Add handling for remote not supporting LOOKUP JOIN on multiple fields

Author: julian-elastic

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/lookup/LookupEnrichQueryGenerator.java

@@ -10,6 +10,10 @@
 import org.apache.lucene.search.Query;
 import org.elasticsearch.core.Nullable;
 
+/**
+ * An interface to generates queries for the lookup and enrich operators.
+ * This interface is used to retrieve queries based on a position index.
+ */
 public interface LookupEnrichQueryGenerator {
 
     /**
@@ -18,6 +22,9 @@ public interface LookupEnrichQueryGenerator {
     @Nullable
     Query getQuery(int position);
 
+    /**
+     * Returns the number of queries in this generator
+     */
     int getPositionCount();
 
 }

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/lookup/QueryList.java

@@ -74,6 +74,7 @@ protected QueryList(
     /**
      * Returns the number of positions in this query list
      */
+    @Override
     public int getPositionCount() {
         return block.getPositionCount();
     }
@@ -87,6 +88,7 @@ public int getPositionCount() {
      */
     public abstract QueryList onlySingleValues(Warnings warnings, String multiValueWarningMessage);
 
+    @Override
     public final Query getQuery(int position) {
         final int valueCount = block.getValueCount(position);
         if (onlySingleValueParams != null && valueCount != 1) {

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/enrich/LookupFromIndexService.java

@@ -251,6 +251,8 @@ public void writeTo(StreamOutput out) throws IOException {
             }
             if (out.getTransportVersion().onOrAfter(TransportVersions.ESQL_LOOKUP_JOIN_ON_MANY_FIELDS)) {
                 planOut.writeCollection(matchFields, (o, matchConfig) -> matchConfig.writeTo(o));
+            } else if (matchFields.size() > 1) {
+                throw new EsqlIllegalArgumentException("LOOKUP JOIN on multiple fields is not supported on remote node");
             }
         }