兆字节(大多数垃圾邮件).

服务器是一个至强四核,2千兆字节的ram(不要问我为什么).只有apache(为什么？)和mysqld正在运行.它是一个旧的免费bsd和mysql 4.1.2(不要问我为什么)

主题:6个问题:188805慢查询:318打开:810刷新表:1打开表:157每秒查询数:7.532

Mysql my.cnf:

[mysqld]
datadir=/usr/local/mysql
log-error=/usr/local/mysql/mysqld.err
pid-file=/usr/local/mysql/mysqld.pid
tmpdir=/var/tmp
innodb_data_home_dir =
innodb_log_files_in_group = 2
join_buffer_size=2M
key_buffer_size=32M
max_allowed_packet=1M
max_connections=800
myisam_sort_buffer_size=32M
query_cache_size=8M
read_buffer_size=2M
sort_buffer_size=2M
table_cache=256
skip-bdb
log-slow-queries = slow.log
long_query_time = 1

#skip-innodb
#default-table-type=innodb
innodb_data_file_path = /usr/local/mysql/ibdata1:10M:autoextend
innodb_log_group_home_dir = /usr/local/mysql/
innodb_buffer_pool_size = 128M
innodb_log_file_size = 16M
innodb_log_buffer_size = 8M
#innodb_flush_log_at_trx_commit=1
#innodb_additional_mem_pool_size=1M
#innodb_lock_wait_timeout=50

log-bin
server-id=201

[isamchk]
key_buffer_size=128M
read_buffer_size=128M
write_buffer_size=128M
sort_buffer_size=128M

[myisamchk]
key_buffer_size=128M[server:~] dmesg | grep memory
real memory  = 2146828288 (2047 MB)
avail memory = 2095534080 (1998 MB)

read_buffer_size=128M
write_buffer_size=128M
sort_buffer_size=128M
tmpdir=/var/tmp

查询只使用一个cpu,top表示25%的cpu时间(所以4个中的1个).

real memory  = 2146828288 (2047 MB)
avail memory = 2095534080 (1998 MB)

62 processes:  2 running, 60 sleeping
CPU states: 25.2% user,  0.0% nice,  1.6% system,  0.0% interrupt, 73.2% idle
Mem: 244M Active, 1430M Inact, 221M Wired, 75M Cache, 112M Buf, 31M Free
Swap: 4096M Total, 1996K Used, 4094M Free

  PID USERNAME     THR PRI NICE   SIZE    RES STATE  C   TIME   WCPU COMMAND
11536 mysql         27  20    0   239M   224M kserel 3 441:16 94.29% mysqld

知道怎么解决吗？

Answer 1

根据我的经验,子查询通常是SQL语句执行时间慢的原因,因此我尽量避免它们.试试这个:

DELETE tname FROM tname INNER JOIN spam ON (tname.old_id = spam.textid);

免责声明:此查询未经过测试,请先备份!:-)

Answer 2

你的选择where id in (select ...)总是表现不佳.

相反,使用一个非常有效的普通连接:

DELETE `text` 
FROM spam
join `text` on `text`.old_id = spam.textid;

首先注意从垃圾邮件中选择,然后加入文本,这将提供最佳性能.