Optimizing Indexes in MySQL: Understanding Rebuild vs. Reorganize and Their Performance Implications

Index optimization is not just about enhancing speed; it’s about striking the balance between thorough overhauls and regular maintenance to keep data access efficient and timely

Rebuilding and reorganizing indexes are operations that optimize the physical storage of index data in a database, improving the performance of some queries. MySQL, especially in the InnoDB storage engine, provides mechanisms to handle these operations. Let’s delve into the differences between the two and their impact on performance.

1. Rebuilding Indexes

  • Definition: Rebuilding an index essentially means creating the index again from scratch. This process drops the existing index and recreates it.
  • Command: This can be done in MySQL using the ALTER TABLE statement with the DROP INDEX and ADD INDEX clauses, or more directly with the ALTER TABLE … FORCE command, which rebuilds the table and all its indexes.
  • Impact:
    • Performance: Rebuilding an index can be resource-intensive. It requires a substantial amount of disk I/O and can be time-consuming, especially for large tables. It can also lock the table, depending on the MySQL version and configuration, which could impact application performance and availability.
    • Disk Space: Rebuilding can free up space that’s wasted by fragmentation. After the rebuild, the index will have a more compact storage layout.

2. Reorganizing Indexes

  • Definition: Reorganizing an index means reshuffling the index pages to optimize storage and access paths. This process retains the existing index but attempts to optimize its layout.
  • Command: In MySQL’s InnoDB storage engine, the OPTIMIZE TABLE command can be used to reorganize a table and its associated indexes.
  • Impact:
    • Performance: Reorganizing is generally less resource-intensive than rebuilding, as it doesn’t need to recreate the index entirely. However, it still requires some disk I/O and might lead to table locks in certain configurations.
    • Disk Space: Reorganizing an index can reclaim unused space and reduce fragmentation, making data access more efficient.

Rebuilding vs. Reorganizing

  • Efficiency: Rebuilding an index is like doing a major overhaul. It’s thorough and can be very effective in optimizing an index, especially if the index is heavily fragmented or has undergone numerous updates, deletions, and insertions. Reorganizing, on the other hand, is more like regular maintenance and is generally faster and less intrusive.
  • Locking: Depending on the MySQL version and the storage engine, both operations can introduce locking. However, with the introduction of online DDL (Data Definition Language) capabilities in InnoDB, many of these operations can be done with minimal locking, allowing for concurrent DML (Data Manipulation Language) operations.

Conclusion

Both rebuilding and reorganizing indexes have their place in a DBA’s toolkit. The choice between them often depends on the specific circumstances and the extent of fragmentation or inefficiency in the index storage. Regular monitoring of index health and performance is key to determining when and which of these operations to perform.

About Shiv Iyer 390 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.