修改MySQL表varchar类型 字段长度锁表情况分析
16lz
2021-05-17
一、修改MySQL表varchar类型 字段长度会锁表嘛？
凭借"经验"给出回答：如果字段长度超过256个字符就会锁表。
这样的回答错误 。看看MySQL 官方文档如何介绍：
Extending VARCHAR column sizeThe number of length bytes(字节) required by a VARCHAR column must remain the same. For VARCHAR columns of 0 to 255 bytes in size, one length byte is required to encode the value.For VARCHAR columns of 256 bytes in size or more, two length bytes are required. As a result, in-place ALTER TABLE only supports increasing VARCHAR column size from 0 to 255 bytes, or from 256 bytes to a greater size. In-place ALTER TABLE does not support increasing the size of a VARCHAR column from less than 256 bytes to a size equal to or greater than 256 bytes. In this case, the number of required length bytes changes from 1 to 2, which is only supported by a table copy (ALGORITHM=COPY).上面的意思如下:
字符串的字段是以字节为单位存储的，utf8 一个字符需要三个字节，utf8mb4 一个字符需要4个字节。对于小于等于255字节以内的长度可以使用一个byte 存储。大于255个字节的长度则需要使用2个byte存储。
online ddl in-place 模式(不锁表)只支持字段的字节长度从0到255之间 或者256到更大值之间变化。
如果修改字段的长度，导致字段的字节长度无法使用 1 byte表示，得使用2个byte才能表示，比如从 240 修改为 256 ，如果在默认字符集为utf8mb4的情况下，varchar(60) 修改为 varchar(64)，则DDL需要以copy模式，也即会锁表，阻塞写操作。
另外就是不支持以 in-place 方式缩小字段长度：
Decreasing VARCHAR size using in-place ALTER TABLE is not supported. Decreasing VARCHAR size requires a table copy (ALGORITHM=COPY).二、实践验证官方提供的说法
演示环境：演示的环境是MySQL5.7.22 二进制安装的主从复制。
下面演示的sql都是在主库上进行执行sql的。
测试表说明：
s_api_filter_abcpool_data表的字符集为utf8mb4，初始字段长度为20 ，80个字节，可以使用1byte表示。分别修改字符串长度为 60--->64--->128。当字段的字节数变动 跨越了256 则会锁表。
测试表结构如下：
root@tidb06 21:59: [test001]> show create table s_api_filter_abcpool_data\G*************************** 1. row *************************** Table: s_api_filter_abcpool_dataCreate Table: CREATE TABLE `s_api_filter_abcpool_data` ( `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '自增主键', `bill_business_id` bigint(20) NOT NULL DEFAULT '0' COMMENT 'API主表单据ID', `finance_core_id` bigint(20) NOT NULL DEFAULT '0' COMMENT 'Pro业务单据ID', `bill_no` varchar(100) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '业务单据号', `bill_type_code` int(10) NOT NULL DEFAULT '0' COMMENT 'Pro业务单据类型,对应api业务单据类型枚举', `bill_type` varchar(64) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '单据名称', `bill_date` varchar(128) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '', `status` tinyint(2) NOT NULL DEFAULT '0' COMMENT '状态，0=未推送，1=已推送', `is_red` tinyint(2) NOT NULL DEFAULT '0' COMMENT '是否红冲 0-否，1-是', `is_wash` tinyint(2) NOT NULL DEFAULT '0' COMMENT '是否冲销 0-否，1-是', `push_date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '推送时间', `order_no` varchar(64) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '订单号', `bill_fund_type` tinyint(2) NOT NULL DEFAULT '0' COMMENT '款项类型', `amount` decimal(10,2) NOT NULL DEFAULT '0.00' COMMENT '金额', `customer_id` int(11) NOT NULL DEFAULT '0' COMMENT '客户ID', `start_date` int(20) NOT NULL DEFAULT '0' COMMENT '起始单元日', `original_start_date` int(20) NOT NULL DEFAULT '0' COMMENT '原起始单元日', `bill_id` bigint(20) NOT NULL DEFAULT '0' COMMENT '账单ID', `original_bill_id` bigint(20) NOT NULL DEFAULT '0' COMMENT '原账单ID', `rule_id` int(20) NOT NULL DEFAULT '0' COMMENT '对应过滤器规则ID', `strategy_code` tinyint(2) NOT NULL DEFAULT '0' COMMENT '策略类型:1-过滤数据中的字段值，2-过滤数据中字段的计算，3-过滤数据中的字段值判断逾期天数-原财务pro过滤池策略，4-通过判断过滤子集的数据，过滤掉父集主表数据', `rule_name` varchar(100) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '对应过滤器名称', `document_info` mediumtext COLLATE utf8mb4_unicode_ci COMMENT 'pro推送单据完整信息', `completion_document_info` mediumtext COLLATE utf8mb4_unicode_ci COMMENT '补全后单据完整信息', `create_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间', `update_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '最后修改时间', PRIMARY KEY (`id`) USING BTREE, KEY `idx_financeCoreId` (`finance_core_id`) USING BTREE COMMENT '核心主键索引', KEY `idx_billBusinessId` (`bill_business_id`) USING BTREE COMMENT 'api业务表单据ID索引', KEY `idx_billNo` (`bill_no`) USING BTREE COMMENT '单据号索引', KEY `idx_startDate` (`start_date`) USING BTREE COMMENT '起始单元日索引', KEY `ide_bill_type_date` (`bill_type_code`,`bill_date`) USING HASH COMMENT '单据类型+操作时间', KEY `idx_create_time` (`create_time`) USING BTREE COMMENT '创建时间索引', KEY `idx_customer_id` (`customer_id`) USING BTREE COMMENT '客户ID索引', KEY `idex_billId` (`bill_id`) USING BTREE COMMENT 'billID索引') ENGINE=InnoDB AUTO_INCREMENT=1387698785715310595 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='API-过滤池存储原AB池数据表'表字符集utf8mb4 和表记录为246万行root@tidb06 22:01: [test001]> select count(*) from s_api_filter_abcpool_data;+----------+| count(*) |+----------+| 2466560 |+----------+开始验证测试:
