[jira] [Created] (HIVE-23891) Using UNION sql clause and speculative execution can cause file duplication in Tez

["George Pachitariu (Jira)" <ji...@apache.org>]

George Pachitariu created HIVE-23891:
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             Summary: Using UNION sql clause and speculative execution can cause file duplication in Tez
                 Key: HIVE-23891
                 URL: https://issues.apache.org/jira/browse/HIVE-23891
             Project: Hive
          Issue Type: Bug
            Reporter: George Pachitariu
            Assignee: George Pachitariu


Hello, 

the specific scenario when this can happen:
 - the execution engine is Tez;
 - speculative execution is on;
 - the query inserts into a table and the last step is a UNION sql clause;

The problem is that Tez creates an extra layer of subdirectories when there is a UNION. Later, when deduplicating, Hive doesn't take that into account and only deduplicates folders but not the files inside.

So for a query like this:
{code:sql}
insert overwrite table union_all
    select * from union_first_part
union all
    select * from union_second_part;
{code}
The folder structure afterwards will be like this (a possible example):
{code:java}
.../union_all/HIVE_UNION_SUBDIR_1/000000_0
.../union_all/HIVE_UNION_SUBDIR_1/000000_1
.../union_all/HIVE_UNION_SUBDIR_2/000000_1
{code}
The attached patch increases the number of folder levels that Hive will check recursively for duplicates (recursively) when we have a UNION in Tez.

Feel free to reach out if you have any questions :).



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