[ES|QL] Add support for byte_length scalar function

docs/changelog/116591.yaml

@@ -0,0 +1,5 @@
+pr: 116591
+summary: "[ES|QL] Add support for `byte_length` scalar function"
+area: ES|QL
+type: feature
+issues: []

docs/reference/esql/functions/string-functions.asciidoc

@@ -9,6 +9,7 @@
 
 // tag::string_list[]
 * <<esql-bit_length>>
+* <<esql-byte_length>>
 * <<esql-concat>>
 * <<esql-ends_with>>
 * <<esql-from_base64>>
@@ -32,6 +33,7 @@
 // end::string_list[]
 
 include::layout/bit_length.asciidoc[]
+include::layout/byte_length.asciidoc[]
 include::layout/concat.asciidoc[]
 include::layout/ends_with.asciidoc[]
 include::layout/from_base64.asciidoc[]

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

@@ -366,12 +366,31 @@ FROM employees
 
 // tag::length-result[]
 first_name:keyword   | last_name:keyword   | fn_length:integer
-Alejandro      |McAlpine       |9              
-Amabile        |Gomatam        |7              
+Alejandro      |McAlpine       |9
+Amabile        |Gomatam        |7
 Anneke         |Preusig        |6
 // end::length-result[]
 ;
 
+docsByteLength
+required_capability: fn_byte_length
+// tag::byteLength[]
+FROM employees
+| KEEP first_name, last_name
+| EVAL fn_byte_length = BYTE_LENGTH(first_name)
+// end::byteLength[]
+| SORT first_name
+| LIMIT 3
+;
+
+// tag::byteLength-result[]
+first_name:keyword   | last_name:keyword   | fn_byte_length:integer
+Alejandro      |McAlpine       |9
+Amabile        |Gomatam        |7
+Anneke         |Preusig        |6
+// end::byteLength-result[]
+;
+
 docsGettingStartedEvalNoColumnName
 // tag::gs-eval-no-column-name[]
 FROM sample_data

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -33,6 +33,11 @@ public enum Cap {
          */
         FN_BIT_LENGTH,
 
+        /**
+         * Support for function {@code BYTE_LENGTH}.
+         */
+        FN_BYTE_LENGTH,
+
         /**
          * Support for function {@code REVERSE}.
          */

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

@@ -118,6 +118,7 @@
 import org.elasticsearch.xpack.esql.expression.function.scalar.spatial.StX;
 import org.elasticsearch.xpack.esql.expression.function.scalar.spatial.StY;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.BitLength;
+import org.elasticsearch.xpack.esql.expression.function.scalar.string.ByteLength;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.Concat;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.EndsWith;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.LTrim;
@@ -308,6 +309,7 @@ private FunctionDefinition[][] functions() {
             // string
             new FunctionDefinition[] {
                 def(BitLength.class, BitLength::new, "bit_length"),
+                def(ByteLength.class, ByteLength::new, "byte_length"),
                 def(Concat.class, Concat::new, "concat"),
                 def(EndsWith.class, EndsWith::new, "ends_with"),
                 def(LTrim.class, LTrim::new, "ltrim"),

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/UnaryScalarFunction.java

@@ -55,6 +55,7 @@
 import org.elasticsearch.xpack.esql.expression.function.scalar.multivalue.AbstractMultivalueFunction;
 import org.elasticsearch.xpack.esql.expression.function.scalar.spatial.StX;
 import org.elasticsearch.xpack.esql.expression.function.scalar.spatial.StY;
+import org.elasticsearch.xpack.esql.expression.function.scalar.string.ByteLength;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.LTrim;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.Length;
 import org.elasticsearch.xpack.esql.expression.function.scalar.string.RLike;
@@ -80,6 +81,7 @@ public static List<NamedWriteableRegistry.Entry> getNamedWriteables() {
         entries.add(Acos.ENTRY);
         entries.add(Asin.ENTRY);
         entries.add(Atan.ENTRY);
+        entries.add(ByteLength.ENTRY);
         entries.add(Cbrt.ENTRY);
         entries.add(Ceil.ENTRY);
         entries.add(Cos.ENTRY);

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/BitLength.java

@@ -40,6 +40,7 @@ public class BitLength extends UnaryScalarFunction {
     @FunctionInfo(
         returnType = "integer",
         description = "Returns the bit length of a string.",
+        note = "All strings are in UTF-8, so a single character can use multiple bytes.",
         examples = @Example(file = "docs", tag = "bitLength")
     )
     public BitLength(