Github user cloud-fan commented on the issue:
https://github.com/apache/spark/pull/13680
One more thought about the format: `UnsafeRow` use 8 bytes to store offset
and length for variable-length type, this is because `UnsafeRow` is
word-aligned, so we can't calculate the element size by subtracting 2 adjacent
offsets, but need an extra length information. This is also why in unsafe row
writer we call `zeroOutPaddingBytes` before we write variable-length field.
However, `UnsafeArrayData` doesn't have this requirement, as it's already a
field of row and itself will be word-aligned. So we can:
1. only use 4 bytes to store offset for variable-length type element, and
calculate the length by subtracting 2 adjacent offsets.
2. still use 8 bytes to store offset and length, and follow unsafe row
writer to `zeroOutPaddingBytes` before write.
Personally I prefer option 1 as it's more compact.
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