ESQL: Have CombineProjections propagate references upwards (elastic#127264)

This will have CombineProjections allow references from the "under" plan
be kept when merging stacked Projections.
This is to prevent field attributes that are dropped by
ReplaceMissingFieldWithNull be used in the resulting plan.

(cherry picked from commit 9626fe5)

Author: bpintea

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/Alias.java

@@ -93,7 +93,8 @@ protected NodeInfo<Alias> info() {
     }
 
     public Alias replaceChild(Expression child) {
-        return new Alias(source(), name(), child, id(), synthetic());
+        // these "nop" replacements can occur on attribute map resolutions having the default return same as the lookup key
+        return child == this.child ? this : new Alias(source(), name(), child, id(), synthetic());
     }
 
     @Override

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

@@ -15,6 +15,7 @@
 import org.elasticsearch.xpack.esql.core.expression.Expression;
 import org.elasticsearch.xpack.esql.core.expression.Expressions;
 import org.elasticsearch.xpack.esql.core.expression.NamedExpression;
+import org.elasticsearch.xpack.esql.core.expression.ReferenceAttribute;
 import org.elasticsearch.xpack.esql.expression.function.grouping.Categorize;
 import org.elasticsearch.xpack.esql.plan.logical.Aggregate;
 import org.elasticsearch.xpack.esql.plan.logical.LogicalPlan;
@@ -128,6 +129,8 @@ private static List<NamedExpression> combineProjections(List<? extends NamedExpr
             if (ne instanceof Alias as) {
                 Expression child = as.child();
                 namedExpressionsBuilder.put(ne.toAttribute(), as.replaceChild(aliasesBuilder.build().resolve(child, child)));
+            } else if (ne instanceof ReferenceAttribute ra) {
+                namedExpressionsBuilder.put(ra, ra);
             }
         }
         List<NamedExpression> replaced = new ArrayList<>();

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

@@ -58,7 +58,8 @@ public LogicalPlan apply(LogicalPlan plan, LocalLogicalOptimizerContext localLog
         // Do not use the attribute name, this can deviate from the field name for union types; use fieldName() instead.
         // Also retain fields from lookup indices because we do not have stats for these.
         Predicate<FieldAttribute> shouldBeRetained = f -> f.field() instanceof PotentiallyUnmappedKeywordEsField
-            || (localLogicalOptimizerContext.searchStats().exists(f.fieldName()) || lookupFieldsBuilder.contains(f));
+            || localLogicalOptimizerContext.searchStats().exists(f.fieldName())
+            || lookupFieldsBuilder.contains(f);
 
         return plan.transformUp(p -> missingToNull(p, shouldBeRetained));
     }
@@ -77,7 +78,7 @@ private LogicalPlan missingToNull(LogicalPlan plan, Predicate<FieldAttribute> sh
             for (int i = 0, size = relationOutput.size(); i < size; i++) {
                 Attribute attr = relationOutput.get(i);
                 NamedExpression projection;
-                if (attr instanceof FieldAttribute f && (shouldBeRetained.test(f) == false)) {
+                if (attr instanceof FieldAttribute f && shouldBeRetained.test(f) == false) {
                     DataType dt = f.dataType();
                     Alias nullAlias = nullLiterals.get(dt);
                     // save the first field as null (per datatype)

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LocalLogicalPlanOptimizerTests.java

@@ -7,6 +7,7 @@
 
 package org.elasticsearch.xpack.esql.optimizer;
 
+import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.index.IndexMode;
@@ -22,6 +23,7 @@
 import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
 import org.elasticsearch.xpack.esql.core.expression.FoldContext;
 import org.elasticsearch.xpack.esql.core.expression.Literal;
+import org.elasticsearch.xpack.esql.core.expression.ReferenceAttribute;
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
@@ -141,10 +143,10 @@ public void testMissingFieldInFilterString() {
 
     /**
      * Expects
-     * Project[[last_name{f}#6]]
+     * Project[[last_name{r}#7]]
      * \_Eval[[null[KEYWORD] AS last_name]]
-     *  \_Limit[10000[INTEGER]]
-     *   \_EsRelation[test][_meta_field{f}#8, emp_no{f}#2, first_name{f}#3, gen..]
+     *   \_Limit[1000[INTEGER],false]
+     *     \_EsRelation[test][_meta_field{f}#9, emp_no{f}#3, first_name{f}#4, gen..]
      */
     public void testMissingFieldInProject() {
         var plan = plan("""
@@ -158,7 +160,7 @@ public void testMissingFieldInProject() {
         var project = as(localPlan, Project.class);
         var projections = project.projections();
         assertThat(Expressions.names(projections), contains("last_name"));
-        as(projections.get(0), FieldAttribute.class);
+        as(projections.get(0), ReferenceAttribute.class);
         var eval = as(project.child(), Eval.class);
         assertThat(Expressions.names(eval.fields()), contains("last_name"));
         var alias = as(eval.fields().get(0), Alias.class);
@@ -171,6 +173,39 @@ public void testMissingFieldInProject() {
         assertThat(Expressions.names(source.output()), not(contains("last_name")));
     }
 
