Why Are High OFFSET Values in MySQL Queries So Slow?

Query Performance Degradation with High OFFSET Values in MySQL

Slow query execution is often attributed to excessive CPU usage or memory consumption. However, in certain cases, even a seemingly innocuous query with a high LIMIT offset can result in significant performance bottlenecks.

The Problem: Increasing OFFSET Impacts Query Speed

Consider a scenario with a large table containing over 16 million records. A query like:

SELECT * FROM large ORDER BY `id` LIMIT 0, 30

executes considerably faster than:

SELECT * FROM large ORDER BY `id` LIMIT 10000, 30

Both queries retrieve the same number of records, but the offset value of 10,000 in the latter query degrades performance.

Understanding the Issue

MySQL uses index scans or full table scans when retrieving data. When an offset value is applied, MySQL must scan a portion of the table to retrieve the starting point for the result set. As the offset value increases, the table scan becomes more extensive, leading to higher execution times.

Optimization Solution: Avoid High OFFSET Values

To optimize such queries, avoid using high OFFSET values. Instead, consider using alternative approaches:

1. Use Incremental Queries: Instead of incrementing the OFFSET by a fixed value, hold the last fetched ID and use it as the starting point for the next query. This approach eliminates the need for a table scan and significantly improves performance.
2. Implement Paging: Divide the result set into pages of a manageable size. Each page can be retrieved using a zero OFFSET value, ensuring optimal performance.

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