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背景
XX实例（一主一从）xxx告警中每天凌晨在报SLA报警，该报警的意思是存在一定的主从延迟（若在此时发生主从切换，需要长时间才可以完成切换，要追延迟来保证主从数据的一致性）
XX实例的慢查询数量最多（执行时间超过1s的sql会被记录），XX应用那方每天晚上在做删除一个月前数据的任务
分析
使用pt-query-digest工具分析最近一周的mysql-slow.log
pt-query-digest --since=148h mysql-slow.log| less
结果第一部分
最近一个星期内，总共记录的慢查询执行花费时间为25403s，最大的慢sql执行时间为266s，平均每个慢sql执行时间5s，平均扫描的行数为1766万
结果第二部分
select arrival_record操作记录的慢查询数量最多有4万多次，平均响应时间为4s，delete arrival_record记录了6次，平均响应时间258s。
select xxx_record语句
select arrival_record 慢查询语句都类似于如下所示，where语句中的参数字段是一样的，传入的参数值不一样
select count(*) from arrival_record where product_id=26 and receive_time between '-03-25 14:00:00' and '-03-25 15:00:00' and receive_spend_ms>=0\G
select arrival_record 语句在mysql中最多扫描的行数为5600万、平均扫描的行数为172万，推断由于扫描的行数多导致的执行时间长
查看执行计划
explain selectcount(*) fromarrival_record whereproduct_id=26and receive_time between '-03-25 14:00:00'and '-03-25 15:00:00'and receive_spend_ms>=0\G;*************************** 1.row ***************************id: 1select_type: SIMPLEtable: arrival_recordpartitions: NULLtype: refpossible_keys: IXFK_arrival_recordkey: IXFK_arrival_recordkey_len: 8ref: constrows: 32261320filtered: 3.70Extra: Using index condition; Using where1row inset, 1warning (0.00sec)
用到了索引IXFK_arrival_record，但预计扫描的行数很多有3000多w行
show index from arrival_record;+----------------+------------+---------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+| Table |Non_unique | Key_name |Seq_in_index | Column_name |Collation | Cardinality |Sub_part | Packed |Null | Index_type |Comment | Index_comment |+----------------+------------+---------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+| arrival_record |0| PRIMARY |1| id |A | 107990720 |NULL | NULL || BTREE || || arrival_record |1| IXFK_arrival_record |1| product_id |A | 1344 |NULL | NULL || BTREE || || arrival_record |1| IXFK_arrival_record |2| station_no |A | 22161 |NULL | NULL |YES | BTREE || || arrival_record |1| IXFK_arrival_record |3| sequence |A | 77233384 |NULL | NULL || BTREE || || arrival_record |1| IXFK_arrival_record |4| receive_time |A | 65854652 |NULL | NULL |YES | BTREE || || arrival_record |1| IXFK_arrival_record |5| arrival_time |A | 73861904 |NULL | NULL |YES | BTREE || |+----------------+------------+---------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+show create table arrival_record;..........arrival_spend_ms bigint(20) DEFAULT NULL,total_spend_ms bigint(20) DEFAULT NULL,PRIMARY KEY (id),KEY IXFK_arrival_record (product_id,station_no,sequence,receive_time,arrival_time) USING BTREE,CONSTRAINT FK_arrival_record_product FOREIGN KEY (product_id) REFERENCES product (id) ON DELETE NO ACTION ON UPDATE NO ACTION) ENGINE=InnoDB AUTO_INCREMENT=614538979DEFAULT CHARSET=utf8 COLLATE=utf8_bin |
该表总记录数约1亿多条，表上只有一个复合索引，product_id字段基数很小，选择性不好
传入的过滤条件 where product_id=26 and receive_time between '-03-25 14:00:00' and '-03-25 15:00:00' and receive_spend_ms>=0 没有station_nu字段，使用不到复合索引 IXFK_arrival_record的product_id,station_no,sequence,receive_time这几个字段
根据最左前缀原则，select arrival_record只用到了复合索引IXFK_arrival_record的第一个字段product_id，而该字段选择性很差，导致扫描的行数很多，执行时间长
receive_time字段的基数大，选择性好，可对该字段单独建立索引，select arrival_record sql就会使用到该索引
