[IMPALA-8423] Add rule to remove useless SELECT node

• Type: Improvement
• Status: In Progress
• Priority: Major
• Resolution: Unresolved
• Component/s: Frontend
• Labels: performance
• Epic Color: ghx-label-6
• Assignee: Unassigned
• Reporter: Quanlong Huang
• Created: 16/Apr/19 13:06
• Updated: 23/Feb/23 07:54

Description

We can add some rules to optimize the plan after we chose a cheapest plan based on cost. For example, one useful rule can be "removing useless SELECT nodes".

Impala will generated a useless SELECT for the following query:

SELECT t.id, t.int_col
FROM functional.alltypestiny t
LEFT JOIN
  (SELECT id, int_col
  FROM functional.alltypestiny) t2
ON (t.id = t2.id)
WHERE t.int_col = t.id
UNION ALL
VALUES (NULL, NULL)

Its single node plan is

PLAN-ROOT SINK
|
00:UNION
|  constant-operands=1
|  row-size=8B cardinality=1
|
04:SELECT
|  predicates: t.id = t.int_col
|  row-size=12B cardinality=0
|
03:HASH JOIN [RIGHT OUTER JOIN]
|  hash predicates: id = t.id
|  runtime filters: RF000 <- t.id
|  row-size=12B cardinality=1
|
|--01:SCAN HDFS [functional.alltypestiny t]
|     HDFS partitions=4/4 files=4 size=460B
|     predicates: t.int_col = t.id
|     row-size=8B cardinality=1
|
02:SCAN HDFS [functional.alltypestiny]
   HDFS partitions=4/4 files=4 size=460B
   runtime filters: RF000 -> id
   row-size=4B cardinality=8

The SELECT node (id=04) is useless since its only predicate "t.id = t.int_col" has been enforced in the SCAN node (id=01) which is the right hand side of the RIGHT OUTER JOIN. The SELECT node won't filter out any more rows.