+    /*
+     * Expects a similar plan to testMissingFieldInProject() above, except for the Alias's child value
+     * Project[[last_name{r}#4]]
+     * \_Eval[[[66 6f 6f][KEYWORD] AS last_name]]
+     *   \_Limit[1000[INTEGER],false]
+     *     \_EsRelation[test][_meta_field{f}#11, emp_no{f}#5, first_name{f}#6, ge..]
+     */
+    public void testReassignedMissingFieldInProject() {
+        var plan = plan("""
+              from test
+            | keep last_name
+            | eval last_name = "foo"
+            """);
+
+        var testStats = statsForMissingField("last_name");
+        var localPlan = localPlan(plan, testStats);
+
+        var project = as(localPlan, Project.class);
+        var projections = project.projections();
+        assertThat(Expressions.names(projections), contains("last_name"));
+        as(projections.get(0), ReferenceAttribute.class);
+        var eval = as(project.child(), Eval.class);
+        assertThat(Expressions.names(eval.fields()), contains("last_name"));
+        var alias = as(eval.fields().get(0), Alias.class);
+        var literal = as(alias.child(), Literal.class);
+        assertThat(literal.value(), is(new BytesRef("foo")));
+        assertThat(literal.dataType(), is(DataType.KEYWORD));
+
+        var limit = as(eval.child(), Limit.class);
+        var source = as(limit.child(), EsRelation.class);
+        assertThat(Expressions.names(source.output()), not(contains("last_name")));
+    }
+
     /**
      * Expects
      * EsqlProject[[first_name{f}#4]]

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LocalPhysicalPlanOptimizerTests.java

@@ -38,8 +38,8 @@
 import org.elasticsearch.xpack.esql.analysis.Verifier;
 import org.elasticsearch.xpack.esql.core.expression.Alias;
 import org.elasticsearch.xpack.esql.core.expression.Expressions;
-import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
 import org.elasticsearch.xpack.esql.core.expression.Literal;
+import org.elasticsearch.xpack.esql.core.expression.ReferenceAttribute;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
 import org.elasticsearch.xpack.esql.core.type.EsField;
@@ -1305,15 +1305,16 @@ private EsQueryExec doTestOutOfRangeFilterPushdown(String query, Analyzer analyz
         return luceneQuery;
     }
 
-    /**
+    /*
      * Expects
      * LimitExec[1000[INTEGER]]
-     * \_ExchangeExec[[],false]
-     *   \_ProjectExec[[_meta_field{f}#8, emp_no{r}#2, first_name{r}#3, gender{f}#4, job{f}#9, job.raw{f}#10, languages{f}#5, first_n
-     * ame{r}#3 AS last_name, long_noidx{f}#11, emp_no{r}#2 AS salary]]
-     *     \_FieldExtractExec[_meta_field{f}#8, gender{f}#4, job{f}#9, job.raw{f}..]
+     * \_ExchangeExec[[_meta_field{f}#8, emp_no{f}#2, first_name{f}#3, gender{f}#4, hire_date{f}#9, job{f}#10, job.raw{f}#11, langua
+     * ges{f}#5, last_name{f}#6, long_noidx{f}#12, salary{f}#7],false]
+     *   \_ProjectExec[[_meta_field{f}#8, emp_no{r}#2, first_name{r}#3, gender{f}#4, hire_date{f}#9, job{f}#10, job.raw{f}#11, langua
+     * ges{f}#5, first_name{r}#3 AS last_name, long_noidx{f}#12, emp_no{r}#2 AS salary]]
+     *     \_FieldExtractExec[_meta_field{f}#8, gender{f}#4, hire_date{f}#9, job{..]<[],[]>
      *       \_EvalExec[[null[INTEGER] AS emp_no, null[KEYWORD] AS first_name]]
-     *         \_EsQueryExec[test], query[][_doc{f}#12], limit[1000], sort[] estimatedRowSize[270]
+     *         \_EsQueryExec[test], indexMode[standard], query[][_doc{f}#13], limit[1000], sort[] estimatedRowSize[278]
      */
     public void testMissingFieldsDoNotGetExtracted() {
         var stats = EsqlTestUtils.statsForMissingField("first_name", "last_name", "emp_no", "salary");
@@ -1340,9 +1341,9 @@ public void testMissingFieldsDoNotGetExtracted() {
             )
         );
         // emp_no
-        assertThat(projections.get(1), instanceOf(FieldAttribute.class));
+        assertThat(projections.get(1), instanceOf(ReferenceAttribute.class));
         // first_name
-        assertThat(projections.get(2), instanceOf(FieldAttribute.class));
+        assertThat(projections.get(2), instanceOf(ReferenceAttribute.class));
 
         // last_name --> first_name
         var nullAlias = Alias.unwrap(projections.get(8));