Database optimization

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  1. Database Optimization for MediaWiki

This article provides a comprehensive guide to database optimization for MediaWiki installations, geared towards beginners. A well-optimized database is crucial for maintaining a fast and responsive wiki, especially as content grows. We’ll cover common issues, practical techniques, and tools to help you improve your wiki's performance. This guide assumes you have basic administrative access to your MediaWiki installation and its database.

Understanding the MediaWiki Database

MediaWiki utilizes a relational database management system (RDBMS), most commonly MySQL/MariaDB, PostgreSQL, or SQLite. The database stores all wiki content, including pages, revisions, user data, categories, and configuration settings. Understanding the database structure is the first step towards optimization.

The core tables include:

  • `page`: Stores information about each wiki page (title, namespace, etc.).
  • `revision`: Contains the actual content of each page revision, along with metadata like author, timestamp, and comment. This table often becomes the largest one.
  • `text`: Stores the wikitext content of each revision.
  • `user`: Holds user account information.
  • `category`: Stores category information.
  • `categorylink`: Links pages to categories.
  • `watchlist`: Stores users' watched pages.
  • `recentchanges`: Logs recent changes to pages, used for the "Recent Changes" page.

Poor database performance directly impacts wiki speed. Slow queries, large table sizes, and inefficient indexing all contribute to a sluggish user experience.

Identifying Performance Bottlenecks

Before diving into optimization, you need to pinpoint where the problems lie. Several methods can help:

1. **Slow Query Log:** Most RDBMSs offer a slow query log. This log records queries that exceed a specified execution time. Analyzing this log reveals the queries causing the most significant delays. For MySQL, enable the slow query log in your `my.cnf` file. For PostgreSQL, configure `log_min_duration_statement`. 2. **MediaWiki Performance Profiling:** MediaWiki has built-in profiling tools. Set `$wgProfilerEnabled = true;` in your `LocalSettings.php` file. This generates detailed reports on script execution times, identifying performance bottlenecks within the MediaWiki code itself. Extension:Profiling provides more advanced features. 3. **Database Server Monitoring:** Monitor CPU usage, memory consumption, disk I/O, and network traffic on your database server. Tools like `top`, `htop`, `iotop`, and database-specific monitoring utilities (e.g., MySQL Workbench, pgAdmin) can provide valuable insights. 4. **Web Server Logs:** Examine your web server (e.g., Apache, Nginx) logs for slow page loads or errors related to database connections. 5. **Utilize `SHOW PROCESSLIST` (MySQL/MariaDB):** This command displays currently running queries, helping you identify long-running operations. 6. **Explain Plans:** Use the `EXPLAIN` statement in your database query language (e.g., `EXPLAIN SELECT * FROM page WHERE title = 'Main Page';`) to analyze how the database executes a query. This reveals whether indexes are being used effectively.

Database Optimization Techniques

Once you've identified the bottlenecks, you can apply various optimization techniques. These are grouped into categories for clarity.

1. Indexing

Indexes are crucial for speeding up queries. They allow the database to quickly locate rows matching specific criteria without scanning the entire table.

  • **Index frequently queried columns:** Columns used in `WHERE` clauses, `JOIN` conditions, and `ORDER BY` clauses are good candidates for indexing.
  • **Composite Indexes:** Create indexes on multiple columns if you frequently query those columns together. The order of columns in a composite index matters.
  • **Avoid Over-Indexing:** Too many indexes can slow down write operations (inserts, updates, deletes) as the database must update the indexes as well.
  • **Analyze Index Usage:** Regularly review your indexes to identify unused or redundant ones. MySQL provides tools like `pt-index-usage` for this purpose.
  • **Specific MediaWiki Indexing Considerations:**
   *   `page.title`: Indexing the `title` column is essential for fast page lookups.
   *   `page.namespace`: Indexing the `namespace` column is important for namespace-specific queries.
   *   `revision.page`: Indexing `revision.page` facilitates efficient retrieval of revisions for a specific page.
   *   `categorylink.cl_from`: Frequently queried for retrieving pages in a category.
   *   `user.user_name`: Indexing the username column is crucial for user authentication and profile lookups.

2. Query Optimization

Writing efficient SQL queries is vital for performance.