现在已经知道了在慢查询中记录的select arrival_record where语句传入的参数字段有 product_id，receive_time，receive_spend_ms，还想知道对该表的访问有没有通过其它字段来过滤了？
神器tcpdump出场的时候到了
使用tcpdump抓包一段时间对该表的select语句
tcpdump -i bond0 -s 0-l-w- dst port 3316| strings | grepselect | egrep -i 'arrival_record'>/tmp/select_arri.log
获取select 语句中from 后面的where条件语句
IFS_OLD=$IFSIFS=$'\n'fori in`cat /tmp/select_arri.log `;doecho${i#*'from'}; done| lessIFS=$IFS_OLD
arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=17andarrivalrec0_.station_no='56742'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=22andarrivalrec0_.station_no='S7100'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=24andarrivalrec0_.station_no='V4631'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=22andarrivalrec0_.station_no='S9466'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=24andarrivalrec0_.station_no='V4205'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=24andarrivalrec0_.station_no='V4105'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=24andarrivalrec0_.station_no='V4506'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=24andarrivalrec0_.station_no='V4617'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=22andarrivalrec0_.station_no='S8356'arrival_record arrivalrec0_ where arrivalrec0_.sequence='-03-27 08:40'andarrivalrec0_.product_id=22andarrivalrec0_.station_no='S8356'select 该表 where条件中有product_id,station_no,sequence字段，可以使用到复合索引IXFK_arrival_record的前三个字段
综上所示，优化方法为，删除复合索引IXFK_arrival_record，建立复合索引idx_sequence_station_no_product_id，并建立单独索引indx_receive_time
delete xxx_record语句
该delete操作平均扫描行数为1.1亿行，平均执行时间是262s
delete语句如下所示，每次记录的慢查询传入的参数值不一样
deletefromarrival_record wherereceive_time < STR_TO_DATE('-02-23', '%Y-%m-%d')\G
执行计划
explainselect* fromarrival_record wherereceive_time < STR_TO_DATE('-02-23', '%Y-%m-%d')\G*************************** 1.row***************************id: 1select_type: SIMPLEtable: arrival_recordpartitions: NULLtype: ALLpossible_keys: NULLkey: NULLkey_len: NULLref: NULLrows: 109501508filtered: 33.33Extra: Usingwhere1rowinset, 1warning(0.00sec)
该delete语句没有使用索引（没有合适的索引可用），走的全表扫描，导致执行时间长
优化方法也是 建立单独索引indx_receive_time(receive_time)
测试
拷贝arrival_record表到测试实例上进行删除重新索引操作
XX实例arrival_record表信息
du -sh /datas/mysql/data/3316/cq_new_cimiss/arrival_record*12K /datas/mysql/data/3316/cq_new_cimiss/arrival_record.frm48G /datas/mysql/data/3316/cq_new_cimiss/arrival_record.ibdselect count() from cq_new_cimiss.arrival_record;+-----------+| count() |+-----------+| 112294946 |+-----------+1亿多记录数SELECTtable_name,CONCAT(FORMAT(SUM(data_length) / 1024/ 1024,2),'M') AS dbdata_size,CONCAT(FORMAT(SUM(index_length) / 1024/ 1024,2),'M') AS dbindex_size,CONCAT(FORMAT(SUM(data_length + index_length) / 1024/ 1024/ 1024,2),'G') AS table_size(G),AVG_ROW_LENGTH,table_rows,update_timeFROMinformation_schema.tablesWHERE table_schema = 'cq_new_cimiss'andtable_name='arrival_record';+----------------+-------------+--------------+------------+----------------+------------+---------------------+| table_name |dbdata_size | dbindex_size |table_size(G) | AVG_ROW_LENGTH |table_rows | update_time |+----------------+-------------+--------------+------------+----------------+------------+---------------------+| arrival_record |18,268.02M | 13,868.05M |31.38G | 175 |109155053| -03-26 12:40:17 |+----------------+-------------+--------------+------------+----------------+------------+---------------------+
