Hacktoberfest 2026: the issues maintainers tagged for October, open and beginner-friendly. Browse Hacktoberfest issues

[Bug]: selector-filtered _changes is slower the first time after creating a Mango index

Open
  • Is something wrong?
#5,926 1 comment 0 reactions 0 assignees View on GitHub

Maintainers usually reply within 1 day

Nobody has claimed this yet.

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
erlang
Domain
databases

Research direction

Reproduce the slowdown with the provided curl commands using a large database, then compare the first and later selector-filtered _changes requests before and after creating the _index entry. Trace the _changes selector-filtering path alongside index creation or update handling. Done means the first filtered _changes request is not delayed by unrelated Mango indexes, with regression coverage for the reproduced case.

Written by the indexing model from the issue text.

Description

bug needs-triage
Version

3.5.1, and main branch

Describe the problem you're encountering

Despite the fact that _changes does not use indexes to speed up selector filters, the first time it is requested after creating a Mango index, it is slower, which possibly indicates it's spending time updating the index.

Expected Behaviour

The time taken for _changes to respond should not depend on the existence or otherwise of Mango indexes.

Steps to Reproduce

To demonstrate this, I created a DB containing a million docs of the form { "_id": "doc-N", "value": N } for N from 1 to 1,000,000. Then I timed how long it took to run this filtered changes request:

# uses this shell function:
cdb () {
  curl -gsu 'admin:admin' "http://127.0.0.1:5984$1" "${@:2}"
}

time ( cdb '/db/_changes?filter=_selector' -X POST -d '{ "selector": { "value": { "$lt": 100 } } }' )

This would take around 12 seconds.

Then I created an index covering the filtered field and re-timed:

cdb '/db/_index' -X POST -d '{ "index": { "fields": ["value"] }, "name": "by-value", "type": "json" }'
time ( cdb '/db/_changes?filter=_selector' -X POST -d '{ "selector": { "value": { "$lt": 100 } } }' )

The first time after creating an index, _changes takes over 30s to complete. On later requests it's back to around 12s, possibly very slightly slower.

This effect shows up regardless of which fields the index covers; even if it does not cover the _changes selector, the _changes request is still slow.

Your Environment

macOS 15, CouchDB 3.5.1, single node, database created with q=2, n=1

Additional Context

No response

Dominant language
Erlang
Stars
7k
Forks
1.1k
Avg merge
1d 23m
Merged PRs (30d)
30

Getting set up

Open in Codespaces

Starts the project's dev container in your browser, under your own GitHub account.

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from apache/couchdb

All issues in apache/couchdb

Similar issues

More Databases issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.