ALGORITHM=INPLACE 方式 修改字段 bill_date 从 varchar(20) 到 varchar(52）
root@tidb06 22:57: [test001]> alter table s_api_filter_abcpool_data modify bill_date varchar(52) NOT NULL DEFAULT '', ALGORITHM=INPLACE;Query OK, 0 rows affected (3 min 7.67 sec)Records: 0 Duplicates: 0 Warnings: 0同时查看当前事务执行情况：
root@tidb06 22:59: [(none)]> select * from information_schema.innodb_trx\G*************************** 1. row *************************** trx_id: 15290 trx_state: RUNNING trx_started: 2021-05-12 22:57:23 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 0 trx_mysql_thread_id: 1905130 trx_query: alter table s_api_filter_abcpool_data modify bill_date varchar(52) NOT NULL DEFAULT '', ALGORITHM=INPLACE trx_operation_state: reading clustered index trx_tables_in_use: 1 trx_tables_locked: 0 trx_lock_structs: 0 trx_lock_memory_bytes: 1136 trx_rows_locked: 0 trx_rows_modified: 0 trx_concurrency_tickets: 0 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 01 row in set (0.00 sec)发现确实没有添加锁当前执行 SQL 的行锁数量0trx_tables_locked: 0事务保留的锁数量0trx_lock_structs: 0ALGORITHM=INPLACE 修改字段 bill_date 从 varchar(52) 到 varchar(64）：
root@tidb06 23:02: [test001]> alter table s_api_filter_abcpool_data modify bill_date varchar(64) NOT NULL DEFAULT '', ALGORITHM=INPLACE;ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.此时只能时采用ALGORITHM=copy方式来修改：此过程非常的慢
root@tidb06 23:02: [test001]> alter table s_api_filter_abcpool_data modify bill_date varchar(64) NOT NULL DEFAULT '', ALGORITHM=copy;root@tidb06 23:17: [test001]> select * from information_schema.INNODB_TRX\G*************************** 1. row *************************** trx_id: 15562 trx_state: RUNNING trx_started: 2021-05-12 23:18:59 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 6756 trx_mysql_thread_id: 1905130 trx_query: alter table s_api_filter_abcpool_data modify bill_date varchar(64) NOT NULL DEFAULT '', ALGORITHM=copy trx_operation_state: fetching rows trx_tables_in_use: 2 trx_tables_locked: 3 trx_lock_structs: 284 trx_lock_memory_bytes: 24784 trx_rows_locked: 6752 trx_rows_modified: 6472 trx_concurrency_tickets: 0 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 01 row in set (0.00 sec)同时查看当前事务执行情况：当前执行 SQL 的行锁数量3 trx_tables_locked: 3 事务保留的锁数量284trx_lock_structs: 284把字段bill_date 扩大到varchar(64)此时查看主库进程：发现在copy to tmp table
root@tidb06 23:20: [test001]> show full processlist\G*************************** 1. row *************************** Id: 37601 User: testrep Host: 172.16.0.246:60158 db: NULLCommand: Binlog Dump Time: 936618 State: Master has sent all binlog to slave; waiting for more updates Info: NULL*************************** 2. row *************************** Id: 1905130 User: root Host: localhost db: test001Command: Query Time: 1019 State: copy to tmp table Info: alter table s_api_filter_abcpool_data modify bill_date varchar(64) NOT NULL DEFAULT '', ALGORITHM=copy*************************** 3. row *************************** Id: 1906899 User: root Host: localhost db: test001Command: Query Time: 0 State: starting Info: show full processlist3 rows in set (0.00 sec)root@tidb06 23:03: [test001]> alter table s_api_filter_abcpool_data modify bill_date varchar(64) NOT NULL DEFAULT '', ALGORITHM=copy;Query OK, 2466560 rows affected (22 min 32.83 sec)Records: 2466560 Duplicates: 0 Warnings: 0此时查看从库会造成延迟：（延迟时间和主库的执行时间差不多）
