Implement suggestions

Author: luigidellaquila

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/rules/logical/PruneEmptyAggregates.java

@@ -15,6 +15,13 @@
 
 import java.util.List;
 
+/**
+ * STATS with no aggregates and no groupings can be replaced with a single, empty row.
+ * This can happen due to expression pruning at optimization time, after all them are dropped,
+ * eg.
+ *
+ * STATS a = count(*) by b | drop a, b
+ */
 public final class PruneEmptyAggregates extends OptimizerRules.OptimizerRule<Aggregate> {
     @Override
     protected LogicalPlan rule(Aggregate agg) {

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/logical/local/CopyingLocalSupplier.java

@@ -20,18 +20,18 @@
 import java.io.IOException;
 
 /**
- * A {@link LocalSupplier} that allways creates a new copy of the {@link Block}s initially provided at creation time.
+ * A {@link LocalSupplier} that allways creates a new copy of the {@link Page} initially provided at creation time.
  * This is created specifically for {@link InlineStats} usage in {@link EsqlSession} for queries that use ROW command.
  *
- * The ROW which gets replaced by {@link ReplaceRowAsLocalRelation} with a {@link LocalRelation} will have its blocks
+ * The ROW which gets replaced by {@link ReplaceRowAsLocalRelation} with a {@link LocalRelation} will have its page
  * used (and released) at least twice:
  * - the {@link LocalRelation} from the left-hand side is used as a source for the right-hand side
  * - the same {@link LocalRelation} is then used to continue the execution of the query on the left-hand side
  *
  * It delegates all its operations to {@link ImmediateLocalSupplier} and, to prevent the double release, it will always
- * create a deep copy of the blocks received in the constructor initially.
+ * create a deep copy of the page received in the constructor initially.
  *
- * Example with the flow and the blocks reuse for a query like "row x = 1 | inline stats y = max(x)"
+ * Example with the flow and the page reuse for a query like "row x = 1 | inline stats y = max(x)"
  * Step 1:
  * Limit[1000[INTEGER],true]
  * \_InlineJoin[LEFT,[],[],[]]

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/session/FieldNameUtils.java

@@ -244,8 +244,9 @@ public static PreAnalysisResult resolveFieldNames(LogicalPlan parsed, boolean ha
             // there cannot be an empty list of fields, we'll ask the simplest and lightest one instead: _index
             return new PreAnalysisResult(IndexResolver.INDEX_METADATA_FIELD, wildcardJoinIndices);
         } else {
-            fieldNames.add(MetadataAttribute.INDEX);
-            return new PreAnalysisResult(fieldNames.stream().flatMap(FieldNameUtils::withSubfields).collect(toSet()), wildcardJoinIndices);
+            HashSet<String> allFields = new HashSet<>(fieldNames.stream().flatMap(FieldNameUtils::withSubfields).collect(toSet()));
+            allFields.add(MetadataAttribute.INDEX);
+            return new PreAnalysisResult(allFields, wildcardJoinIndices);
         }
     }