[C++] Add "AppendEmptyValue" builder APIs for use inside StructBuilder::AppendNull

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StructBuilder should probably also add "UnsafeAppendNull" so that there is the option of using the Unsafe* operations on the children

Reporter: Wes McKinney / @wesm
Assignee: Ji Liu / @tianchen92

PRs and other links:

• GitHub Pull Request #7887

_{Note: This issue was originally created as ARROW-9304. Please see the migration documentation for further details.}

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Ji Liu / @tianchen92:
Hi Wes, have you started work on this issue? if not, i would like to take this one. Also have some questions

i. Now the builder usages are: call structBuilder appendNull, then call child builder appendNull, this issue meant to call child builder appendNull inside structBuilder#appendNull?

ii. Why we need AppendEmptyValue API, instead of just using appendNull?

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Wes McKinney / @wesm:
Calling AppendNull triggers creation of a validity bitmap on the child fields which may not be needed if the child fields do not have any nulls.

Perhaps let's call the API AppendEmpty

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Ji Liu / @tianchen92:
Thanks for clarification, can i take it? :)

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Wes McKinney / @wesm:
Sure please go ahead

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Ji Liu / @tianchen92:
Hi @wesm, I worked on this issue today, but still confused about what’s the use case of this API after reading the code:

#1 when call StructBuilder#AppendNull, shouldn’t all child fields slot at this index be null? then what’s the meaning of "Calling AppendNull triggers creation of a validity bitmap on the child fields which may not be needed if the child fields do not have any nulls", or it just means child null bitmap is the same with parent?

#2 what’s the correct behavior of AppendEmpty? append zero data, length_++ or maybe null_count_++(or null_count_ should always 0)?

#3 It’s better to give a detail example, like

Struct<utf8, int32> [{"joe", 1}, {null, 2}, null, {"mark", 4}] 

to explain how we use AppendEmpty :)

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Wes McKinney / @wesm:
There is no need for the child arrays to be null when the top-level struct slot is null, so only a placeholder "empty" value needs to be appended. This is discussed in the columnar format documentation

http://arrow.apache.org/docs/format/Columnar.html#struct-layout

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Antoine Pitrou / @pitrou:
This is also worthwhile doing on union arrays (especially sparse).

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Antoine Pitrou / @pitrou:
Issue resolved by pull request 7887
#7887