root@tidb05 23:27: [(none)]> show slave status\G.................................. Seconds_Behind_Master: 51Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 172160247 Master_UUID: da9d5c3e-a8aa-11eb-9c17-00163e0eced4 Master_Info_File: mysql.slave_master_info SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: copy to tmp table Master_Retry_Count: 86400..................................root@tidb05 23:30: [(none)]> show full processlist;+---------+-------------+-----------+---------+---------+--------+----------------------------------+--------------------------------------------------------------------------------------------------------+| Id | User | Host | db | Command | Time | State | Info |+---------+-------------+-----------+---------+---------+--------+----------------------------------+--------------------------------------------------------------------------------------------------------+| 33 | system user | | NULL | Connect | 937180 | Waiting for master to send event | NULL || 34 | system user | | test001 | Connect | 1581 | copy to tmp table | alter table s_api_filter_abcpool_data modify bill_date varchar(64) NOT NULL DEFAULT '', ALGORITHM=copy || 1872763 | root | localhost | NULL | Query | 0 | starting | show full processlist |+---------+-------------+-----------+---------+---------+--------+----------------------------------+--------------------------------------------------------------------------------------------------------+3 rows in set (0.00 sec)nline ddl in-place 模式 字节数大于256时，也是不会锁表的
验证如下：
root@tidb06 23:32: [test001]> alter table s_api_filter_abcpool_data modify bill_date varchar(128) NOT NULL DEFAULT '', ALGORITHM=inplace;Query OK, 0 rows affected (2 min 16.39 sec)Records: 0 Duplicates: 0 Warnings: 0root@tidb06 23:32: [test001]> select * from information_schema.INNODB_TRX\G*************************** 1. row *************************** trx_id: 15684 trx_state: RUNNING trx_started: 2021-05-12 23:32:31 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 0 trx_mysql_thread_id: 1905130 trx_query: alter table s_api_filter_abcpool_data modify bill_date varchar(128) NOT NULL DEFAULT '', ALGORITHM=inplace trx_operation_state: reading clustered index trx_tables_in_use: 1 trx_tables_locked: 0 trx_lock_structs: 0 trx_lock_memory_bytes: 1136 trx_rows_locked: 0 trx_rows_modified: 0 trx_concurrency_tickets: 0 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 01 row in set (0.00 sec)另外就是不支持以 ALGORITHM=inplace 方式缩小字段长度。 降低varchar长度，只能采用ALGORITHM=COPY的方式
Decreasing VARCHAR size using in-place ALTER TABLE is not supported. Decreasing VARCHAR size requires a table copy (ALGORITHM=COPY).验证如下：
order_no varchar(64) 长度从64降低到32
不支持ALGORITHM=inplace方式:
root@tidb06 22:17: [test001]> alter table s_api_filter_abcpool_data modify order_no varchar(32) NOT NULL DEFAULT '', ALGORITHM=inplace;ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.耗时22分钟，主库期间一直锁表：
root@tidb06 22:33: [test001]> alter table s_api_filter_abcpool_data modify order_no varchar(32) NOT NULL DEFAULT '', ALGORITHM=copy;Query OK, 2466560 rows affected (22 min 23.97 sec)Records: 2466560 Duplicates: 0 Warnings: 0root@tidb06 22:34: [(none)]> select * from information_schema.INNODB_TRX\G*************************** 1. row *************************** trx_id: 49734 trx_state: RUNNING trx_started: 2021-05-16 22:34:11 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 9587 trx_mysql_thread_id: 2590351 trx_query: alter table s_api_filter_abcpool_data modify order_no varchar(32) NOT NULL DEFAULT '', ALGORITHM=copy trx_operation_state: inserting trx_tables_in_use: 2 trx_tables_locked: 3 trx_lock_structs: 1572 trx_lock_memory_bytes: 139472 trx_rows_locked: 9583 trx_rows_modified: 8015 trx_concurrency_tickets: 0 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 01 row in set (0.00 sec)此时从库也出现了 锁表，延迟,cp临时表
Seconds_Behind_Master: 857 Slave_SQL_Running_State: copy to tmp table trx_tables_locked: 3 trx_lock_structs: 254参考文档连接：
https://mp.weixin.qq.com/s/R9j58nSXgKNK94PYMB-Otg
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