How Does MySQL's OFFSET in LIMIT Queries Affect Performance, and How Can It Be Optimized?

Impact of OFFSET in MySQL LIMIT Queries on Query Performance

When working with large datasets in MySQL, using LIMIT with an OFFSET can significantly impact query performance. This is particularly evident when utilizing the PRIMARY KEY as the ORDER BY criteria. For instance, the following query with a smaller OFFSET:

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

executes much faster than a query with a larger OFFSET:

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

This is true even though both queries retrieve the same number of records (30).

Understanding OFFSET's Impact

To understand this phenomenon, we need to examine MySQL's query execution plan. When using ORDER BY with an OFFSET, MySQL needs to first sort the entire table by the specified criteria. It then skips the specified number of records (OFFSET) and retrieves the desired number of records (LIMIT). This sorting process becomes increasingly expensive as the OFFSET increases and the table size grows.

Optimization Technique

To optimize such queries, it is advisable to utilize a range-based retrieval approach rather than relying solely on OFFSET. This can be achieved by holding the last record's ID from the previous fetch and adding a WHERE condition:

WHERE id > lastId LIMIT 0, 30

By maintaining a zero OFFSET and retrieving records based on the previous set's last ID, we effectively eliminate the expensive sorting operation. Consequently, the query performance will improve drastically, especially for large tables.

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