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https://issues.apache.org/jira/browse/IMPALA-8026?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16734325#comment-16734325
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Tim Armstrong commented on IMPALA-8026:
---------------------------------------
Actually the hash join approach doesn't quite work, because we depend on
num_rows_returned_ being updated since ReachedLimit() is checked inside many
loops. But we can remove the rows_returned_counter_ maintenance to the
GetNext() function so that it's only updated in one place.
> 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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