Does it make sense to use an index that will have a low cardinality?
Yes, even a low cardinality index can yield performance gains under certain conditions, such as when combined with other columns in a composite index. It particularly proves effective in read-intensive environments where recurrent access patterns align with the indexed fields:
Here, even though status has limited distinct values, combining it with lastAccessed offers a faster, more efficient multilevel search than a complete table scan.
An in-depth breakdown: Under what circumstances do low cardinality indexes shine?
Use Case - Performance benefits for order-by/limit clauses
Low cardinality indexes are typically not universal problem solvers, but shine bright like a diamond in specific use cases such as ORDER BY with LIMIT clauses. They can speed things up significantly, making the query execution feel just like a Sunday drive.
The rare jewel - Infrequently occurring value fields
In scenarios where fields contain infrequently occurring values with an overall low cardinality, an index becomes beneficial. Though creating the index may not affect DML operations substantially, unless it's a major performance bottleneck, it's probably a safe bet.
Composite index - the power combo
When using composite indexes including a low cardinality field, it is about the company it keeps. Ensure it is paired with other high-selectivity fields to reduce disk I/O operations and make your application run like a gazelle.
Test, test, and test again!
Remember: Benchmarking is key. Testing both with and without the index provides invaluable insights into real-world efficiency. Based on your datasets and queries, the same index can either prove a boon or have zero impact.
Context is key - Database specifics
Your database engine matters. Fans of MySQL 5, take a bow – the query optimizer might decide not to use the index if more suitable indexes are available or if the table is small. Just like you, it prefers the path of least resistance.
Making the right call - Trade-offs in index usage
Before you slap an index on a column, consider the balance between the burden of a bigger index size against the gains of avoiding a full table scan. Sometimes, reading more rows with an index can be a sneaky ninja who beats the full table scan bouncer.
Keeping things sorted - Indexes and ordering
Indexes on low-cardinality fields can still find their moments of glory, especially when dealing with sorting operations involving ORDER BY clauses. Testing code puts you in the driver's seat when deciding to maintain such an index.
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