[ES|QL] Add a standard deviation function

docs/changelog/116531.yaml

@@ -1,5 +1,5 @@
 pr: 116531
-summary: "[ES|QL] Add a standard deviation function"
+summary: "Add a standard deviation function"
 area: ES|QL
 type: enhancement
 issues: []

x-pack/plugin/esql/qa/testFixtures/src/main/resources/stats.csv-spec

@@ -2951,3 +2951,38 @@ FROM employees
 STD_DEV(languages):double
 null
 ;
+
+stdDevMultiValue
+required_capability: std_dev
+FROM employees
+| STATS STD_DEV(salary_change)
+;
+
+STD_DEV(salary_change):double
+7.062226788733394
+;
+
+stdDevFilter
+required_capability: std_dev
+FROM employees
+| STATS greater_than = STD_DEV(salary_change) WHERE languages > 3
+, less_than = STD_DEV(salary_change) WHERE languages <= 3
+, salary = STD_DEV(salary * 2)
+, count = COUNT(*) BY gender
+;
+
+greater_than:double | less_than:double  | salary:double      | count:long | gender:keyword
+6.949207097931448   | 7.127229475750027 | 27921.220736207077 | 10         | null
+6.975232333891946   | 6.604807075547775 | 26171.331109641273 | 57         | M
+6.4543266953142835  | 7.57786788789264  | 29045.770666969744 | 33         | F
+;
+
+stdDevRow
+required_capability: std_dev
+ROW a = [1,2,3], b = 5
+| STATS STD_DEV(a), STD_DEV(b)
+;
+
+STD_DEV(a):double | STD_DEV(b):double
+0.816496580927726 | 0.0
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/StdDev.java

@@ -28,6 +28,8 @@
 import java.util.List;
 
 import static java.util.Collections.emptyList;
+import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.DEFAULT;
+import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.isType;
 
 public class StdDev extends AggregateFunction implements ToAggregator {
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(Expression.class, "StdDev", StdDev::new);
@@ -68,6 +70,17 @@ public DataType dataType() {
         return DataType.DOUBLE;
     }
 
+    @Override
+    protected Expression.TypeResolution resolveType() {
+        return isType(
+            field(),
+            dt -> dt.isNumeric() && dt != DataType.UNSIGNED_LONG,
+            sourceText(),
+            DEFAULT,
+            "numeric except unsigned_long or counter types"
+        );
+    }
+
     @Override
     protected NodeInfo<StdDev> info() {
         return NodeInfo.create(this, StdDev::new, field(), filter());