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Posted to issues-all@impala.apache.org by "Tim Armstrong (JIRA)" <ji...@apache.org> on 2019/01/04 16:41:00 UTC
[jira] [Commented] (IMPALA-8026) Actual row counts for nested loop
join are meaningless
[ https://issues.apache.org/jira/browse/IMPALA-8026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16734320#comment-16734320 ]
Tim Armstrong commented on IMPALA-8026:
---------------------------------------
The logic for num_rows_returned_ and rows_returned_counter_ in NestedLoopJoinNode is way overcomplicated and spread all over the source file. I suspect the bug is in there. I scanned through it and NestedLoopJoinNode::FindBuildMatches() looks suspicious - it's incrementing rows_returned_counter_ by output_batch->num_rows(), but it can be called multiple times for the same output batch, so will double-count previous rows in the batch.
PartitionedHashJoinNode::GetNext() has a much simpler and more efficient implementation of maintaining those counters that we can use.
> Actual row counts for nested loop join are meaningless
> ------------------------------------------------------
>
> Key: IMPALA-8026
> URL: https://issues.apache.org/jira/browse/IMPALA-8026
> Project: IMPALA
> Issue Type: Bug
> Components: Backend
> Affects Versions: Impala 3.1.0
> Reporter: Paul Rogers
> Priority: Major
>
> Consider this extract from a query plan:
> {noformat}
> Operator #Rows Est. #Rows
> --------------------------------------------------------------
> …
> | 10:HASH JOIN 9.53M 18.14K
> | |--19:EXCHANGE 1 1
> | | 00:SCAN HDFS 1 1
> | 06:NESTED LOOP JOIN 4.88B 863.84K
> | |--18:EXCHANGE 1 1
> | | 04:SCAN HDFS 1 1
> | 05:HASH JOIN 9.53M 863.84K
> {noformat}
> If the above is to be believed, the 06 nested loop join produced 5 billion rows. But, the actual number is far too huge for that: joining 1 row with 10 million rows cannot produce 500 times that number of rows.
> It appears that the nested loop join actually processed and returned the 9.5 million rows, since that is the same number produced by the 10 hash join which joins a single row with the output of the nested loop join.
> Because this same bogus result appears across multiple plans, it is likely that the actual number is completely wrong and bears no relation to the number of rows actually returned.
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