Does using a composite primary key in MySQL significantly impact insert, update, and select operations for a table with ~1 million rows, 200 inserts/second, and 200 selects/second?

Performance Implications of Composite Primary Keys in MySQL

In MySQL, a composite primary key combines multiple fields to uniquely identify rows in a table. While composite primary keys offer advantages for data integrity and certain types of queries, they also raise concerns about potential performance impacts.

The provided context describes a table with a composite primary key consisting of three fields in MySQL 5.1. With around 200 inserts and 200 selects per second, and a table size of approximately 1 million rows, the question arises whether the composite primary key affects the performance of these operations.

Impact on Inserts and Updates

The answer to this question is that the performance difference between using a composite primary key and a simple auto-incrementing integer (INT) primary key is negligible for inserts and updates. Both types of primary keys will perform similarly, especially with the limited number of inserts and updates described in the context.

Effect on Selects

The impact of a composite primary key on select operations depends on various factors. InnoDB tables are implicitly clustered on the primary key value, meaning that searches for rows based on the primary key will be faster than using a secondary index. However, this benefit is only realized if both fields in the composite primary key are included in the query's WHERE clause.

For instance, if the table layout includes a composite primary key of (col1, col2), and the query only searches for rows based on col1, the search will not take advantage of the primary key and will instead use the secondary index (if one exists) or perform a full table scan.

Comparison to Auto-Incrementing Field

If an auto-incrementing field is used as a "fake" primary key, additional lookups are required during SELECT operations. In this scenario, the engine must first find the corresponding row pointer in the index for the composite key (col1, col2) and then use that row pointer to retrieve the actual row from the table. This process is slightly more time-consuming compared to using a composite primary key, especially if the index for the composite key is not optimally designed.

Conclusion

In MySQL, composite primary keys can have potential performance implications on SELECT operations, particularly if not all fields in the composite key are used in the query's WHERE clause. However, for the given scenario with a moderate number of inserts, updates, and selects, the difference between using a composite primary key and an auto-incrementing field is likely to be minimal.

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