[Parquet] Minor: Update comments in page decompressor

[alamb]

Which issue does this PR close?

• follow on to POC: Avoid a copy for uncompressed pages #8756

Rationale for this change

@etseidl comments: #8756 (comment)

Not relevant to this PR, but I think this TODO has largely been addressed by #8376 which enabled skipping the decoding of the page statistics.

While I was in here, I also wanted to capture the learning based on @mapleFU 's comment #8756 (comment)

The code looks good to me but the I don't know if the comment "not compressed" can be replaced, if decompress_buffer is called and decompressed_size == 0 , seems that it generally means something like "this page only have levels, but not have non-null values"? ( Point me out if I'm wrong)

What changes are included in this PR?

Include some comments

Are these changes tested?

No (there are no code changes)

Are there any user-facing changes?

No, this is internal comments only. No code / behavior changes

[mapleFU]

Generally this is ok to me, but I don't know would some page being just be empty (with num-rows == 0), I know currently most writer would not write that.

[alamb]

When I did some code coverage testing, I found it was rare but not impossible (it happens 3 times in the parquet-rs suite: #8756 (comment))

[mapleFU]

Would page without values or some word better here?

[etseidl]

Would page without values or some word better here?

I think the current wording is correct. In this case num_values is non-zero, so there are values, but they all just happen to be null (and thus are not encoded in the data).

I'd think the ways decompressed_size can be zero are 1) v2 page with all null values, 2) v1 page with no nesting and all nulls at the top level (so D is always 0). Second case would have definition level data.

[mapleFU]

I think the current wording is correct. In this case num_values is non-zero, so there are values, but they all just happen to be null (and thus are not encoded in the data).

I mean, I don't know would a case that just num_rows ( not num_values ) == 0 would exists