SET unmapped_fields="nullify";
from languages_lookup_non_unique_key
| lookup join languages_lookup on language_code 
| keep language_code, language_name, foobar

EsqlIllegalArgumentException: unsupported logical plan node [Join]
        at org.elasticsearch.xpack.esql.planner.mapper.MapperUtils.unsupported(MapperUtils.java:202)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.mapBinary(Mapper.java:199)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.mapInner(Mapper.java:71)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.mapUnary(Mapper.java:90)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.mapInner(Mapper.java:67)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.mapUnary(Mapper.java:90)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.mapInner(Mapper.java:67)
        at org.elasticsearch.xpack.esql.planner.mapper.Mapper.map(Mapper.java:58)
        at org.elasticsearch.xpack.esql.session.EsqlSession.physicalPlan(EsqlSession.java:1250)
        at org.elasticsearch.xpack.esql.session.EsqlSession.optimizedPhysicalPlan(EsqlSession.java:1262)
        at org.elasticsearch.xpack.esql.session.EsqlSession.logicalPlanToPhysicalPlan(EsqlSession.java:1196)
        at org.elasticsearch.xpack.esql.session.EsqlSession.executeSubPlans(EsqlSession.java:465)
        at org.elasticsearch.xpack.esql.session.EsqlSession.executeOptimizedPlan(EsqlSession.java:395)
        at org.elasticsearch.xpack.esql.session.EsqlSession$1.lambda$onResponse$3(EsqlSession.java:319)
        at org.elasticsearch.server@9.4.0-SNAPSHOT/org.elasticsearch.action.ActionListenerImplementations$ResponseWrappingActionListener.onResponse(ActionListenerImplementations.java:261)