表的開銷由MySQL內(nèi)部數(shù)據(jù)管理產(chǎn)生，常見原因：1.刪除大量行；2.更新變長(zhǎng)字段導(dǎo)致空間減少；3.頻繁增刪操作。 phpMyAdmin顯示此信息用於提示可優(yōu)化的表。修復(fù)方法為使用“Optimize table”功能，該操作重建表並回收空間，適用於MyISAM和InnoDB引擎，建議定期執(zhí)行以保持?jǐn)?shù)據(jù)庫(kù)效率。

When you see the "Overhead" column in phpMyAdmin's table overview, it's pointing out tables that have some wasted space due to how MySQL manages data internally. This overhead typically happens after a lot of updates, deletes, or inserts — especially when rows are changed in size and leave behind unused space.

What causes overhead in a table?

MySQL stores data in blocks (called pages), and when rows are deleted or updated, gaps can be left behind. Here are a few common situations that create overhead:

• You delete a bunch of rows from a table.
• You update variable-length fields like VARCHAR or TEXT , and the new values take up less space than before.
• You run many INSERT and DELETE operations on a MyISAM or older InnoDB table.

This isn't a problem with your data integrity, but it does mean there's extra space being used inefficiently.

Why does phpMyAdmin show this?

phpMyAdmin pulls this info directly from MySQL's internal table status reports. When you look at the table list, it shows overhead because it's giving you a quick way to spot tables that might benefit from optimization.

You'll often see overhead reported for tables using the MyISAM storage engine, since it doesn't clean up after itself as well. If you're using InnoDB , which is more modern and efficient, you might still see overhead sometimes, but it tends to manage space better automatically.

How do you fix it?

If you see a number under the Overhead column, the usual fix is to optimize the table. In phpMyAdmin, you can do this by:

• Going to the table in question
• Clicking the checkbox next to it (or multiple tables)
• Choosing “Optimize table” from the dropdown menu

Behind the scenes, this runs an OPTIMIZE TABLE command, which basically rebuilds the table to reclaim unused space.

Here's what actually happens during optimization:

• MySQL creates a new copy of the table without the gaps.
• It reindexes everything so it's compact and efficient.
• Then it swaps the old version with the new one.

It's safe to do this on most tables, especially if they're not under constant heavy use. Just note that optimizing large tables can take a little time and may lock the table briefly, depending on your MySQL version and storage engine.

For small sites or low-traffic databases, running optimize every few months is usually enough. For high-traffic apps with lots of writes, you might want to check this once in a while just to keep things tidy.

基本上就這些。

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