wjjc1017 opened a new issue, #69489: URL: https://github.com/apache/doris/issues/69489
### Search before asking - [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues. ### Version Doris Nereids `master` ### What's Wrong? `fe/fe-core/src/main/java/org/apache/doris/nereids/rules/rewrite/EliminateGroupBy.java` treats sample and population dispersion functions identically: ```java // lines 71-72 private static final ImmutableSet<Class<? extends Expression>> supportedDevLikeFunctions = ImmutableSet.of(Stddev.class, StddevSamp.class, Variance.class, VarianceSamp.class); // lines 133-136 } else if (supportedDevLikeFunctions.contains(f.getClass())) { Expression expr = ifNullElse(f.child(0), new NullLiteral(DoubleType.INSTANCE), new DoubleLiteral(0)); newOutput.add((NamedExpression) ne.withChildren(expr)); } ``` When grouping by a unique, non-null key each group has exactly `n = 1` row. Population variance/stddev (`VAR_POP` / `STDDEV_POP`) divide by `n = 1` and indeed return `0.0`. **Sample** variance/stddev (`VAR_SAMP` / `STDDEV_SAMP`) divide by `n - 1 = 0` and must return `NULL`; the rewrite silently returns `0.0`. ### What You Expected? - Expected: `(1, NULL, NULL)`, `(2, NULL, NULL)` (each group is a single row). - Actual: `(1, 0.0, 0.0)`, `(2, 0.0, 0.0)`. (`STDDEV` / `VARIANCE` — i.e. population — correctly return `0.0` and must keep doing so.) ### How to Reproduce? ```sql CREATE TABLE t (k INT, x INT) DUPLICATE KEY(k) DISTRIBUTED BY HASH(k) BUCKETS 1; INSERT INTO t VALUES (1, 5), (2, 7); SELECT k, STDDEV_SAMP(x), VAR_SAMP(x) FROM t GROUP BY k; ``` ### Anything Else? ### Root cause `supportedDevLikeFunctions` conflates population and sample variants. The empty-group/one-row compensation `ifNullElse(arg, NULL, 0.0)` is only valid for the population functions. ### Suggested fix Split the set: keep `Stddev` / `Variance` in `supportedDevLikeFunctions`, and handle `StddevSamp` / `VarianceSamp` separately by rewriting to `NULL` (not `0.0`) when the group is a single row — or simply exclude the sample variants from `canRewrite` and keep the aggregate. ### Are you willing to submit PR? - [ ] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct) -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
