[Discover] [ES|QL] fix: stats by previous aggregated field - #290259

…0259) (#290297)

# Backport

This will backport the following commits from `main` to `9.5`:
- [[Discover] [ES|QL] fix: stats by previous aggregated field
(#290259)](#290259)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Sebastian Delle
Donne","email":"sebastian.delledonne@elastic.co"},"sourceCommit":{"committedDate":"2026-09-10T11:59:28Z","message":"[Discover]
[ES|QL] fix: stats by previous aggregated field (#290259)\n\n- Closes
https://github.com/elastic/kibana/issues/290060\n## Summary\n\nFixes an
unhandled `TypeError: undefined is not an object
(evaluating\n'groupFieldNode.arg')` thrown by `getESQLStatsQueryMeta`
(used by\nDiscover's ES|QL \"cascade documents\" breakdown feature) when
a query has\ntwo `STATS` commands and the second groups `BY` a field
that the first\n`STATS` declared as an **aggregate** alias rather than a
`BY`-grouping\nalias.\n\n### Repro\n\n```esql\nFROM
kibana_sample_data_logs\n| STATS x = MAX(bytes)\n| STATS c = COUNT(*) BY
x\n```\n\n### Solution\n`x` should be treated as a regular column, the
code was incorrectly\nbranching into a special case as `x` was defined
in the previous `STATS`\ncommand (expecting to be found as new group
definition).\nAlso added a catch to avoid buggy UI when analysing the
query fails,\n(discover was held in a permanent loading state before the
fix)\n\n#### Before\n<img width=\"1724\" height=\"945\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/17ba47a5-84f3-48fc-b09f-bb6263d1ae07\"\n/>\n\n####
After\n<img width=\"1723\" height=\"902\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/c137f927-01e3-4705-8d0d-ad50663f08e0\"\n/>\n\n\n\n###
Checklist\n- [x] [Unit or
functional\ntests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)\nwere
updated or added to match the most common
scenarios","sha":"d5e1a4e37970834570aaa52d0488081ded16123c","branchLabelMapping":{"^v9.6.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Feature:Discover","release_note:fix","Team:DataDiscovery","Team:ESQL","backport:version","v9.6.0","v9.5.5"],"title":"[Discover]
[ES|QL] fix: stats by previous aggregated
field","number":290259,"url":"https://github.com/elastic/kibana/pull/290259","mergeCommit":{"message":"[Discover]
[ES|QL] fix: stats by previous aggregated field (#290259)\n\n- Closes
https://github.com/elastic/kibana/issues/290060\n## Summary\n\nFixes an
unhandled `TypeError: undefined is not an object
(evaluating\n'groupFieldNode.arg')` thrown by `getESQLStatsQueryMeta`
(used by\nDiscover's ES|QL \"cascade documents\" breakdown feature) when
a query has\ntwo `STATS` commands and the second groups `BY` a field
that the first\n`STATS` declared as an **aggregate** alias rather than a
`BY`-grouping\nalias.\n\n### Repro\n\n```esql\nFROM
kibana_sample_data_logs\n| STATS x = MAX(bytes)\n| STATS c = COUNT(*) BY
x\n```\n\n### Solution\n`x` should be treated as a regular column, the
code was incorrectly\nbranching into a special case as `x` was defined
in the previous `STATS`\ncommand (expecting to be found as new group
definition).\nAlso added a catch to avoid buggy UI when analysing the
query fails,\n(discover was held in a permanent loading state before the
fix)\n\n#### Before\n<img width=\"1724\" height=\"945\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/17ba47a5-84f3-48fc-b09f-bb6263d1ae07\"\n/>\n\n####
After\n<img width=\"1723\" height=\"902\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/c137f927-01e3-4705-8d0d-ad50663f08e0\"\n/>\n\n\n\n###
Checklist\n- [x] [Unit or
functional\ntests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)\nwere
updated or added to match the most common
scenarios","sha":"d5e1a4e37970834570aaa52d0488081ded16123c"}},"sourceBranch":"main","suggestedTargetBranches":["9.5"],"targetPullRequestStates":[{"branch":"main","label":"v9.6.0","branchLabelMappingKey":"^v9.6.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/290259","number":290259,"mergeCommit":{"message":"[Discover]
[ES|QL] fix: stats by previous aggregated field (#290259)\n\n- Closes
https://github.com/elastic/kibana/issues/290060\n## Summary\n\nFixes an
unhandled `TypeError: undefined is not an object
(evaluating\n'groupFieldNode.arg')` thrown by `getESQLStatsQueryMeta`
(used by\nDiscover's ES|QL \"cascade documents\" breakdown feature) when
a query has\ntwo `STATS` commands and the second groups `BY` a field
that the first\n`STATS` declared as an **aggregate** alias rather than a
`BY`-grouping\nalias.\n\n### Repro\n\n```esql\nFROM
kibana_sample_data_logs\n| STATS x = MAX(bytes)\n| STATS c = COUNT(*) BY
x\n```\n\n### Solution\n`x` should be treated as a regular column, the
code was incorrectly\nbranching into a special case as `x` was defined
in the previous `STATS`\ncommand (expecting to be found as new group
definition).\nAlso added a catch to avoid buggy UI when analysing the
query fails,\n(discover was held in a permanent loading state before the
fix)\n\n#### Before\n<img width=\"1724\" height=\"945\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/17ba47a5-84f3-48fc-b09f-bb6263d1ae07\"\n/>\n\n####
After\n<img width=\"1723\" height=\"902\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/c137f927-01e3-4705-8d0d-ad50663f08e0\"\n/>\n\n\n\n###
Checklist\n- [x] [Unit or
functional\ntests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)\nwere
updated or added to match the most common
scenarios","sha":"d5e1a4e37970834570aaa52d0488081ded16123c"}},{"branch":"9.5","label":"v9.5.5","branchLabelMappingKey":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Sebastian Delle Donne <sebastian.delledonne@elastic.co>