How to Handle Redis Slow Queries: Logs, Commands, and Best Practices

Redis slow queries can degrade performance, and the first step is to locate them using the slow query log.

Slow Query Log

Retrieve Slow Query Log

showlog get [N]

The log contains four fields: log ID, timestamp, command duration, and the executed command with its arguments.

Get Current Length of Slow Query Log List

showlog len

Clear Slow Query Log

showlog reset

Command Complexity of Slow Queries

Slow operations are related to command complexity. You can check command complexities at https://redis.io/commands . For example, the keys command has O(N) complexity and should be avoided in production.

Replace with Efficient Commands

If many slow commands are identified, replace them with more efficient alternatives:

Use SSCAN instead of SMEMBERS to iterate over set members and avoid blocking.

Perform sorting, intersection, and union operations on the client side rather than using SORT, SUNION, or SINTER commands.

The KEYS command scans all keys and is generally discouraged in production environments.

Best Practices

Two main configuration options control the slow query log: slowlog-max-len and slowlog-log-slower-than.

slowlog-max-len

Purpose: Maximum number of entries stored in the slow query log.

Default: 128 entries (source code confirms this default).

Recommendation: Increase the length in production (e.g., >1000) to avoid truncating long commands and to limit memory usage.

slowlog-log-slower-than

Purpose: Threshold (in microseconds) for logging a command as slow.

Default: 10 ms.

Recommendation: Adjust based on traffic; for high‑load scenarios, set to 1 ms.

Persisting the Log

Persist the slow query log to a database to prevent loss and enable visual query interfaces for operations troubleshooting.