zoucao created FLINK-26982:
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Summary: strike a balance between reuse the same RelNode and
project/filter/limit/partition push down
Key: FLINK-26982
URL: https://issues.apache.org/jira/browse/FLINK-26982
Project: Flink
Issue Type: Improvement
Components: Table SQL / Planner
Reporter: zoucao
Now, Flink has effective reuse logic to reuse the same RelNode and subplan, but
it will lose efficacy in some situations, like project/filter/limit/partition
push down, if one of them is enabled, the new source is not the same from old
one, so the source can not be reused anymore.
For some complicated SQL, many views will be created from the same table, and
the scan RelNode can not be reused, such that many of the same threads about
reading source data will be created in one task, which will cause the memory
problem and sometimes will cause reading amplification.
Should we do something to enforce reusing decided by users themselves?
The following SQL shows the situation proposed above.
{code:java}
create table fs(
a int,
b string,
c bigint
) PARTITIONED by ( c )with (
'connector' = 'filesystem',
'format' = 'csv',
'path' = 'file:///tmp/test'
);
select * from
(select * from fs limit 1)
union all
(select * from fs where a = 2)
union all
(select 1, b, c from fs)
union all
(select 1, b, c from fs where c = 1)
{code}
== Optimized Execution Plan ==
{code:java}
Union(all=[true], union=[a, b, c])
:- Union(all=[true], union=[a, b, c])
: :- Union(all=[true], union=[a, b, c])
: : :- Limit(offset=[0], fetch=[1])
: : : +- Exchange(distribution=[single])
: : : +- TableSourceScan(table=[[default_catalog, default_database, fs,
limit=[1]]], fields=[a, b, c])
: : +- Calc(select=[CAST(2 AS INTEGER) AS a, b, c], where=[(a = 2)])
: : +- TableSourceScan(table=[[default_catalog, default_database, fs,
filter=[=(a, 2)]]], fields=[a, b, c])
: +- Calc(select=[CAST(1 AS INTEGER) AS EXPR$0, b, c])
: +- TableSourceScan(table=[[default_catalog, default_database, fs,
project=[b, c], metadata=[]]], fields=[b, c])
+- Calc(select=[CAST(1 AS INTEGER) AS EXPR$0, b, CAST(1 AS BIGINT) AS c])
+- TableSourceScan(table=[[default_catalog, default_database, fs,
partitions=[{c=1}], project=[b], metadata=[]]], fields=[b])
{code}
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