  • **Avoid `SELECT *`:** Only select the columns you need. Retrieving unnecessary data increases I/O and network traffic.
  • **Use `WHERE` clauses effectively:** Filter data as early as possible in the query.
  • **Optimize `JOIN` operations:** Ensure that `JOIN` conditions are indexed and that the join order is optimal. Consider using `INNER JOIN` instead of `LEFT JOIN` when appropriate.
  • **Avoid `LIKE` with leading wildcards:** `LIKE '%keyword'` is slow because it cannot use indexes. `LIKE 'keyword%'` is much faster.
  • **Use `LIMIT`:** When you only need a subset of results, use the `LIMIT` clause to restrict the number of rows returned.
  • **Subqueries vs. Joins:** Sometimes, rewriting a subquery as a `JOIN` can improve performance.
  • **Prepared Statements:** Use prepared statements to avoid repeatedly parsing the same query.
  • **Consider using stored procedures:** For complex, frequently executed tasks, stored procedures can offer significant performance gains.

3. Database Schema Optimization

While less common, sometimes schema changes can improve performance.

  • **Data Types:** Use the most appropriate data types for each column. Avoid using overly large data types when smaller ones suffice.
  • **Normalization:** Ensure your database schema is properly normalized to reduce data redundancy and improve data integrity. However, excessive normalization can sometimes lead to performance issues due to increased `JOIN` operations. A balance is key.
  • **Partitioning (Advanced):** For very large tables, consider partitioning the table into smaller, more manageable pieces. This can improve query performance and simplify maintenance. This is a more advanced technique and requires careful planning.

4. Caching

Caching reduces the load on the database by storing frequently accessed data in memory.

  • **MediaWiki Caching:** MediaWiki has several built-in caching mechanisms, including:
   *   **ObjectCache:**  A central caching system used by MediaWiki. Configure it to use a fast caching backend like Memcached or Redis.  Caching provides detailed information.
   *   **ParserCache:** Caches the output of the parser, reducing the need to re-parse wikitext.
   *   **QueryCache:** Caches the results of database queries. (Depreciated in newer versions, focus on ObjectCache and external caching.)
  • **Database Query Cache:** MySQL has a query cache, but it can sometimes cause performance issues under heavy write loads. Consider disabling it if it's not providing significant benefits.
  • **Web Server Cache:** Use a web server cache (e.g., Varnish, Nginx caching) to cache static content and frequently accessed pages.
  • **Reverse Proxy Cache:** A reverse proxy like Varnish can cache entire pages, reducing the load on both the web server and the database.

5. Maintenance Tasks

Regular maintenance tasks are essential for maintaining database performance.

  • **Optimize Tables:** Use the `OPTIMIZE TABLE` statement (MySQL) or `VACUUM FULL` (PostgreSQL) to reclaim unused space and defragment tables. Schedule this task during off-peak hours. Maintenance tasks provides further information.
  • **Analyze Tables:** Use the `ANALYZE TABLE` statement (MySQL) or `ANALYZE` (PostgreSQL) to update table statistics. This helps the query optimizer make better decisions.
  • **Purge Old Revisions:** MediaWiki stores a history of all page revisions. Over time, this can consume significant storage space. Configure `$wgArchiveThreshold` in `LocalSettings.php` to automatically archive old revisions.
  • **Remove Unused Data:** Delete old logs, temporary files, and other unnecessary data.
  • **Regular Backups:** Always maintain regular database backups to protect against data loss.

6. Database Server Configuration

Optimizing the database server configuration can significantly improve performance.

  • **Memory Allocation:** Allocate sufficient memory to the database server. The amount of memory required depends on the size of your database and the number of concurrent users.
  • **Buffer Pool Size (MySQL):** Increase the `innodb_buffer_pool_size` to cache frequently accessed data in memory.
  • **Shared Buffers (PostgreSQL):** Increase `shared_buffers`.
  • **Connection Limits:** Configure the maximum number of database connections to handle the expected load.
  • **Disk I/O:** Use fast storage devices (e.g., SSDs) to improve disk I/O performance.
  • **Network Configuration:** Ensure that the database server and web server are on the same network and have a low-latency connection.

7. MediaWiki Specific Configuration

  • **$wgMainCacheType:** Set this to a fast caching backend like Memcached or Redis.
  • **$wgParserCacheTtl:** Adjust the time-to-live (TTL) for the parser cache.
  • **$wgJobRunLimit:** Controls the number of jobs processed per script execution. Adjust this value based on server resources.
  • **$wgRateLimits:** Configure rate limits to prevent abuse and protect against denial-of-service attacks.

Tools and Resources


Conclusion

Database optimization is an ongoing process. Regularly monitor your wiki's performance, identify bottlenecks, and apply appropriate optimization techniques. By following the guidelines outlined in this article, you can significantly improve the speed and responsiveness of your MediaWiki installation, providing a better experience for your users. Remember to always back up your database before making any significant changes.

Main Page Manual:Configuration Manual:Caching Manual:Maintenance tasks Extension:Profiling Help:Contents MediaWiki MySQL PostgreSQL SQLite

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