Resolving MySQL Error 1258 – SQLSTATE: HY000 (ER_ZLIB_Z_BUF_ERROR)

When working with MySQL, encountering Error 1258 – SQLSTATE: HY000 (ER_ZLIB_Z_BUF_ERROR) can be a sign of trouble with data compression processes. This error indicates that there’s insufficient room in the output buffer, which could suggest that the length of uncompressed data might have been corrupted. Understanding and fixing this issue requires a careful approach to ensure data integrity and system performance.

Understanding Error 1258

Error 1258 is related to the ZLIB compression library used within MySQL for compressing and decompressing data. When MySQL reports this error, it’s often because the system tried to uncompress data but found that the buffer allocated for the uncompressed data was too small. This can happen due to incorrect buffer size settings or corrupt data.

Diagnosing the Issue

To diagnose this issue, you need to identify where and how the compression or decompression is being used. This could be within table compression, file compression, or during data transfer between the server and clients. Once you’ve located the source of the compression, you can start investigating the buffer sizes and the integrity of the data.

Examples and Sample Code

Here are some examples and sample code that can help illustrate potential fixes for Error 1258:

Example 1: Adjusting Buffer Sizes

If you’re using custom code to handle compressed data, ensure that the buffer sizes are adequate. For example, when using the COMPRESS() function, you might need to increase the buffer size for the UNCOMPRESS() function:

SELECT UNCOMPRESS(compressed_data) FROM my_table;

If this query results in Error 1258, you may need to look into the application code that is handling the uncompressed data and adjust the buffer sizes accordingly.

Example 2: Verifying Data Integrity

Corrupted data can also cause this error. If you suspect that your compressed data might be corrupted, you should verify its integrity. This might involve re-compressing the data or restoring it from a backup.

Example 3: Configuration Settings

MySQL’s configuration settings may also influence buffer sizes. For instance, the max_allowed_packet variable determines the maximum size of one packet or any generated/intermediate string. If this size is too small, it could lead to Error 1258 during operations that involve large amounts of data.

To address this, you can increase the max_allowed_packet size in the MySQL configuration file (my.cnf or my.ini):


After making changes, you would need to restart the MySQL server for the new settings to take effect.


Error 1258 in MySQL is a signal that there’s an issue with data compression or decompression operations. To resolve this error, you should:

  1. Check the application code for correct buffer size allocations.
  2. Ensure the integrity of your compressed data.
  3. Review and adjust MySQL configuration settings if necessary.

By carefully examining these areas, you can diagnose the root cause of the ER_ZLIB_Z_BUF_ERROR and take appropriate action to fix it, thereby maintaining the smooth operation of your MySQL databases. Remember to always back up your data before making any changes to prevent accidental data loss.

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