Only rename when absolutely needed to avoid a name conflict

Author: julian-elastic

x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/generator/command/pipe/LookupJoinGenerator.java

@@ -40,8 +40,8 @@ public CommandDescription generate(
         String lookupIdxName = lookupIdx.idxName();
         int joinColumnsCount = randomInt(lookupIdx.keys().size() - 1) + 1; // at least one column must be used for the join
         List<LookupIdxColumn> joinColumns = ESTestCase.randomSubsetOf(joinColumnsCount, lookupIdx.keys());
-        List<String> keyNames = new ArrayList<>();
-        List<String> joinOn = new ArrayList<>();
+        List<String> joinOnLeft = new ArrayList<>();
+        List<String> joinOnRight = new ArrayList<>();
         Set<String> usedColumns = new HashSet<>();
         for (LookupIdxColumn joinColumn : joinColumns) {
             String idxKey = joinColumn.name();
@@ -58,10 +58,10 @@ public CommandDescription generate(
                 usedColumns.add(key.name());
                 usedColumns.add(idxKey);
             }
-            keyNames.add(key.name());
-            joinOn.add(idxKey);
+            joinOnLeft.add(key.name());
+            joinOnRight.add(idxKey);
         }
-        if (keyNames.isEmpty()) {
+        if (joinOnLeft.isEmpty()) {
             return EMPTY_DESCRIPTION;
         }
         StringBuilder stringBuilder = new StringBuilder();
@@ -71,17 +71,17 @@ public CommandDescription generate(
 
         if (useExpressionJoin) {
             // Get all right side column names (from lookup index)
-            Set<String> rightColumnNames = lookupIdx.keys()
+            Set<String> allRightColumnNames = lookupIdx.keys()
                 .stream()
                 .map(LookupIdxColumn::name)
                 .collect(java.util.stream.Collectors.toSet());
 
             // Generate rename commands for ALL left columns that exist in right side
             Set<String> allLeftColumnNames = previousOutput.stream().map(Column::name).collect(java.util.stream.Collectors.toSet());
             Map<String, String> leftColumnName2Renamed = new HashMap<>();
-            // Rename left columns that conflict with right side columns (either in lookup index or used in join)
+            // Rename due to co1 == col2, the conflict is on col2 name
             for (String leftColumnName : allLeftColumnNames) {
-                if (rightColumnNames.contains(leftColumnName)) {
+                if (joinOnRight.contains(leftColumnName)) {
                     String renamedColumn = leftColumnName + "_left";
                     stringBuilder.append("| rename ");
                     stringBuilder.append(leftColumnName);
@@ -91,13 +91,26 @@ public CommandDescription generate(
                 }
             }
 
+            // Rename due to co1 == col2, the conflict is on col1 name
+            // but only rename if it wasn't renamed already
+            for (String rightColumnName : allRightColumnNames) {
+                if (joinOnLeft.contains(rightColumnName) && leftColumnName2Renamed.containsKey(rightColumnName) == false) {
+                    String renamedColumn = rightColumnName + "_left";
+                    stringBuilder.append("| rename ");
+                    stringBuilder.append(rightColumnName);
+                    stringBuilder.append(" as ");
+                    stringBuilder.append(renamedColumn);
+                    leftColumnName2Renamed.put(rightColumnName, renamedColumn);
+                }
+            }
+
             // Generate expression join syntax
             stringBuilder.append(" | lookup join ").append(lookupIdxName).append(" on ");
 
             // Add join conditions for all columns
-            for (int i = 0; i < keyNames.size(); i++) {
-                String leftColumnName = keyNames.get(i);
-                String rightColumnName = joinOn.get(i);
+            for (int i = 0; i < joinOnLeft.size(); i++) {
+                String leftColumnName = joinOnLeft.get(i);
+                String rightColumnName = joinOnRight.get(i);
 
                 // Only == and != are allowed, as the rest of the operators don's support all types
                 String[] booleanOperators = { "==", "!=" };
@@ -112,16 +125,16 @@ public CommandDescription generate(
             }
         } else {
             // Generate field-based join (original behavior)
-            for (int i = 0; i < keyNames.size(); i++) {
+            for (int i = 0; i < joinOnLeft.size(); i++) {
                 stringBuilder.append("| rename ");
-                stringBuilder.append(keyNames.get(i));
+                stringBuilder.append(joinOnLeft.get(i));
                 stringBuilder.append(" as ");
-                stringBuilder.append(joinOn.get(i));
+                stringBuilder.append(joinOnRight.get(i));
             }
             stringBuilder.append(" | lookup join ").append(lookupIdxName).append(" on ");
-            for (int i = 0; i < keyNames.size(); i++) {
-                stringBuilder.append(joinOn.get(i));
-                if (i < keyNames.size() - 1) {
+            for (int i = 0; i < joinOnLeft.size(); i++) {
+                stringBuilder.append(joinOnRight.get(i));
+                if (i < joinOnLeft.size() - 1) {
                     stringBuilder.append(", ");
                 }
             }