Understanding min_examined_row_limit and Its Role in MySQL Query Efficiency

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Summary: Understanding min_examined_row_limit in MySQL Query Efficiency

min_examined_row_limit is a MySQL system variable that influences query optimization by dictating the minimum number of rows a table must have before the optimizer considers using an index. By default, it is set to zero, allowing index usage even on small tables. However, adjusting this value can improve performance in scenarios where index scans are inefficient, such as low-selectivity indexes or small result sets.

This variable interacts with the cost-based optimizer (CBO) to estimate execution costs. If a table has fewer rows than the threshold, the optimizer may favor a full table scan over an index scan. This is particularly useful when the overhead of index lookups outweighs the benefits of filtering rows. The variable is session-level by default, meaning changes apply only to the current session unless configured persistently in the MySQL configuration file.

To optimize effectively, use tools like EXPLAIN to analyze execution plans and iteratively adjust min_examined_row_limit. However, be cautious with persistent changes, as they can impact other queries. This variable should be part of a broader optimization strategy, including indexing, query design, and other optimizer settings.

In summary, min_examined_row_limit is a powerful tool for fine-tuning MySQL performance, but its optimal value depends on your specific database schema, data characteristics, and query workload.

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