Remove folding from Percentile

Author: julian-elastic

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

@@ -15,8 +15,8 @@
 import org.elasticsearch.compute.aggregation.PercentileDoubleAggregatorFunctionSupplier;
 import org.elasticsearch.compute.aggregation.PercentileIntAggregatorFunctionSupplier;
 import org.elasticsearch.compute.aggregation.PercentileLongAggregatorFunctionSupplier;
+import org.elasticsearch.xpack.esql.EsqlIllegalArgumentException;
 import org.elasticsearch.xpack.esql.core.expression.Expression;
-import org.elasticsearch.xpack.esql.core.expression.FoldContext;
 import org.elasticsearch.xpack.esql.core.expression.Literal;
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
@@ -34,6 +34,7 @@
 import java.util.List;
 
 import static java.util.Collections.singletonList;
+import static org.elasticsearch.core.Strings.format;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.FIRST;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.SECOND;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.isFoldable;
@@ -169,7 +170,12 @@ protected AggregatorFunctionSupplier doubleSupplier() {
     }
 
     private int percentileValue() {
-        return ((Number) percentile.fold(FoldContext.small() /* TODO remove me */)).intValue();
+        if (percentile() instanceof Literal literal) {
+            return ((Number) literal.value()).intValue();
+        }
+        throw new EsqlIllegalArgumentException(
+            format(null, "Percentile value must be a constant integer in [{}], found [{}]", source(), percentile)
+        );
     }
 
     @Override

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/230_folding.yml

@@ -559,3 +559,25 @@ Query using TERM function on name but with non constant query:
   - match: { error.type: "verification_exception" }
   - contains: { error.reason: "second argument of [TERM(name, salary)] must be [string], found value [salary] type [integer]" }
 
+---
+
+Percentile with foldable value:
+  - do:
+      esql.query:
+        body:
+          query: |
+            ROW x = 100 | STATS p = PERCENTILE(x, 19.8 + 13.4454545) | LIMIT 1
+  - match: { columns.0.name: "p" }
+  - match: { values.0.0: 100 }
+
+---
+
+Percentile with non-foldable value:
+  - do:
+      catch: bad_request
+      esql.query:
+        body:
+          query: |
+            FROM employees | STATS p = PERCENTILE(salary, salary + 1)  | LIMIT 1
+  - match: { error.type: "verification_exception" }
+  - contains: { error.reason: "second argument of [PERCENTILE(salary, salary + 1)] must be a constant, received [salary + 1]" }