[GitHub] [hive] kasakrisz commented on a change in pull request #1323: HIVE-23723. Limit operator pushdown through LOJ (amagyar)

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kasakrisz commented on a change in pull request #1323:
URL: https://github.com/apache/hive/pull/1323#discussion_r461675704



##########
File path: ql/src/java/org/apache/hadoop/hive/ql/optimizer/OrderlessLimitPushDownOptimizer.java
##########
@@ -0,0 +1,132 @@
+/*
+ *
+ *  * 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.hadoop.hive.ql.optimizer;
+
+import static org.apache.hadoop.hive.ql.optimizer.topnkey.TopNKeyProcessor.copyDown;
+import static org.apache.hadoop.hive.ql.optimizer.topnkey.TopNKeyPushdownProcessor.moveDown;
+
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Stack;
+
+import org.apache.hadoop.hive.ql.exec.CommonJoinOperator;
+import org.apache.hadoop.hive.ql.exec.LimitOperator;
+import org.apache.hadoop.hive.ql.exec.Operator;
+import org.apache.hadoop.hive.ql.exec.ReduceSinkOperator;
+import org.apache.hadoop.hive.ql.lib.DefaultGraphWalker;
+import org.apache.hadoop.hive.ql.lib.DefaultRuleDispatcher;
+import org.apache.hadoop.hive.ql.lib.Node;
+import org.apache.hadoop.hive.ql.lib.NodeProcessorCtx;
+import org.apache.hadoop.hive.ql.lib.RuleRegExp;
+import org.apache.hadoop.hive.ql.lib.SemanticGraphWalker;
+import org.apache.hadoop.hive.ql.lib.SemanticNodeProcessor;
+import org.apache.hadoop.hive.ql.lib.SemanticRule;
+import org.apache.hadoop.hive.ql.parse.ParseContext;
+import org.apache.hadoop.hive.ql.parse.SemanticException;
+import org.apache.hadoop.hive.ql.plan.JoinCondDesc;
+import org.apache.hadoop.hive.ql.plan.JoinDesc;
+import org.apache.hadoop.hive.ql.plan.OperatorDesc;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Push LIMIT without an Order By through Selects and Left Outer Joins
+ */
+public class OrderlessLimitPushDownOptimizer extends Transform {
+  private static final Logger LOG = LoggerFactory.getLogger(OrderlessLimitPushDownOptimizer.class);
+
+  @Override
+  public ParseContext transform(ParseContext pctx) throws SemanticException {
+    Map<SemanticRule, SemanticNodeProcessor> opRules = new LinkedHashMap<SemanticRule, SemanticNodeProcessor>();
+    opRules.put(
+            new RuleRegExp("LIMIT push down", LimitOperator.getOperatorName() + "%"),
+            new LimitPushDown());
+    SemanticGraphWalker walker = new DefaultGraphWalker(new DefaultRuleDispatcher(null, opRules, null));
+    walker.startWalking(new ArrayList<>(pctx.getTopOps().values()), null);
+    return pctx;
+  }
+
+  private static class LimitPushDown implements SemanticNodeProcessor {
+    @Override
+    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx procCtx, Object... nodeOutputs) throws SemanticException {
+      ReduceSinkOperator reduceSink = findReduceSink(stack);
+      if (reduceSink != null && !reduceSink.getConf().hasOrderBy()) { // LIMIT + ORDER BY handled by TopNKey push down

Review comment:
       Should we also check whether the ReduceSink also has a topn value greater then 0?
   ```
   reduceSink.getConf().getTopN() > 0
   ```




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