Fix stats aggregation returning zero results with size:0

[prudhvigodithi]

Description

While testing intra segment #20202 noticed the bug where for stats aggs collectRange method uses maximum instead of max variable. See the SumAggregator which works correctly.

Before fix

curl -s "localhost:9200/big5/_search?pretty" -H 'Content-Type: application/json' -d' { "size": 0, "aggs": { "metrics_stats": { "stats": { "field": "metrics.size" } } } }'
{
  "took" : 3,
  "timed_out" : false,
  "terminated_early" : true,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 10000,
      "relation" : "gte"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "metrics_stats" : {
      "count" : 0,
      "min" : null,
      "max" : null,
      "avg" : null,
      "sum" : 0.0
    }
  }
}

After fix

curl -s "localhost:9200/big5/_search?pretty" -H 'Content-Type: application/json' -d' { "size": 0, "aggs": { "metrics_stats": { "stats": { "field": "metrics.size" } } } }'
{
  "took" : 1080,
  "timed_out" : false,
  "terminated_early" : true,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 10000,
      "relation" : "gte"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "metrics_stats" : {
      "count" : 116000000,
      "min" : 1000.0,
      "max" : 9999.0,
      "avg" : 2438.516547112069,
      "sum" : 2.82867919465E11
    }
  }
}

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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📝 Walkthrough

Walkthrough

Modified the collectRange method in StatsAggregator to use the explicit max parameter instead of a stored maximum value from maxes.get(0) when determining the range of documents to process. A corresponding test was added to validate the per-leaf range collection behavior.

Changes

Cohort / File(s)	Summary
Core Logic server/src/main/java/org/opensearch/search/aggregations/metrics/StatsAggregator.java	Modified loop termination condition in collectRange from iterating while doc < maxes.get(0) to doc < max, switching from stored value to explicit parameter-based range boundary.
Test Coverage server/src/test/java/org/opensearch/search/aggregations/metrics/StatsAggregatorTests.java	Added testCollectRange() method with supporting imports (LeafReaderContext, LeafBucketCollector) to validate per-leaf collection logic by building expected statistics through direct value summation and asserting count, sum, min, max, and average.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

bug, Search:Aggregations

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing stats aggregation to return correct results when size:0 is specified.
Description check	✅ Passed	The description includes problem explanation with code reference and before/after examples, but the Related Issues section is incomplete and the Check List items are not marked.

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[github-actions]

❌ Gradle check result for 374d98a: FAILURE

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[github-actions]

❌ Gradle check result for 374d98a: FAILURE

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[github-actions]

❌ Gradle check result for 8b2d689: FAILURE

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[github-actions]

❌ Gradle check result for 8b2d689: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 8b2d689: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.23%. Comparing base (be7b387) to head (8b2d689).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20427      +/-   ##
============================================
- Coverage     73.28%   73.23%   -0.05%     
- Complexity    71825    71868      +43     
============================================
  Files          5793     5793              
  Lines        328844   328846       +2     
  Branches      47343    47343              
============================================
- Hits         240978   240827     -151     
- Misses        68571    68743     +172     
+ Partials      19295    19276      -19     

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[prudhvigodithi]

Thanks @bowenlan-amzn, adding @asimmahmood1 @jainankitk

[asimmahmood1]

Good find! I'm surprised it wasn't caught in testing.

Looks like this was introduced in 3.4: #20067

I think Codecov report should've caught that is line isn't tested:

https://app.codecov.io/gh/opensearch-project/OpenSearch/pull/20427?dropdown=coverage&src=pr&el=h1&utm_medium=referral&utm_source=github&utm_content=checks&utm_campaign=pr+comments&utm_term=opensearch-project

[jainankitk]

Thanks @prudhvigodithi for fixing this. Hopefully we don't have similar issue for other aggregators