Do it this way instead

Author: nik9000

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

@@ -100,9 +100,6 @@ public ExpressionEvaluator.Factory toEvaluator(ToEvaluator toEvaluator) {
     @Override
     public PushedBlockLoaderExpression tryPushToFieldLoading(SearchStats stats) {
         if (field instanceof FieldAttribute f) {
-            if (stats.hasDocValues(f.fieldName()) == false) {
-                return null;
-            }
             BlockLoaderWarnings warnings = new BlockLoaderWarnings(DriverContext.WarningsMode.COLLECT, source());
             return new PushedBlockLoaderExpression(f, new BlockLoaderFunctionConfig.Named("LENGTH", warnings));
         }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/stats/SearchContextStats.java

@@ -168,8 +168,13 @@ public boolean supportsLoaderConfig(
         for (SearchExecutionContext context : contexts) {
             MappedFieldType ft = context.getFieldType(name.string());
             if (ft == null) {
-                // Missing fields are always null, no matter what we try to push.
-                continue;
+                /*
+                 * Missing fields are always null no matter what we try to push so they
+                 * should work, but we need this check here to prevent actually pushing
+                 * to a LOOKUP JOIN. If the field comes from a LOOKUP JOIN  then it'll
+                 * show up as missing here. And we can't push to those fields. Yet.
+                 */
+                return false;
             }
             if (ft.supportsBlockLoaderConfig(config, preference) == false) {
                 // If any one field doesn't support the loader config we'll disable pushing the expression to the field