Compute output of LookupJoinExec dynamically

LookupJoinExec should not assume its output but instead compute it from
- Its input fields from the left
- The fields added from the lookup index

Currently, LookupJoinExec's output is determined when the logical plan
is mapped to a physical one, and thereafter the output cannot be changed
anymore.
This makes it impossible to have late materialization of fields from the
left hand side via field extractions, because we are forced to extract
*all* fields before the LookupJoinExec, otherwise we do not achieve the
prescribed output.

Avoid that by tracking only which fields the LookupJoinExec will
add from the lookup index instead of tracking the whole output (that was
only correct for the logical plan).

Author: alex-spies

x-pack/plugin/esql/qa/server/multi-clusters/src/javaRestTest/java/org/elasticsearch/xpack/esql/ccq/MultiClusterSpecIT.java

@@ -47,7 +47,7 @@
 import static org.elasticsearch.xpack.esql.EsqlTestUtils.classpathResources;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.INLINESTATS;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.INLINESTATS_V2;
-import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.JOIN_LOOKUP_V2;
+import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.JOIN_LOOKUP_V3;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.JOIN_PLANNING_V1;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.METADATA_FIELDS_REMOTE_TEST;
 import static org.elasticsearch.xpack.esql.qa.rest.EsqlSpecTestCase.Mode.SYNC;
@@ -125,7 +125,7 @@ protected void shouldSkipTest(String testName) throws IOException {
         assumeFalse("INLINESTATS not yet supported in CCS", testCase.requiredCapabilities.contains(INLINESTATS.capabilityName()));
         assumeFalse("INLINESTATS not yet supported in CCS", testCase.requiredCapabilities.contains(INLINESTATS_V2.capabilityName()));
         assumeFalse("INLINESTATS not yet supported in CCS", testCase.requiredCapabilities.contains(JOIN_PLANNING_V1.capabilityName()));
-        assumeFalse("LOOKUP JOIN not yet supported in CCS", testCase.requiredCapabilities.contains(JOIN_LOOKUP_V2.capabilityName()));
+        assumeFalse("LOOKUP JOIN not yet supported in CCS", testCase.requiredCapabilities.contains(JOIN_LOOKUP_V3.capabilityName()));
     }
 
     private TestFeatureService remoteFeaturesService() throws IOException {

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

@@ -5,7 +5,7 @@
 
 //TODO: this sometimes returns null instead of the looked up value (likely related to the execution order)
 basicOnTheDataNode-Ignore
-required_capability: join_lookup_v2
+required_capability: join_lookup_v3
 
 FROM employees
 | EVAL language_code = languages
@@ -22,7 +22,7 @@ emp_no:integer | language_code:integer | language_name:keyword
 ;
 
 basicRow-Ignore
-required_capability: join_lookup
+required_capability: join_lookup_v3
 
 ROW language_code = 1
 | LOOKUP JOIN languages_lookup ON language_code
@@ -33,7 +33,7 @@ language_code:keyword  | language_name:keyword
 ;
 
 basicOnTheCoordinator
-required_capability: join_lookup_v2
+required_capability: join_lookup_v3
 
 FROM employees
 | SORT emp_no
@@ -51,7 +51,7 @@ emp_no:integer | language_code:integer | language_name:keyword
 
 //TODO: this sometimes returns null instead of the looked up value (likely related to the execution order)
 subsequentEvalOnTheDataNode-Ignore
-required_capability: join_lookup_v2
+required_capability: join_lookup_v3
 
 FROM employees
 | EVAL language_code = languages
@@ -69,7 +69,7 @@ emp_no:integer | language_code:integer | language_name:keyword | language_code_x
 ;
 
 subsequentEvalOnTheCoordinator
-required_capability: join_lookup_v2
+required_capability: join_lookup_v3
 
 FROM employees
 | SORT emp_no

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

@@ -521,7 +521,7 @@ public enum Cap {
         /**
          * LOOKUP JOIN
          */
-        JOIN_LOOKUP_V2(Build.current().isSnapshot()),
+        JOIN_LOOKUP_V3(Build.current().isSnapshot()),
 
         /**
          * Fix for https://github.com/elastic/elasticsearch/issues/117054

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/PhysicalOptimizerRules.java

@@ -7,8 +7,6 @@
 
 package org.elasticsearch.xpack.esql.optimizer;
 
