How to choose gh-ost or pt-online-schema-change? 1. pt-online-schema-change belongs to Percona Toolkit, has a long history and good community support; 2. gh-ost is lighter and supports triggerless mode, suitable for high concurrency or large table scenarios. Core process during use: 1. Create a new table and apply a new schema; 2. Copy the original table data; 3. Synchronize incremental changes (trigger or binlog); 4. Replace the original table. Notes include: 1. Ensure the index and foreign keys are correct; 2. Pay attention to the short locks in the switching phase; 3. Leave enough disk space; 4. Monitor copy delays. Common error checks: 1. Check lock waiting and deadlock; 2. Check for unique key conflicts; 3. Confirm the account permissions; 4. gh-ost requires confirmation that binlog is ROW mode. In addition, peak business operations should be avoided, and impact assessments should be made, environmental drills should be tested, and rollback plans should be prepared.

Modifying the MySQL table structure online is a common requirement, especially in production environments. Using ALTER TABLE directly can easily lock tables, affecting service availability. At this time, you need to use tools like gh-ost or pt-online-schema-change to achieve "online" changes.

Both tools can complete DDL operations without locking tables, but the usage methods and principles are slightly different. Let’s talk about their practical application and precautions from several perspectives.

How to choose: gh-ost or pt-online-schema-change?

Both are based on the idea of "creating temporary table data migration trigger synchronization", but they are different in detail processing.

• pt-online-schema-change is part of Percona Toolkit. It has a longer history, good community support, and relatively comprehensive functions.
• gh-ost is an open source tool for GitHub. It is lighter in design and supports triggerless mode (implemented through binlog), which has more advantages for certain high concurrency or large table scenarios.

If your environment allows triggers, you can choose both; if you want to reduce the impact on the main library, or if you cannot create triggers, gh-ost is more suitable.

Key steps and precautions when using

No matter which tool it is, the core process is:

1. Create a new table and apply a new schema change
2. Copy the original table data to the new table
3. Synchronize incremental changes to the original table (via trigger or binlog)
4. Replace the original table

But in actual implementation, you should pay attention to these points:

• Index and Foreign Keys : Make sure the new table structure is correct, especially if the foreign key constraints should be preserved or adjusted.
• Row lock problem : Although the table will not be locked for a long time, there will be short lock writes during the switching phase.
• Disk space : It will take up extra space during operation, especially large tables, remember to reserve them.
• Replication Delay : If replication is enabled, binlog synchronization may cause delays and requires monitoring.

Common errors and troubleshooting suggestions

When encountering problems during use, you can start from these aspects:

• Check whether there are locks waiting or deadlocks occur (via SHOW ENGINE INNODB STATUS )
• Check if synchronization fails due to unique key conflicts
• Ensure that the database account has sufficient permissions (such as creating tables, deleting tables, triggers, etc.)
• If you are using gh-ost, make sure that the binlog format is ROW mode and there are correct filtering rules.

Also, do not operate during peak business periods to avoid affecting performance.

Beyond the tool: What else do you need to consider?

In addition to technical means, there are a few things you should do before going online:

• Evaluate the scope of changes in advance, such as whether it involves index reconstruction and whether it will affect the query plan
• Perform the test environment to observe resource consumption and time consumption
• There is a rollback solution, and it can be quickly recovered if there is an error.

Some companies will encapsulate a layer of automated scripts, combining health checks and notification mechanisms to simplify such operations.

Basically that's it. Both tools can solve the pain points of online DDL well. The key is to understand their working mechanism and use it in the right scenario.

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