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From "ASF GitHub Bot (Jira)" <>
Subject [jira] [Work logged] (HIVE-23365) Put RS deduplication optimization under cost based decision
Date Mon, 01 Jun 2020 16:35:00 GMT


ASF GitHub Bot logged work on HIVE-23365:

                Author: ASF GitHub Bot
            Created on: 01/Jun/20 16:34
            Start Date: 01/Jun/20 16:34
    Worklog Time Spent: 10m 
      Work Description: jcamachor commented on a change in pull request #1035:

File path: ql/src/java/org/apache/hadoop/hive/ql/optimizer/correlation/
@@ -197,6 +186,27 @@ public static boolean merge(ReduceSinkOperator cRS, ReduceSinkOperator
pRS, int
     return true;
+  private static long estimateReducers(HiveConf conf, ReduceSinkOperator rs) {
+    // TODO: Check if we can somehow exploit the logic in SetReducerParallelism
+    if (rs.getConf().getNumReducers() > 0) {
+      return rs.getConf().getNumReducers();
+    }
+    int constantReducers = conf.getIntVar(HiveConf.ConfVars.HADOOPNUMREDUCERS);
+    if (constantReducers > 0) {
+      return constantReducers;
+    }
+    long inputTotalBytes = 0;
+    List<Operator<?>> rsSiblings = rs.getChildOperators().get(0).getParentOperators();
+    for (Operator<?> sibling : rsSiblings) {
+      if (sibling.getStatistics() != null) {

Review comment:
       I am wondering, if you do not have stats available for any of the siblings, we should
probably skip the parallelism check, and thus, fallback to previous behavior.

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Issue Time Tracking

    Worklog Id:     (was: 439593)
    Time Spent: 40m  (was: 0.5h)

> Put RS deduplication optimization under cost based decision
> -----------------------------------------------------------
>                 Key: HIVE-23365
>                 URL:
>             Project: Hive
>          Issue Type: Improvement
>          Components: Physical Optimizer
>            Reporter: Jesus Camacho Rodriguez
>            Assignee: Stamatis Zampetakis
>            Priority: Major
>              Labels: pull-request-available
>         Attachments: HIVE-23365.01.patch, HIVE-23365.02.patch, HIVE-23365.03.patch, HIVE-23365.04.patch,
>          Time Spent: 40m
>  Remaining Estimate: 0h
> Currently, RS deduplication is always executed whenever it is semantically correct. However,
it could be beneficial to leave both RS operators in the plan, e.g., if the NDV of the second
RS is very low. Thus, we would like this decision to be cost-based. We could use a simple
heuristic that would work fine for most of the cases without introducing regressions for existing
cases, e.g., if NDV for partition column is less than estimated parallelism in the second
RS, do not execute deduplication.

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