[IMPALA-8263] Planner failed to flip build/probe side of join

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Not A Bug
• Affects Version/s: Impala 3.1.0
• Component/s: Frontend
• Epic Color: ghx-label-4
• Assignee: Unassigned
• Reporter: Paul Rogers
• Created: 28/Feb/19 06:24
• Updated: 20/Sep/19 21:56
• Resolved: 20/Sep/19 21:56

Description

TPC-H query 8 was reviewed after applying the changes proposed for DRILL-8014. (See tpch-all.test.) The revised plan has better cardinality number, but contains this odd structure:

11:HASH JOIN [INNER JOIN]
|  hash predicates: o_custkey = c_custkey
|  row-size=139B cardinality=39.66K
|
|--04:SCAN HDFS [tpch.customer]
|     row-size=10B cardinality=150.00K
|
10:HASH JOIN [INNER JOIN]
|  hash predicates: o_orderkey = l_orderkey
|  row-size=129B cardinality=39.66K

As I understand it, the planner should flip the left and right sides of an inner join if the right side (the 04 scan) has a larger cardinality than the left (join 10) side. That flip did not happen in this case, causing the join to build a hash table about 4 times larger than necessary.

Perhaps there is some other constraint. Investigate to determine if the behavior is correct (and if so why), or the source of incorrect behavior.