磁盘占用空间48G，mysql中该表大小为31G，存在17G左右的碎片，大多由于删除操作造成的（记录被删除了，空间没有回收)
备份还原该表到新的实例中，删除原来的复合索引，重新添加索引进行测试
mydumper并行压缩备份
user=rootpasswd=xxxxsocket=/datas/mysql/data/3316/mysqld.sockdb=cq_new_cimisstable_name=arrival_recordbackupdir=/datas/dump_$table_namemkdir -p $backupdirnohup echo `date +%T`&& mydumper -u $user -p $passwd -S $socket -B $db -c -T $table_name -o $backupdir -t 32-r 2000000&& echo `date +%T`&
并行压缩备份所花时间（52s）和占用空间（1.2G，实际该表占用磁盘空间为48G，mydumper并行压缩备份压缩比相当高！）
Starteddumpat: -03-2612:46:04........Finisheddumpat: -03-2612:46:56du-sh/datas/dump_arrival_record/1.2G/datas/dump_arrival_record/
拷贝dump数据到测试节点
scp -rp /datas/dump_arrival_record root@10.230.124.19:/datas
多线程导入数据
time myloader -u root -S /datas/mysql/data/3308/mysqld.sock -P 3308 -p root -B test-d /datas/dump_arrival_record -t 32
real 126m42.885s
user 1m4.543s
sys 0m4.267s
逻辑导入该表后磁盘占用空间
du -h -d 1 /datas/mysql/data/3308/test/arrival_record.*12K /datas/mysql/data/3308/test/arrival_record.frm30G /datas/mysql/data/3308/test/arrival_record.ibd没有碎片，和mysql的该表的大小一致
cp-rp /datas/mysql/data/3308/datas
分别使用online DDL和 pt-osc工具来做删除重建索引操作
先删除外键，不删除外键，无法删除复合索引，外键列属于复合索引中第一列
nohup bash /tmp/ddl_index.sh &-04-04-10:41:39 beginstopmysqld_3308-04-04-10:41:41beginrm -rf datadir andcp -rp datadir_bak-04-04-10:46:53startmysqld_3308-04-04-10:46:59onlineddlbegin-04-04-11:20:34onlie ddlstop-04-04-11:20:34beginstopmysqld_3308-04-04-11:20:36beginrm -rf datadir andcp -rp datadir_bak-04-04-11:22:48startmysqld_3308-04-04-11:22:53pt-osc begin-04-04-12:19:15pt-osc stoponlineddl花费时间为34分钟，pt-osc花费时间为57分钟，使用onlne ddl时间约为pt-osc工具时间的一半
做DDL 参考
实施
由于是一主一从实例，应用是连接的vip，删除重建索引采用online ddl来做。停止主从复制后，先在从实例上做（不记录binlog），主从切换，再在新切换的从实例上做（不记录binlog)
function red_echo () {local what="$*"echo -e "$(date +%F-%T) ${what}"}function check_las_comm(){if [ "$1" != "0" ];thenred_echo "$2"echo "exit 1"exit 1fi}red_echo "stop slave"mysql -uroot -p$passwd --socket=/datas/mysql/data/${port}/mysqld.sock -e"stop slave"check_las_comm "$?" "stop slave failed"red_echo "online ddl begin"mysql -uroot -p$passwd --socket=/datas/mysql/data/${port}/mysqld.sock -e"set sql_log_bin=0;select now() as ddl_start;ALTER TABLE $db_.\`${table_name}\` DROP FOREIGN KEY FK_arrival_record_product,drop index IXFK_arrival_record,add index idx_product_id_sequence_station_no(product_id,sequence,station_no),add index idx_receive_time(receive_time);select now() as ddl_stop" >>${log_file} 2>& 1red_echo "onlie ddl stop"red_echo "add foreign key"mysql -uroot -p$passwd --socket=/datas/mysql/data/${port}/mysqld.sock -e"set sql_log_bin=0;ALTER TABLE $db_.${table_name} ADD CONSTRAINT _FK_${table_name}_product FOREIGN KEY (product_id) REFERENCES cq_new_cimiss.product (id) ON DELETE NO ACTION ON UPDATE NO ACTION;" >>${log_file} 2>& 1check_las_comm "$?" "add foreign key error"red_echo "add foreign key stop"red_echo "start slave"mysql -uroot -p$passwd --socket=/datas/mysql/data/${port}/mysqld.sock -e"start slave"check_las_comm "$?" "start slave failed"
执行时间
-04-08-11:17:36 stop slave
mysql: [Warning] Using a password on the command line interface can be insecure.
