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 size

The 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_data
Create 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: 1
trx_last_foreign_key_error: NULL
 trx_adaptive_hash_latched: 0
 trx_adaptive_hash_timeout: 0
          trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 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0
trx_tables_locked: 0
事务保留的锁数量0
trx_lock_structs: 0

ALGORITHM=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: 1
trx_last_foreign_key_error: NULL
 trx_adaptive_hash_latched: 0
 trx_adaptive_hash_timeout: 0
          trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 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284
trx_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: NULL
Command: 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: test001
Command: 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: test001
Command: Query
   Time: 0
  State: starting
   Info: show full processlist
3 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: 51
Master_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: 0

root@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: 1
trx_last_foreign_key_error: NULL
 trx_adaptive_hash_latched: 0
 trx_adaptive_hash_timeout: 0
          trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 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: 0

root@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: 1
trx_last_foreign_key_error: NULL
 trx_adaptive_hash_latched: 0
 trx_adaptive_hash_timeout: 0
          trx_is_read_only: 0
trx_autocommit_non_locking: 0
1 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