[GitHub] [doris] morrySnow commented on a diff in pull request #14730: [feature](nereids)merge proj-proj in post process

[GitBox <gi...@apache.org>]

morrySnow commented on code in PR #14730:
URL: https://github.com/apache/doris/pull/14730#discussion_r1036937981


##########
fe/fe-core/src/main/java/org/apache/doris/nereids/processor/post/MergeProjectPostProcessor.java:
##########
@@ -0,0 +1,60 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package org.apache.doris.nereids.processor.post;
+
+import org.apache.doris.nereids.CascadesContext;
+import org.apache.doris.nereids.rules.rewrite.logical.MergeProjects;
+import org.apache.doris.nereids.trees.expressions.Alias;
+import org.apache.doris.nereids.trees.expressions.Expression;
+import org.apache.doris.nereids.trees.expressions.NamedExpression;
+import org.apache.doris.nereids.trees.plans.Plan;
+import org.apache.doris.nereids.trees.plans.physical.PhysicalProject;
+
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * merge consecutive projects
+ */
+public class MergeProjectPostProcessor extends PlanPostProcessor {
+
+    @Override
+    public PhysicalProject visitPhysicalProject(PhysicalProject<? extends Plan> project, CascadesContext ctx) {
+        Plan child = project.child();
+        child = child.accept(this, ctx);
+        if (child instanceof PhysicalProject) {
+            Map<Expression, Expression> aliasMap = getAliasProjections((PhysicalProject) child);
+            List<NamedExpression> projections = project.getProjects();
+            projections = projections.stream()
+                    .map(e -> MergeProjects.ExpressionReplacer.INSTANCE.visit(e, aliasMap))
+                    .map(NamedExpression.class::cast)
+                    .collect(Collectors.toList());
+            return project.withProjectionsAndChild(projections, child.child(0));
+        }
+        return project;

Review Comment:
   could we abstract a function for merging project list? this function looks like
   ```
   List< NamedExpression> func(List< NamedExpression> top, List< NamedExpression> bottom)
   ```
   and then both merge logical and physical project to call it to do merge job



##########
fe/fe-core/src/main/java/org/apache/doris/nereids/rules/rewrite/logical/MergeProjects.java:
##########
@@ -92,7 +95,7 @@ public Expression visit(Expression expr, Map<Expression, Expression> substitutio
                 Slot ref = ((SlotReference) expr).withQualifier(Collections.emptyList());
                 if (substitutionMap.containsKey(ref)) {
                     Alias res = (Alias) substitutionMap.get(ref);
-                    return res.child();
+                    return new Alias(res.child(), ref.getName());

Review Comment:
   the new Alias is useless and will intro more complexity to expression process. because when we come here the slot is not the root of this expr tree.



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