How to add fields to mysql large tables without stopping

Keywords: MySQL Database Stored Procedure socket

Adding fields to a table of ten million or more levels in MySQL has always been a headache. In this case, how to deal with it? This paper uses three common scenarios to illustrate the case.

1. Environmental preparation

Database version: 5.7.25-28 (Percona branch)

Server configuration: three centos 7 virtual machines with 2CPU 2G memory

Database architecture: 1 master-slave MHA architecture (in order to facilitate the demonstration of master-slave switching scenario, if GTID is enabled, then two nodes can be used). For the construction of MHA, please refer to this article MySQL highly available MHA cluster deployment

Prepare test table: create a 2kw record table. For quick creation methods, please refer to Quickly create a continuous number

This time, we slightly modified the stored procedure and added several more fields. The stored procedure is as follows:

CREATE  PROCEDURE `sp_createNum`(cnt INT )
    DROP TABLE  if exists  tb_add_columns;
    CREATE TABLE if not exists tb_add_columns(id int primary key,col1 int,col2 varchar(32));
    INSERT INTO tb_add_columns(id,col1,col2) SELECT i  as id ,i%7 as col1,md5(i) as col2;
    WHILE i < cnt DO
        INSERT INTO tb_add_columns(id,col1,col2) SELECT id + i   as id ,( id + i) %7 as col1,md5( id + i) as col2  FROM tb_add_columns WHERE id <=cnt - i ;
        SET i = i*2;

Call the stored procedure to complete the creation of test table and test data.

mysql> call sp_createNum(20000000);

2. Add fields directly

Usage scenario: when the system is not busy or the table has few accesses, such as meeting ONLINE DDL, it can be added directly.

Simulation scenario: create a test script, access a random record of the table every 10s, and then add fields to the table

The access script is as follows:

# gjc

for i in  {1..1000000000}                    # Access times 1000000000, adjust as needed
    id=$RANDOM                          #Generate random number    
    mysql -uroot -p'123456' --socket=/data/mysql3306/tmp/mysql.sock  -e "select  a.*,now() from  testdb.tb_add_columns a where id = "$id     # Access data
    sleep 10                            #  Pause for 10s

Run script


Add fields to table

mysql> alter table  testdb.tb_add_columns add col3 int;

At this time, the access is normal.

The scenario attached with ONLINE DDL is as follows. It is suggested that DBA s should be clear about it

(image reproduced in )


(image reproduced in )

3. Use tools to add online

Although the table can still be read and written when Online DDL adds fields, the tool Pt OSC or GH OST is used most for large table operations in production scenarios.

This paper mainly introduces Pt OSC (PT online schema change) to add fields. This command is the most frequently used one in Percona Toolkit

For the installation and main use of Percona Toolkit, please refer to Five minutes to learn how to install and use Percona Toolkit

Add field

root@mha1 ~]# pt-online-schema-change --alter "ADD COLUMN  col4  int" h=localhost,P=3306,p=123456,u=root,D=testdb,t=tb_add_columns,S=/data/mysql3306/tmp/mysql.sock  --charset=utf8mb4 --execute

The main process is as follows:

1> Cannot connect to A=utf8mb4,P=3306,S=/data/mysql3306/tmp/mysql.sock,h=,p=...,u=root
1> Cannot connect to A=utf8mb4,P=3306,S=/data/mysql3306/tmp/mysql.sock,h=,p=...,u=root
No slaves found.  See --recursion-method if host mha1 has slaves.  #  Because socket is used to connect to the database and the root remote connection account is not configured, this prompt will appear

