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https://issues.apache.org/jira/browse/FLINK-3848?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15650023#comment-15650023
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Jark Wu edited comment on FLINK-3848 at 11/9/16 7:07 AM:
---------------------------------------------------------
Hi [~tonycox] , I have looked into your work and think you may have
misunderstood this issue.
The {{ProjectableTableSource}} interface is for projection push-down. Such as
we have a Table with 5 columns ( a,b,c,d,e) but the query only need column
{{a}}.
{code}
SELECT a FROM MyTable
{code}
Currently, the physical plan is a TableScan to read all columns out , and then
apply a FlatMap to emit column {{a}}. If we have {{ProjectableTableSource}},
the TableScan should only read the column {{a}} out. It is a performance
optimization.
Users do not need to do something like
{{prepareCsvSource.setProjection(Array("id"))}} to manually specify the
projection columns.
Correct me if I'm wrong [~fhueske].
was (Author: jark):
Hi [~tonycox] , I have looked into your work and think you may have
misunderstood this issue.
The {{ProjectableTableSource}} interface is for projection push-down. Such as
we have a Table with 5 columns ( a,b,c,d,e) but the query only need column a.
{code}
SELECT a FROM MyTable
{code}
Currently, the physical plan will be a TableScan to read all columns out , and
then apply a FlatMap to emit column a. If we have {{ProjectableTableSource}},
the TableScan should only read the column {{a}} out. It is a performance
optimization.
So users do not need to do something like
{{prepareCsvSource.setProjection(Array("id"))}} to manually specify the
projection columns.
Correct me if I'm wrong [~fhueske].
> Add ProjectableTableSource interface and translation rule
> ---------------------------------------------------------
>
> Key: FLINK-3848
> URL: https://issues.apache.org/jira/browse/FLINK-3848
> Project: Flink
> Issue Type: New Feature
> Components: Table API & SQL
> Reporter: Fabian Hueske
> Assignee: Anton Solovev
>
> Add a {{ProjectableTableSource}} interface for {{TableSource}} implementation
> that support projection push-down.
> The interface could look as follows
> {code}
> def trait ProjectableTableSource {
> def setProjection(fields: Array[String]): Unit
> }
> {code}
> In addition we need Calcite rules to push a projection into a TableScan that
> refers to a {{ProjectableTableSource}}. We might need to tweak the cost model
> as well to push the optimizer in the right direction.
> Moreover, the {{CsvTableSource}} could be extended to implement
> {{ProjectableTableSource}}.
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