KhushJain opened a new pull request, #4691:
URL: https://github.com/apache/solr/pull/4691
https://issues.apache.org/jira/browse/SOLR-18328
# Description
Added support for the `std(col)` (standard deviation) metric in `rollup()`
streaming expressions.
The `std(col)` metric was non-functional; the `StdMetric` class was a stub
copy-pasted from `MeanMetric` — `update()` was a no-op, `getValue()` returned
`null`, and `newInstance()` even returned a `MeanMetric` instead of a
`StdMetric`.
# Solution
Implemented `StdMetric` to compute the sample standard deviation:
**`StdMetric.java`**:
1. `update()` now accumulates `count`, `sum`, and `sumSq` from the column
value (handling `Double`/`Float`/`Integer`/`Long`, skipping nulls/other types).
2. `getValue()` computes the sample standard deviation `sqrt((n*sumSq -
sum^2) / (n*(n-1)))`, returning `0.0` when `count <= 1`, and honoring
`outputLong` (rounds to a `long` when set).
3. Fixed `newInstance()` to return a `StdMetric` (was incorrectly returning
`MeanMetric`), and replaced the unused `doubleSum`/`longSum` fields with
`sum`/`sumSq`.
# Tests
- **`StreamExpressionTest.testRollupStdMetric`** (new): validates `std(a_i)`
and `std(a_f)` alongside `count(*)` in a `rollup(... over="a_s" ...)`
expression across multiple groups.
- **`StreamingTest.testRollupStream`**: added `StdMetric("a_i")` /
`StdMetric("a_f")` to the metrics array and asserted expected values per group.
# Checklist
Please review the following and check all that apply:
- [x] I have reviewed the guidelines for [How to
Contribute](https://github.com/apache/solr/blob/main/CONTRIBUTING.md) and my
code conforms to the standards described there to the best of my ability.
- [x] I have created a Jira issue and added the issue ID to my pull request
title.
- [x] I have given Solr maintainers
[access](https://help.github.com/en/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork)
to contribute to my PR branch. (optional but recommended, not available for
branches on forks living under an organisation)
- [x] I have developed this patch against the `main` branch.
- [x] I have run `./gradlew check`.
- [x] I have added tests for my changes.
- [x] I have added documentation for the [Reference
Guide](https://github.com/apache/solr/tree/main/solr/solr-ref-guide)
- [x] I have added a [changelog
entry](https://github.com/apache/solr/blob/main/dev-docs/changelog.adoc) for my
change
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