# A software update is available:
Operation, tries, wait:
  analyze_table, 10, 1                                     
  copy_rows, 10, 0.25                                       
  create_triggers, 10, 1                      
  drop_triggers, 10, 1
  swap_tables, 10, 1
  update_foreign_keys, 10, 1
Altering `testdb`.`tb_add_columns`...
Creating new table...                                     #  Create an intermediate table named "_ Original table name_ new"
Created new table testdb._tb_add_columns_new OK.           
Altering new table...                                     #  Modify the table, that is, add fields to the new table. Since the new table has no data, it will be added soon
Altered `testdb`.`_tb_add_columns_new` OK.                  
2020-06-20T12:23:43 Creating triggers...                  #  Create trigger, which is used to automatically synchronize data changes (add, modify, delete) in the original table to the new table during copying the original table to the new table
2020-06-20T12:23:43 Created triggers OK.
2020-06-20T12:23:43 Copying approximately 19920500 rows... # Copy data. The database volume is in the statistical information. It is not accurate
Copying `testdb`.`tb_add_columns`:  11% 03:50 remain       #  Copy data in batches (how much data is copied in each batch according to the table size). During the copying process, you can use show processlist to see the corresponding sql
Copying `testdb`.`tb_add_columns`:  22% 03:22 remain
Copying `testdb`.`tb_add_columns`:  32% 03:10 remain
Copying `testdb`.`tb_add_columns`:  42% 02:45 remain
Copying `testdb`.`tb_add_columns`:  51% 02:21 remain
Copying `testdb`.`tb_add_columns`:  62% 01:48 remain
Copying `testdb`.`tb_add_columns`:  72% 01:21 remain
Copying `testdb`.`tb_add_columns`:  81% 00:53 remain
Copying `testdb`.`tb_add_columns`:  91% 00:24 remain
2020-06-20T12:28:40 Copied rows OK.                       # Copy data complete
2020-06-20T12:28:40 Analyzing new table...                # Optimize new tables
2020-06-20T12:28:40 Swapping tables...                    # Exchange table name, change the original table to "_ Original table name_ old "and change the new table name to the original table name
2020-06-20T12:28:41 Swapped original and new tables OK.    
2020-06-20T12:28:41 Dropping old table...                 #  Delete the old table (or add parameters without deleting the old table)
2020-06-20T12:28:41 Dropped old table `testdb`.`_tb_add_columns_old` OK.
2020-06-20T12:28:41 Dropping triggers...                  # Delete trigger 
2020-06-20T12:28:41 Dropped triggers OK.
Successfully altered `testdb`.`tb_add_columns`.            # complete

In the process of modification, reading and writing are not affected. You can write a program containing reading and writing

Note: whether you add fields directly or with Pt OSC, you need to obtain the metadata lock of the table before you can add it (including when Pt OSC creates triggers and exchanges table names at last). Therefore, if a table is a hot table, it cannot be added if it is read and written frequently or occupied by other sessions when it is added.

For example: lock a record

Add fields with Pt OSC, and you will find that you are stuck in the step of creating trigger all the time

At this time, view the corresponding SQL and wait for metadata lock

It's the same with adding directly, for example

When the lock waiting is reached, an error will be reported to discard the added field

mysql> alter table  testdb.tb_add_columns add col5 int;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

In this case, you need to wait for the system to be added when it is not busy, or use the subsequent master-slave switch after creating the slave library

4. Modify in the slave database first, and then switch between master and slave

Usage scenario: if a table in the above example has a large amount of data and is a hot table (read and write are particularly frequent), you can consider adding it to the slave database first, then switching between the master and slave, and then adding fields to several other nodes after switching.

First, add from the library (in this article, add from the alternative node)

mysql> alter table  testdb.tb_add_columns add col5 int;
Query OK, 0 rows affected (1 min 1.91 sec)
Records: 0  Duplicates: 0  Warnings: 0

Switch between master and slave

Use MHA script to switch Online

masterha_master_switch  --conf=/etc/masterha/app1.conf --master_state=alive  --orig_master_is_new_slave --new_master_host=  --new_master_port=3306

Add fields to other nodes after switching

/* Add to the original master database  */
mysql>  alter table  testdb.tb_add_columns add col5 int;
Query OK, 0 rows affected (1 min 8.36 sec)
Records: 0  Duplicates: 0  Warnings: 0

/* Another added from the library  */
mysql>  alter table  testdb.tb_add_columns add col5 int;
Query OK, 0 rows affected (1 min 8.64 sec)
Records: 0  Duplicates: 0  Warnings: 0

This completes the field addition.

5. Summary

There are three ways to add or modify fields in MySQL production environment. There are many precautions in actual use, so we need to summarize them.

  • Add directly

If the reading and writing of the table is not frequent and the data volume is small (usually within 1G or a million), just add it directly (you can learn about online ddl)

  • Use pt_osc add

If the table is large but the reading and writing is not too large, and you want to try not to affect the reading and writing of the original table, you can use percona tools to add, which is equivalent to creating a new table with added fields, then dropping the data of the original table to copy to the new table, and the data during the copying of historical data will also be synchronized to the new table, finally deleting the original table, renaming the new table to the name of the original table, and adding fields

  • First add from library and then switch from master to slave

If a table has a large amount of data and is a hot table (read and write are particularly frequent), you can consider adding it to the slave database first, then switching between the master and slave, and then adding fields to several other nodes after switching

Posted by romilbm on Fri, 19 Jun 2020 23:20:57 -0700