-import org.elasticsearch.xpack.esql.core.expression.Expression;
-import org.elasticsearch.xpack.esql.core.util.ReflectionUtils;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.OptimizerRules.TransformDirection;
 import org.elasticsearch.xpack.esql.plan.physical.PhysicalPlan;
 import org.elasticsearch.xpack.esql.rule.ParameterizedRule;
@@ -62,34 +60,4 @@ public final PhysicalPlan apply(PhysicalPlan plan) {
 
         protected abstract PhysicalPlan rule(SubPlan plan);
     }
-
-    public abstract static class OptimizerExpressionRule<E extends Expression> extends Rule<PhysicalPlan, PhysicalPlan> {
-
-        private final TransformDirection direction;
-        // overriding type token which returns the correct class but does an uncheck cast to LogicalPlan due to its generic bound
-        // a proper solution is to wrap the Expression rule into a Plan rule but that would affect the rule declaration
-        // so instead this is hacked here
-        private final Class<E> expressionTypeToken = ReflectionUtils.detectSuperTypeForRuleLike(getClass());
-
-        public OptimizerExpressionRule(TransformDirection direction) {
-            this.direction = direction;
-        }
-
-        @Override
-        public final PhysicalPlan apply(PhysicalPlan plan) {
-            return direction == TransformDirection.DOWN
-                ? plan.transformExpressionsDown(expressionTypeToken, this::rule)
-                : plan.transformExpressionsUp(expressionTypeToken, this::rule);
-        }
-
-        protected PhysicalPlan rule(PhysicalPlan plan) {
-            return plan;
-        }
-
-        protected abstract Expression rule(E e);
-
-        public Class<E> expressionToken() {
-            return expressionTypeToken;
-        }
-    }
 }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/rules/physical/local/InsertFieldExtraction.java

@@ -104,15 +104,15 @@ private static Set<Attribute> missingAttributes(PhysicalPlan p) {
         var missing = new LinkedHashSet<Attribute>();
         var inputSet = p.inputSet();
 
-        // FIXME: the extractors should work on the right side as well
+        // TODO: We need to extract whatever fields are missing from the left hand side.
         // skip the lookup join since the right side is always materialized and a projection
         if (p instanceof LookupJoinExec join) {
-            // collect fields used in the join condition
             return Collections.emptySet();
         }
 
         var input = inputSet;
         // collect field attributes used inside expressions
+        // TODO: Rather than going over all expressions manually, this should just call .references()
         p.forEachExpression(TypedAttribute.class, f -> {
             if (f instanceof FieldAttribute || f instanceof MetadataAttribute) {
                 if (input.contains(f) == false) {

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/logical/join/Join.java

@@ -11,7 +11,7 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.xpack.esql.core.expression.Attribute;
-import org.elasticsearch.xpack.esql.core.expression.NamedExpression;
+import org.elasticsearch.xpack.esql.core.expression.AttributeSet;
 import org.elasticsearch.xpack.esql.core.expression.ReferenceAttribute;
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
@@ -23,12 +23,9 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
-import java.util.Set;
-import java.util.stream.Collectors;
 
 import static org.elasticsearch.xpack.esql.expression.NamedExpressions.mergeOutputAttributes;
 import static org.elasticsearch.xpack.esql.plan.logical.join.JoinTypes.LEFT;
-import static org.elasticsearch.xpack.esql.plan.logical.join.JoinTypes.RIGHT;
 
 public class Join extends BinaryPlan {
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(LogicalPlan.class, "Join", Join::new);
@@ -100,6 +97,19 @@ public List<Attribute> output() {
         return lazyOutput;
     }
 
