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https://issues.apache.org/jira/browse/FLINK-33996?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17803626#comment-17803626
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Jeyhun Karimov commented on FLINK-33996:
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Thanks for reporting the issue [~hackergin] and thanks for your comment
[~xuyangzhong]. I mostly agree with [~xuyangzhong]. An alternative (or simpler)
solution would be to add an extra check in {{FlinkRelUtil::mergeable}}
function, since all projection/calc merging rules use the result of this
function before merging.
[~hackergin] IMO adding a new rule would be tricky since
- We will need to ingest the new rule to multiple places in the optimization
process (codebase), because 1) project and calc merging rules are used several
times and 2) the order of the new optimization rule will matter
- If we add a new rule, and if the two project/calc nodes already merged (by
the existing merging rules), it will be non-trivial for our new rule to
rollback (seperate the merged project/calc nodes)
What do you guys think? Also, please feel free to check/review the PR.
> Support disabling project rewrite when multiple exprs in the project
> reference the same sub project field.
> ----------------------------------------------------------------------------------------------------------
>
> Key: FLINK-33996
> URL: https://issues.apache.org/jira/browse/FLINK-33996
> Project: Flink
> Issue Type: Improvement
> Components: Table SQL / Runtime
> Affects Versions: 1.18.0
> Reporter: Feng Jin
> Priority: Major
> Labels: pull-request-available
>
> When multiple top projects reference the same bottom project, project rewrite
> rules may result in complex projects being calculated multiple times.
> Take the following SQL as an example:
> {code:sql}
> create table test_source(a varchar) with ('connector'='datagen');
> explan plan for select a || 'a' as a, a || 'b' as b FROM (select
> REGEXP_REPLACE(a, 'aaa', 'bbb') as a FROM test_source);
> {code}
> The final SQL plan is as follows:
> {code:sql}
> == Abstract Syntax Tree ==
> LogicalProject(a=[||($0, _UTF-16LE'a')], b=[||($0, _UTF-16LE'b')])
> +- LogicalProject(a=[REGEXP_REPLACE($0, _UTF-16LE'aaa', _UTF-16LE'bbb')])
> +- LogicalTableScan(table=[[default_catalog, default_database,
> test_source]])
> == Optimized Physical Plan ==
> Calc(select=[||(REGEXP_REPLACE(a, _UTF-16LE'aaa', _UTF-16LE'bbb'),
> _UTF-16LE'a') AS a, ||(REGEXP_REPLACE(a, _UTF-16LE'aaa', _UTF-16LE'bbb'),
> _UTF-16LE'b') AS b])
> +- TableSourceScan(table=[[default_catalog, default_database, test_source]],
> fields=[a])
> == Optimized Execution Plan ==
> Calc(select=[||(REGEXP_REPLACE(a, 'aaa', 'bbb'), 'a') AS a,
> ||(REGEXP_REPLACE(a, 'aaa', 'bbb'), 'b') AS b])
> +- TableSourceScan(table=[[default_catalog, default_database, test_source]],
> fields=[a])
> {code}
> It can be observed that after project write, regex_place is calculated twice.
> Generally speaking, regular expression matching is a time-consuming operation
> and we usually do not want it to be calculated multiple times. Therefore, for
> this scenario, we can support disabling project rewrite.
> After disabling some rules, the final plan we obtained is as follows:
> {code:sql}
> == Abstract Syntax Tree ==
> LogicalProject(a=[||($0, _UTF-16LE'a')], b=[||($0, _UTF-16LE'b')])
> +- LogicalProject(a=[REGEXP_REPLACE($0, _UTF-16LE'aaa', _UTF-16LE'bbb')])
> +- LogicalTableScan(table=[[default_catalog, default_database,
> test_source]])
> == Optimized Physical Plan ==
> Calc(select=[||(a, _UTF-16LE'a') AS a, ||(a, _UTF-16LE'b') AS b])
> +- Calc(select=[REGEXP_REPLACE(a, _UTF-16LE'aaa', _UTF-16LE'bbb') AS a])
> +- TableSourceScan(table=[[default_catalog, default_database,
> test_source]], fields=[a])
> == Optimized Execution Plan ==
> Calc(select=[||(a, 'a') AS a, ||(a, 'b') AS b])
> +- Calc(select=[REGEXP_REPLACE(a, 'aaa', 'bbb') AS a])
> +- TableSourceScan(table=[[default_catalog, default_database,
> test_source]], fields=[a])
> {code}
> After testing, we probably need to modify these few rules:
> org.apache.flink.table.planner.plan.rules.logical.FlinkProjectMergeRule
> org.apache.flink.table.planner.plan.rules.logical.FlinkCalcMergeRule
> org.apache.flink.table.planner.plan.rules.logical.FlinkProjectCalcMergeRule
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