ddl_start
-04-0811:17:36
ddl_stop
-04-0811:45:13
-04-08-11:45:13 onlie ddl stop
-04-08-11:45:13add foreign key
mysql: [Warning] Using a password on the command line interface can be insecure.
-04-08-12:33:48 add foreign key stop
-04-08-12:33:48start slave
再次查看delete 和select语句的执行计划
explain selectcount(*) fromarrival_record wherereceive_time < STR_TO_DATE('-03-10', '%Y-%m-%d')\G*************************** 1. row ***************************id: 1select_type: SIMPLEtable: arrival_recordpartitions: NULLtype: rangepossible_keys: idx_receive_timekey: idx_receive_timekey_len: 6ref: NULLrows: 7540948filtered: 100.00Extra: Using where; Using indexexplain selectcount(*) fromarrival_record whereproduct_id=26and receive_time between '-03-25 14:00:00'and '-03-25 15:00:00'and receive_spend_ms>=0\G;*************************** 1.row ***************************id: 1select_type: SIMPLEtable: arrival_recordpartitions: NULLtype: rangepossible_keys: idx_product_id_sequence_station_no,idx_receive_timekey: idx_receive_timekey_len: 6ref: NULLrows: 291448filtered: 16.66Extra: Using index condition; Using where都使用到了idx_receive_time 索引，扫描的行数大大降低
索引优化后
delete 还是花费了77s时间
deletefromarrival_record wherereceive_time < STR_TO_DATE('-03-10', '%Y-%m-%d')\G
delete 语句通过receive_time的索引删除300多万的记录花费77s时间*
delete大表优化为小批量删除
应用端已优化成每次删除10分钟的数据（每次执行时间1s左右），xxx中没在出现SLA（主从延迟告警）
另一个方法是通过主键的顺序每次删除20000条记录
#得到满足时间条件的最大主键ID#通过按照主键的顺序去 顺序扫描小批量删除数据#先执行一次以下语句SELECT MAX(id) INTO @need_delete_max_id FROM `arrival_record` WHERE receive_time<'-03-01' ;DELETE FROM arrival_record WHERE id<@need_delete_max_id LIMIT 20000;select ROW_COUNT(); #返回20000#执行小批量delete后会返回row_count(), 删除的行数#程序判断返回的row_count()是否为0，不为0执行以下循环，为0退出循环，删除操作完成DELETE FROM arrival_record WHERE id<@need_delete_max_id LIMIT 20000;select ROW_COUNT();#程序睡眠0.5s
总结
表数据量太大时，除了关注访问该表的响应时间外，还要关注对该表的维护成本（如做DDL表更时间太长，delete历史数据）。
对大表进行DDL操作时，要考虑表的实际情况（如对该表的并发表，是否有外键）来选择合适的DDL变更方式。
对大数据量表进行delete，用小批量删除的方式，减少对主实例的压力和主从延迟。
作者：jia-xin
原文：/YangJiaXin/p/10828244.html
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