[jira] [Created] (IMPALA-8002) Unstable join ordering for equivalent tables

["Paul Rogers (JIRA)" <ji...@apache.org>]

Paul Rogers created IMPALA-8002:
-----------------------------------

             Summary: Unstable join ordering for equivalent tables
                 Key: IMPALA-8002
                 URL: https://issues.apache.org/jira/browse/IMPALA-8002
             Project: IMPALA
          Issue Type: Bug
          Components: Frontend
    Affects Versions: Impala 3.1.0
            Reporter: Paul Rogers


Consider the following test: {{PlannerTest.testJoins()}}:

{noformat}
select t1.d, t2.d
from functional.nulltable t1, functional.nulltable t2, functional.nulltable t3
where t1.d IS DISTINCT FROM t2.d
and t3.a != t2.g
---- PLAN
PLAN-ROOT SINK
|
04:NESTED LOOP JOIN [INNER JOIN]
|  predicates: t3.a != t2.g
|
|--02:SCAN HDFS [functional.nulltable t3]
|     partitions=1/1 files=1 size=18B
|
03:NESTED LOOP JOIN [INNER JOIN]
|  predicates: t1.d IS DISTINCT FROM t2.d
|
|--00:SCAN HDFS [functional.nulltable t1]
|     partitions=1/1 files=1 size=18B
|
01:SCAN HDFS [functional.nulltable t2]
   partitions=1/1 files=1 size=18B
{noformat}

Despite no changes in the planner code, on one run the order flipped to the above. Previously, 01 was t1 and 00 was t2.

Likely, the behavior when two tables are equivalent has some kind of non-determinism, perhaps storing candidates in a Java Set or Map with undefined order.



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