fix ifnull bug (#4832)

[ti-chi-bot]

This is an automated cherry-pick of #4832

Signed-off-by: xufei xufeixw@mail.ustc.edu.cn

What problem does this PR solve?

Issue Number: close #4829

Problem Summary:

What is changed and how it works?

The root cause is for
ifnull(col1, col2)
TiFlash should rewrite it to
multiIf(isnull(col1), col2, assumeNotNull(col1))
but it rewrite it to
multiIf(isnull(col1), assumeNotNull(col2), col1)

Why unit test does not find this bug:
Because unit test and DAGExpressionAnalyze does not share the same rewrite code, and unit test rewrite it correctly. We should enhance the test framework to ensure the function rewrite logic is the same between unit test and DAGExpressionAnalyze.(#4830)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Fix ifNull bug if the second argument contains null

[ti-chi-bot]

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[windtalker]

Close as there is no more version based on 6.0 branch