Is selective key export from BGSAVE / PSYNC possible?

[otheng03]

Background

In large Valkey deployments, it is sometimes need to extract a subset of keys based on a custom condition (for example, a hash applied to the key name), while minimizing impact on the running instance.

Using SCAN (or similar keyspace iteration) doesn't seem to be ideal in this case, as it still requires iterating over the entire keyspace and may take a long time or introduce overhead when the number of keys is very large.

Commands like BGSAVE and PSYNC are attractive because they use background processes and allow dumping data efficiently without significantly slowing down Valkey’s main operations. However, they currently dump all keys, which then requires additional post-processing to filter the keys of interest.

Question

I’m looking for a way to extract only a subset of keys that satisfy a specific condition from a snapshot taken at a point in time, ideally without scanning the entire keyspace by SCAN command in the main Valkey process.

For example, consider a scenario like:

• Extract only keys where hash_function(key) == 1000
• The goal is to complete this within a few seconds
• Avoid putting significant CPU or latency pressure on the primary Valkey process

At the moment, the options seem to be:

• Use SCAN and filter client-side → Requires SCAN commands iteration and can be slow at scale.
• Use BGSAVE or PSYNC and post-process the full dump offline → Efficient for the server, but still requires dumping all keys.

My questions are:

• Are there recommended methods or extensions (modules, hooks, etc.) for achieving conditional key extraction more efficiently?
• If this is intentionally unsupported, is the main reason architectural (e.g., design trade-offs)?

Thank you for the reading. Any guidance or confirmation would be greatly appreciated.