Implement BitLength function push down

[Willendless]

Feature Request

Describe the feature you'd like:

We'd like to support BitLength function push down.

Teachability, Documentation, Adoption, Migration Strategy:

At least one integration test and several unit tests.

[johnhaxx7]

/assign

[johnhaxx7]

Hi @Willendless , @ywqzzy, can you give some hint on how to run integration tests?
I tried to run some scripts in this dir https://github.com/pingcap/tiflash/blob/master/tests/README.md
but seems like it's missing a test build at tiflash/tests/fullstack-test/../../tests/.build/tiflash

[ywqzzy]

Hi @Willendless , @ywqzzy, can you give some hint on how to run integration tests? I tried to run some scripts in this dir https://github.com/pingcap/tiflash/blob/master/tests/README.md but seems like it's missing a test build at tiflash/tests/fullstack-test/../../tests/.build/tiflash

If you want to test your own code in integration test:

1. First, you need to build your own tiflash binary in {your_work_space}/build.
2. Second, you need to run your tidb cluster locally using tiup playgroud. You need to change the binary of TiFlash in tiup config, tiup playground You can ref to this doc to run tiup playgroud.tiupYou can ref to this doc to install tiup and understand what it is.
3. Third, you need to check {your_work_space}/tests/_env.sh to make the port and build dir right.
4. Finally, you can run your tests using commands like "./run-test.sh fullstack-test2/ddl".

[isHuangXin]

I am very interested in this issue and want to try it. Moreover, I have successfully built the tiflash project so far. Plz assign me.

[JaySon-Huang]

/unassign

Unassign this issue due to no more comments/PR and no response to #5092 (comment)

[JaySon-Huang]

@isHuangXin you can leave a comment to assign yourself when you are free

[isHuangXin]

/assign

[isHuangXin]

I'm already starting to debug the whole process, but I'm feeling confused.

As the document describes that the pushdown function is initiated from the TiDB side. The scalarExprSupportedByFlash in /tidb/expression/expression.go will determine which functions can be pushed down to TiFlash for execution.

But I didn't find ast.bitLength in scalarExprSupportedByFlash function. Instead, I found ast.BitLength in scalarExprSupportedByTiKV. Does this mean that I should implement the ast.length function in tidb first, and then push down the ast.bitLength function in tidb to tiflash.