When working with MySQL databases, one common task is reclaiming disk space and defragmenting tables. The typical solution that most of us have turned to is OPTIMIZE TABLE. While this sounds like a simple, quick fix, there are a few myths and things we often overlook that can lead to confusion. Let’s break it down.
The Basics: OPTIMIZE TABLE and ALTER TABLE
To reclaim space or defragment a table in MySQL, the go-to commands are usually:
OPTIMIZE TABLE <table_name>;or OPTIMIZE TABLE [table_name_1], [table_name_2]or via sudo mysqlcheck -o [schema] [table] -u [username] -p [password]
But before we dive into the myths, let’s clarify what happens when you run these commands.
OPTIMIZE TABLE Overview
OPTIMIZE TABLEis essentially a shorthand forALTER TABLE <table_name> ENGINE=InnoDBfor InnoDB tables. It works by rebuilding the table, compacting data, and reclaiming unused space.- In MySQL 5.6.17 and later, the command works online, meaning it allows concurrent reads and writes during the rebuild, with some exceptions (brief locking during initial and final stages). Prior to 5.6.17, the table was locked for the entire duration of the operation, causing application downtime.
Myth #1: OPTIMIZE TABLE Is Always Quick
- No:
OPTIMIZE TABLEcan indeed take a long time for large tables, especially if there are a lot of inserts, deletes, or updates. This is true when rebuilding the table. For larger datasets, the I/O load can be significant.
mysql> OPTIMIZE TABLE my_large_table;
+----------------------------+--------+----------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+----------------------------+--------+----------+----------+----------+
| mydb.my_large_table | optimize | ok | Table optimized |
+----------------------------+--------+----------+----------+----------+
In the output, the values under each column heading would show:
- Table: The table that was optimized (e.g., yourdb.customers).
- Op: The operation performed (optimize).
- Msg_type: Type of message, usually status.
- Msg_text: The result of the operation, such as OK or a specific message (e.g., “Table is already up to date”).
If the table is already optimized or doesn’t require optimization, the output might look like this:
+------------------+----------+----------+-----------------------------+
| Table | Op | Msg_type | Msg_text |
+------------------+----------+----------+-----------------------------+
| yourdb.customers | optimize | note | Table is already up to date |
+------------------+----------+----------+-----------------------------+
Below screenshot explains possible values of msg_text etc.

Real-Time Example Validation:
- MySQL logs can show something like this:
[Note] InnoDB: Starting online optimize table my_large_table [Note] InnoDB: Table optimized successfully
However, for larger tables, it is critical to consider the additional I/O load during the rebuild. For example:bash [Note] InnoDB: Rebuilding index my_large_table_idx [Note] InnoDB: Table rebuild completed in 300 seconds
Note: In order to get more detailed information its good to verify PROCESSLIST or SLOW QUERY LOG (if enabled).
Myth #2: OPTIMIZE TABLE Doesn’t Block Other Operations
- Yes/No: This myth is partly true and partly false depending on the MySQL version.
- For MySQL 5.5 and earlier: The table is locked for writes, but concurrent reads are allowed.
- For MySQL 5.6.16 and earlier: Same as above .. concurrent reads are allowed, but writes are blocked.
- For MySQL 5.6.17 and later: Concurrent reads and writes are allowed during the rebuild process, but the table still needs to be briefly locked during the initial and final phases. There is a brief lock required to start the process, which is often overlooked.
Real-Time Example for MySQL 5.6.17+:
[Note] InnoDB: Starting online optimize table my_large_table
[Note] InnoDB: Table optimized successfully
Although reads and writes are allowed during this process, you might still experience short bursts of lock at the start and end of the operation.
Myth #3: You Don’t Need to Worry About Disk Space
- No: You need sufficient disk space before running
OPTIMIZE TABLE. If you’re running low on space, you could encounter errors or performance issues during the rebuild process. - There are few bugs as well which might could occur if disk space is insufficient. Additionally, there’s also temporary disk space required during the rebuild process. Running
OPTIMIZE TABLEwith insufficient space could fail silently, leading to issues down the line.
Best Practice:
Ensure that your disk has at least as much free space as the table you’re optimizing, as a copy of the table is created temporarily during the rebuild.
Myth #4: ALTER TABLE with Row Format Is Always the Solution
- No:
ALTER TABLE ... ROW_FORMAT=COMPRESSEDor other formats can help optimize space, but it may not always result in savings, especially for certain data types (like BLOBs or large text fields). It can also introduce overhead on the CPU if you’re using compression.
In some cases, switching to a compressed format can actually increase the size of the table, depending on the type of data stored.
Real-Time Example:
- For a table like
customer_data:ALTER TABLE customer_data ROW_FORMAT=COMPRESSED;Depending on the types of columns and data (e.g., BLOBs or TEXT), compression might not always yield the expected results.
Myth #5: You Only Need to Optimize Tables When They Get Slow
- No: This is another common misconception. Regular optimization is crucial to ensure long-term performance, especially for heavily modified tables. Tables that undergo a lot of updates or deletions can become fragmented over time, even without obvious performance degradation.
Optimizing periodically can help prevent gradual performance loss.
Real-Time Example:
- If you have an
orderstable:mysql> OPTIMIZE TABLE orders;Over time, especially with frequentUPDATEorDELETEoperations, fragmentation can slow down access, even if it’s not immediately noticeable.
Main Pointerss ..
- OPTIMIZE TABLE is a helpful tool but not a one-size-fits-all solution.
- It requires sufficient disk space and careful consideration of your MySQL version and storage engine (InnoDB vs. MyISAM).
- In MySQL 5.6.17 and later, online optimizations are possible, but brief locking still occurs during the process.
- For MyISAM tables, there’s no escaping the full lock during optimization.
- Always assess the potential overhead (I/O and CPU usage) before running the operation, especially on larger datasets.
By breaking these myths, you can make better decisions when using OPTIMIZE TABLE to keep your database healthy without causing unnecessary downtime or performance hits.
Hope It Helped!
Prashant Dixit
Database Architect @RENAPS
Reach us at : https://renaps.com/