+    public List<Attribute> rightOutputFields() {
+        AttributeSet leftInputs = left().outputSet();
+
+        List<Attribute> rightOutputFields = new ArrayList<>();
+        for (Attribute attr : output()) {
+            if (leftInputs.contains(attr) == false) {
+                rightOutputFields.add(attr);
+            }
+        }
+
+        return rightOutputFields;
+    }
+
     /**
      * Combine the two lists of attributes into one.
      * In case of (name) conflicts, specify which sides wins, that is overrides the other column - the left or the right.
@@ -108,18 +118,11 @@ public static List<Attribute> computeOutput(List<Attribute> leftOutput, List<Att
         JoinType joinType = config.type();
         List<Attribute> output;
         // TODO: make the other side nullable
-        Set<String> matchFieldNames = config.matchFields().stream().map(NamedExpression::name).collect(Collectors.toSet());
         if (LEFT.equals(joinType)) {
-            // right side becomes nullable and overrides left except for match fields, which we preserve from the left
-            List<Attribute> rightOutputWithoutMatchFields = rightOutput.stream()
-                .filter(attr -> matchFieldNames.contains(attr.name()) == false)
-                .toList();
+            // right side becomes nullable and overrides left except for join keys, which we preserve from the left
+            AttributeSet rightKeys = new AttributeSet(config.rightFields());
+            List<Attribute> rightOutputWithoutMatchFields = rightOutput.stream().filter(attr -> rightKeys.contains(attr) == false).toList();
             output = mergeOutputAttributes(rightOutputWithoutMatchFields, leftOutput);
-        } else if (RIGHT.equals(joinType)) {
-            List<Attribute> leftOutputWithoutMatchFields = leftOutput.stream()
-                .filter(attr -> matchFieldNames.contains(attr.name()) == false)
-                .toList();
-            output = mergeOutputAttributes(leftOutputWithoutMatchFields, rightOutput);
         } else {
             throw new IllegalArgumentException(joinType.joinName() + " unsupported");
         }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/physical/LookupJoinExec.java

@@ -19,7 +19,6 @@
 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
 
@@ -30,43 +29,43 @@ public class LookupJoinExec extends BinaryExec implements EstimatesRowSize {
         LookupJoinExec::new
     );
 
-    private final List<Attribute> matchFields;
     private final List<Attribute> leftFields;
     private final List<Attribute> rightFields;
-    private final List<Attribute> output;
-    private List<Attribute> lazyAddedFields;
+    /**
+     * These cannot be computed from the left + right outputs, because
+     * {@link org.elasticsearch.xpack.esql.optimizer.rules.physical.local.ReplaceSourceAttributes} will replace the {@link EsSourceExec} on
+     * the right hand side by a {@link EsQueryExec}, and thus lose the information of which fields we'll get from the lookup index.
+     */
+    private final List<Attribute> addedFields;
+    private List<Attribute> lazyOutput;
 
     public LookupJoinExec(
         Source source,
         PhysicalPlan left,
         PhysicalPlan lookup,
-        List<Attribute> matchFields,
         List<Attribute> leftFields,
         List<Attribute> rightFields,
-        List<Attribute> output
+        List<Attribute> addedFields
     ) {
         super(source, left, lookup);
-        this.matchFields = matchFields;
         this.leftFields = leftFields;
         this.rightFields = rightFields;
-        this.output = output;
+        this.addedFields = addedFields;
     }
 
     private LookupJoinExec(StreamInput in) throws IOException {
         super(Source.readFrom((PlanStreamInput) in), in.readNamedWriteable(PhysicalPlan.class), in.readNamedWriteable(PhysicalPlan.class));
-        this.matchFields = in.readNamedWriteableCollectionAsList(Attribute.class);
         this.leftFields = in.readNamedWriteableCollectionAsList(Attribute.class);
         this.rightFields = in.readNamedWriteableCollectionAsList(Attribute.class);
-        this.output = in.readNamedWriteableCollectionAsList(Attribute.class);
+        this.addedFields = in.readNamedWriteableCollectionAsList(Attribute.class);
     }
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
-        out.writeNamedWriteableCollection(matchFields);
         out.writeNamedWriteableCollection(leftFields);
         out.writeNamedWriteableCollection(rightFields);
-        out.writeNamedWriteableCollection(output);
+        out.writeNamedWriteableCollection(addedFields);
     }
 
     @Override
@@ -78,10 +77,6 @@ public PhysicalPlan lookup() {
         return right();
     }
 
