Redis and SQL databases, such as MySQL or PostgreSQL, have different use cases and therefore varying performances under different circumstances.
Redis is an in-memory data store that can be used as a database, cache, and message broker. Because it's in-memory, it's incredibly fast, allowing operations to execute in microseconds. It supports simple types like strings, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs, geospatial indexes, etc. But keep in mind, being an in-memory database, the cost of memory can become a limiting factor for large data sets, while persistence options can affect performance.
SQL databases, on the other hand, are typically disk-based relational databases that provide strong ACID compliance and support complex queries with joins. They're excellent for situations where you have complex relations between entities or need to perform multi-step transactions. Performance can be slower compared to in-memory solutions, but modern SQL databases have sophisticated optimizations and caching techniques to speed up read-heavy workloads.
In summary, the choice between Redis and a SQL database depends largely on your specific use case and requirements. If you need high-speed access to simple data types and structures, Redis can provide superior performance. If you require complex queries, relationships, and ACID compliance, a SQL database may be more suitable. Often, both are used together in an application for caching and persistent storage.