Fix empty array handling in arraySplit

[hczhcz]

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• Bug Fix

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Fix #7708 empty array handling in arraySplit

[tavplubix]

In case like SELECT arraySplit(x -> 0, []) column mapped is ColumnConst and mapped->size() is zero. However we access it's value in if (column_cut_const->getValue<UInt8>()) (line 68). It works because nested column actually has size 1 and ColumnConst just returns value of nested column without any checks, but it looks very suspicious.

[tavplubix]

May be it's better to return [[]] for empty array to make result more consistent with arraySplit(x -> 0, [1, 2, 3, 4, 5]), which returns [[1,2,3,4,5]] (array, which contains the source array)?

[alexey-milovidov]

Ok for now.

[hczhcz]

@tavplubix
...->getValue<UInt8>() is used in many high-order functions (arrayFilter, arrayCount, arrayAll, etc).
It might be good to make developers aware the suspicious thing by adding some comments & docs.

[nikitamikhaylov]

Conflicts with 19.16