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Tim Armstrong commented on IMPALA-8026: --------------------------------------- [~Paul.Rogers] you're right, that's all it's doing. The implementation is complicated a bit because some nodes try to return early, before they've constructed a full output batch, if they hit a limit, and the limit checks depend on updating num_rows_returned_. This kind-of makes sense since if there are selective predicates it may require an indefinitely long time to produce the net row in the batch. It could be simplified for sure. > 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 > Assignee: Tim Armstrong > 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. -- This message was sent by Atlassian JIRA (v7.6.3#76005) --------------------------------------------------------------------- To unsubscribe, e-mail: issues-all-unsubscr...@impala.apache.org For additional commands, e-mail: issues-all-h...@impala.apache.org