What is fragmentation in MySQL?

What is fragmentation in MySQL?

Fragmentation means that the physical ordering of the index pages on the disk is not close to the index ordering of the records on the pages, or that there are many unused pages in the 64-page blocks that were allocated to the index. One symptom of fragmentation is that a table takes more space than it “should” take.

How do I defrag a table in MySQL?

How to Optimize MySQL Tables and Defragment to Recover Space

1. Identify Tables for Optimization. The first step is to identify whether you have fragmentation on your MySQL database.
2. Defrag using OPTIMIZE TABLE command.
3. Defrag using mysqlcheck command.
4. Defrag All Tables or All Databases.
5. After Optimization.

How do I fix SQL fragmentation?

You can fix index fragmentation by rebuilding or defragmenting the index. If the fragmentation level is low, you can defragment the index. If it’s high, then you should rebuild the index. You can use SQL Server Management Studio (SSMS) or T-SQL to get started managing index fragmentation.

What is Data_free in MySQL?

The DATA_FREE column shows the free space in bytes for InnoDB tables. For MySQL Cluster, DATA_FREE shows the space allocated on disk for, but not used by, a Disk Data table or fragment on disk.

How can we reduce fragmentation?

You can reduce fragmentation and improve read-ahead performance by using one of the following:

1. Dropping and re-creating an index.
2. Rebuilding an index by using the DBCC DBREINDEX statement.
3. Defragmenting an index by using the DBCC INDEXDEFRAG statement.

Does rebuilding indexes improve performance?

As you start to do inserts, index performance will actually improve for a time as the free-space pages are used, and then start to deteriorate as index fragmentation begins. Eventually the fragmentation in your index will be worse than it was after you completed your index rebuild, and performance can only get worse.

What is Information_schema tables in MySQL?

Chapter 1 INFORMATION_SCHEMA Tables INFORMATION_SCHEMA provides access to database metadata, information about the MySQL server such as the name of a database or table, the data type of a column, or access privileges. Other terms that are sometimes used for this information are data dictionary and system catalog.

What is Data_length in MySQL?

DATA_LENGTH. For MyISAM , DATA_LENGTH is the length of the data file, in bytes. For InnoDB , DATA_LENGTH is the approximate amount of space allocated for the clustered index, in bytes. Specifically, it is the clustered index size, in pages, multiplied by the InnoDB page size.

Does MySQL optimize queries?

You can optimize single-query transactions for InnoDB tables, using the technique in Section 8.5. 3, “Optimizing InnoDB Read-Only Transactions”. Avoid transforming the query in ways that make it hard to understand, especially if the optimizer does some of the same transformations automatically.

How do I cache a MySQL query?

MySQL determines the queries to cache by examining the query_cache_type variable. Setting this value to 0 or OFF prevents caching or retrieval of cached queries. You can also set it to 1 to enable caching for all queries except for ones beginning with the SELECT SQL_NO_CACHE statement.

What is mysql table fragmentation and how can I reduce it?

MySQL tables, including MyISAM and InnoDB, two of the most common types, experience fragmentation as data is inserted and deleted randomly. Fragmentation can leave large holes in your table, blocks which must be read when scanning the table. Optimizing your table can therefore make full table scans and range scans more efficient.

What are the signs of fragmentation in InnoDB?

All InnoDB data and indexes are stored in B-trees , and their fill factor may vary from 50% to 100%. Another symptom of fragmentation is that a table scan such as this takes more time than it “should” take:

What is index fragmentation?

Fragmentation means that the physical ordering of the index pages on the disk is not close to the index ordering of the records on the pages, or that there are many unused pages in the 64-page blocks that were allocated to the index. One symptom of fragmentation is that a table takes more space than it “should” take.

How do I defragment a mySQL table?

For more information, see Section 15.12, “InnoDB and Online DDL” . Another way to perform a defragmentation operation is to use mysqldump to dump the table to a text file, drop the table, and reload it from the dump file.