feat(binder): convert to constant lookup when case when expression is in simple form

[xzhseh]

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What's changed and what's your intention?

As titled, related: #14317, specifically the following pattern.

In addition, if users write a case-when expression that can be converted to the simple form, we can also normalize it in optimizer easily:

SELECT a,
       CASE WHEN a=1 THEN 'one'
            WHEN a=2 THEN 'two'
            ELSE 'other'
       END
    FROM test;

Also, base branch is set to #15077 to avoid any potential merge conflict, should merge after merging base branch.

Planner tests & e2e_test will be added later.

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[BugenZhao]

Can you elaborate on the relationship between this PR and #14965? BTW, would you like to have a try of https://graphite.dev for the stacking workflow? 😄

[xzhseh]

Can you elaborate on the relationship between this PR and #14965?

#14965 aims to optimize the pattern like below, e.g.,

select case 1 when 1 then 114514 (...) else 1919810 end;

to a simple constant (i.e., 114514 in the above example) during frontend binding phase, while this PR is particularly for detecting some pattern that can be transformed to optimizable case-when expression (e.g., constant lookup), an example would be the pattern in the above description.

BTW, would you like to have a try of https://graphite.dev for the stacking workflow? 😄

I heard of graphite before (but never actually try it) and I just realize how powerful it is, will start using it today. 😄
Thanks for the recommendation!

[TennyZhuang]

Can you add some planner test? to cover most branches of your function.

[xzhseh]

Note here, only if the amount of arms exceeds the pre-set limit (i.e., 30), we will try the transformation. In other words, the case-when expression as below will operate normally. (only 2 arms excluding the fallback arm)

select
    case 
        when c1 = 1 then (...)
        when c1 = 2 then (...)
        else (...)
    end
from t1;

[TennyZhuang]

LGTM