-    public List<Attribute> matchFields() {
-        return matchFields;
-    }
-
     public List<Attribute> leftFields() {
         return leftFields;
     }
@@ -91,29 +86,26 @@ public List<Attribute> rightFields() {
     }
 
     public List<Attribute> addedFields() {
-        if (lazyAddedFields == null) {
-            AttributeSet set = outputSet();
-            set.removeAll(left().output());
-            for (Attribute m : matchFields) {
-                set.removeIf(a -> a.name().equals(m.name()));
+        return addedFields;
+    }
+
+    @Override
+    public List<Attribute> output() {
+        if (lazyOutput == null) {
+            lazyOutput = new ArrayList<>(left().output());
+            for (Attribute attr : addedFields) {
+                lazyOutput.add(attr);
             }
-            lazyAddedFields = new ArrayList<>(set);
-            lazyAddedFields.sort(Comparator.comparing(Attribute::name));
         }
-        return lazyAddedFields;
+        return lazyOutput;
     }
 
     @Override
     public PhysicalPlan estimateRowSize(State state) {
-        state.add(false, output);
+        state.add(false, output());
         return this;
     }
 
-    @Override
-    public List<Attribute> output() {
-        return output;
-    }
-
     @Override
     public AttributeSet inputSet() {
         // TODO: this is a hack since the right side is always materialized - instead this should
@@ -129,12 +121,12 @@ protected AttributeSet computeReferences() {
 
     @Override
     public LookupJoinExec replaceChildren(PhysicalPlan left, PhysicalPlan right) {
-        return new LookupJoinExec(source(), left, right, matchFields, leftFields, rightFields, output);
+        return new LookupJoinExec(source(), left, right, leftFields, rightFields, addedFields);
     }
 
     @Override
     protected NodeInfo<? extends PhysicalPlan> info() {
-        return NodeInfo.create(this, LookupJoinExec::new, left(), right(), matchFields, leftFields, rightFields, output);
+        return NodeInfo.create(this, LookupJoinExec::new, left(), right(), leftFields, rightFields, addedFields);
     }
 
     @Override
@@ -148,15 +140,12 @@ public boolean equals(Object o) {
         if (super.equals(o) == false) {
             return false;
         }
-        LookupJoinExec hash = (LookupJoinExec) o;
-        return matchFields.equals(hash.matchFields)
-            && leftFields.equals(hash.leftFields)
-            && rightFields.equals(hash.rightFields)
-            && output.equals(hash.output);
+        LookupJoinExec other = (LookupJoinExec) o;
+        return leftFields.equals(other.leftFields) && rightFields.equals(other.rightFields) && addedFields.equals(other.addedFields);
     }
 
     @Override
     public int hashCode() {
-        return Objects.hash(super.hashCode(), matchFields, leftFields, rightFields, output);
+        return Objects.hash(super.hashCode(), leftFields, rightFields, addedFields);
     }
 }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/LocalExecutionPlanner.java

@@ -583,8 +583,8 @@ private PhysicalOperation planLookupJoin(LookupJoinExec join, LocalExecutionPlan
         if (localSourceExec.indexMode() != IndexMode.LOOKUP) {
             throw new IllegalArgumentException("can't plan [" + join + "]");
         }
-        List<Layout.ChannelAndType> matchFields = new ArrayList<>(join.matchFields().size());
-        for (Attribute m : join.matchFields()) {
+        List<Layout.ChannelAndType> matchFields = new ArrayList<>(join.leftFields().size());
+        for (Attribute m : join.leftFields()) {
             Layout.ChannelAndType t = source.layout.get(m.id());
             if (t == null) {
                 throw new IllegalArgumentException("can't plan [" + join + "][" + m + "]");
@@ -604,7 +604,7 @@ private PhysicalOperation planLookupJoin(LookupJoinExec join, LocalExecutionPlan
                 lookupFromIndexService,
                 matchFields.getFirst().type(),
                 localSourceExec.index().name(),
-                join.matchFields().getFirst().name(),
+                join.leftFields().getFirst().name(),
                 join.addedFields().stream().map(f -> (NamedExpression) f).toList(),
                 join.source()
             ),

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/mapper/LocalMapper.java

@@ -120,15 +120,7 @@ private PhysicalPlan mapBinary(BinaryPlan binary) {
                 );
             }
             if (right instanceof EsSourceExec source && source.indexMode() == IndexMode.LOOKUP) {
-                return new LookupJoinExec(
-                    join.source(),
-                    left,
-                    right,
-                    config.matchFields(),
-                    config.leftFields(),
-                    config.rightFields(),
-                    join.output()
-                );
+                return new LookupJoinExec(join.source(), left, right, config.leftFields(), config.rightFields(), join.rightOutputFields());
             }
         }