commit 6f9d9514c3c765c6e593ce0bf146041f3e561c29
author Aman Sinha <amsinha@cloudera.com> Sat Sep 19 23:02:01 2020 -0700
committer Impala Public Jenkins <impala-public-jenkins@cloudera.com> Tue Sep 22 00:41:28 2020 +0000
tree 46b508fcb6e67bf91d2b9638017710c0039af90d
parent 79b17db83a2f97a409e468c5dd5cfaeb9454f5dd

IMPALA-10179: After join inversion inherit the left child's parallelism

The planner makes a cost based decision to invert a join - i.e swap
left and right child - based on cardinality, row size and parallelism
of both sides. The join node's numNodes and numInstances attributes were
originally set based on the left child's values. These were not getting
reset after the inversion and the plan shows the original value,
although the backend actually uses the correct value from the left
input..so this mainly affects planner estimates.

This patch fixes this behavior by inheriting the new left child's
parallelism (numNodes and numInstances) after inversion. A common case
where the join inputs get flipped is when the original query has a
left semi join (IN, EXISTS etc.) and the subquery has a large
cardinality compared to the outer query. This gets converted to a
right semi join.  This pattern occurs for example in TPC-DS q10, q35,
q69.

Testing:
 - Updated the plans for several TPC-DS queries to reflect the modified
   parallelism.  11 queries had the join fragment 'instances' increase
   from either 1 or 2 to 6 (for planner tests hosts = 3 and mt_dop = 2).

   There were also 4 queries where the parallelism reduced. Based on an
   initial check, these changes seem expected.

   TODO: Check if this reduction is expected based on the costing. One
   option may be to use a max(left parallelism, right parallelism) value
   to avoid unintended side effects.

 - Couple of TPC-H queries also had increase in parallelism.
 - Ran e2e TPC-DS queries.
 - TODO: test performance on larger scale factor

Change-Id: I01ba559034ad76f7ccee41f237c81b29d8402950
Reviewed-on: http://gerrit.cloudera.org:8080/16480
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>