MQE: fix issue where subqueries could return series with no points

[charleskorn]

What this PR does

This PR fixes an issue where MQE could return invalid results if a subquery was evaluated as the top-level expression for an instant query.

If a series had all of its samples filtered out, MQE would return a series with no samples, for example the query (avg(foo) > 100)[10m:30s] could return a result like this:

{
  "status": "success",
  "data": {
    "resultType": "matrix",
    "result": [
      {
        "metric": {
          "env": "prod"
        }
      }
    ]
  }
}

It should instead not return the series at all:

{
  "status": "success",
  "data": {
    "resultType": "matrix",
    "result": [
    ]
  }
}

Which issue(s) this PR fixes or relates to

(none)

Checklist

• Tests updated.
• [n/a] Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [n/a] about-versioning.md updated with experimental features.

[jhesketh]

lgtm

However do we have a test where this happens with a sub-query at not the top of the query. I believe it should be fine, but I think we should have a test for it.

[charleskorn]

However do we have a test where this happens with a sub-query at not the top of the query. I believe it should be fine, but I think we should have a test for it.

Do you mean something like last_over_time((metric > Inf)[20s:10s]), or something like ((metric > Inf)[20s:10s])[30s:20s]?

[jhesketh]

Do you mean something like last_over_time((metric > Inf)[20s:10s]), or something like ((metric > Inf)[20s:10s])[30s:20s]?

I meant the former, but why not both? ¯_(ツ)_/¯

[charleskorn]

Do you mean something like last_over_time((metric > Inf)[20s:10s]), or something like ((metric > Inf)[20s:10s])[30s:20s]?

I meant the former, but why not both? ¯_(ツ)_/¯

Added in 961f2e5

[jhesketh]

Thanks!

[grafanabot]

The backport to r314 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-9998-to-r314 origin/r314
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 1afa9f6750da32c166a55427f0b9cbea227d67da
# Push it to GitHub
git push --set-upstream origin backport-9998-to-r314
git switch main
# Remove the local backport branch
git branch -D backport-9998-to-r314

Then, create a pull request where the base branch is r314 and the compare/head branch is backport-9998-to-r314.

[grafanabot]

The backport to r315 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-9998-to-r315 origin/r315
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 1afa9f6750da32c166a55427f0b9cbea227d67da
# Push it to GitHub
git push --set-upstream origin backport-9998-to-r315
git switch main
# Remove the local backport branch
git branch -D backport-9998-to-r315

Then, create a pull request where the base branch is r315 and the compare/head branch is backport-9998-to-r315.

[grafanabot]

The backport to r316 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-9998-to-r316 origin/r316
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 1afa9f6750da32c166a55427f0b9cbea227d67da
# Push it to GitHub
git push --set-upstream origin backport-9998-to-r316
git switch main
# Remove the local backport branch
git branch -D backport-9998-to-r316

Then, create a pull request where the base branch is r316 and the compare/head branch is backport-9998-to-r316.

[grafanabot]

The backport to r317 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-9998-to-r317 origin/r317
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 1afa9f6750da32c166a55427f0b9cbea227d67da
# Push it to GitHub
git push --set-upstream origin backport-9998-to-r317
git switch main
# Remove the local backport branch
git branch -D backport-9998-to-r317

Then, create a pull request where the base branch is r317 and the compare/head branch is backport-9998-to-r317.