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

Hello, I am Wufan.

“Redis 有慢查询怎么办？”

First, locate slow queries using the slow query log.

Slow Query Log

Retrieve Slow Query Log

slowlog get [N]

The log contains four fields: entry ID, timestamp, execution time, and the command with its arguments.

Get Current Length of Slow Query Log List

showlog len

Clear Slow Query Log

showlog reset

Complexity of Slow Commands

The slowness of a command is related to its algorithmic complexity. You can check command complexities at the official Redis command reference: https://redis.io/commands When Redis performance degrades, use the Redis log or the latency monitor tool to identify slow requests, then consult the documentation to see if the command has high complexity. For example, the keys command has O(N) complexity, as shown in the figure below.

Replace with Efficient Commands

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

When you need all members of a set, use SSCAN iteratively instead of SMEMBERS to avoid returning a large payload at once.

For sorting, intersection, or union operations, perform them in the client rather than using SORT, SUNION, or SINTER to prevent slowing down the Redis instance.

The KEYS command scans all stored key‑value pairs, causing high latency and is generally discouraged in production.

Best Practices

Two main configurations 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 value: 128 (as seen in the source code).

Configuration recommendation: In production, increase the length to avoid truncating long commands; a value of 1000 or more is common.

slowlog-log-slower-than

Purpose: Threshold (in microseconds) for a command to be considered slow and logged.

Default value: 10 ms.

Configuration recommendation: Adjust based on traffic; high‑throughput environments may set it to 1 ms.

Persisting the Log

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

References: Redis 6.0 source code, Redis development and operations articles.