[FLINK-23555] Improve common subexpression elimination by using local references

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Fix Version/s: 1.20.0
• Component/s: Table SQL / Planner
• Assignee: Unassigned
• Reporter: weibowen
• Created: 30/Jul/21 06:24
• Updated: 11/Mar/24 12:44

Description

When we write a sql like 

select udf2(udf1(field), udf3(udf1(field) ...

udf1(field) will be invoked twice. However once udf1 has a bad performance, it will have a huge impact to the whole task. More times invoked, huger impact.

I hope that whatever how many times udf1(field) writed in sql, Flink will take advantage of common subexpression elimination and only invoke it once.

Then i do some work on this, and the attachment tells the result.

 

The sql.png shows the sql logic,

and i read source from kafka and sink into blackhole. The parallelism is 1.

The udf `testcse` do nothing except sleeping 20 milliseconds, while the udf `testcse2`, `testcse3` and `testcse4` are the same udf with different alias which completely do nothing.

As expected, the performance after optimization is approximately 3 times than before since I write `testcse(sid)` 3 times in sql.

before:

after: