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[Parquet] Minor: Update comments in page decompressor #8764

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Merged
merged 2 commits into from
Nov 6, 2025
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[Parquet] Minor: Update comments in page decompressor #8764

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2ba0324
Update comments in page decompressor
alamb Nov 1, 2025
cae8d57
Update parquet/src/file/serialized_reader.rs
alamb Nov 6, 2025
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4 changes: 2 additions & 2 deletions parquet/src/file/serialized_reader.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -387,8 +387,6 @@ pub(crate) fn decode_page(
can_decompress = header_v2.is_compressed.unwrap_or(true);
}

// TODO: page header could be huge because of statistics. We should set a
// maximum page header size and abort if that is exceeded.
let buffer = match decompressor {
Some(decompressor) if can_decompress => {
let uncompressed_page_size = usize::try_from(page_header.uncompressed_page_size)?;
Expand All @@ -398,6 +396,8 @@ pub(crate) fn decode_page(
let decompressed_size = uncompressed_page_size - offset;
let mut decompressed = Vec::with_capacity(uncompressed_page_size);
decompressed.extend_from_slice(&buffer[..offset]);
// decompressed size of zero corresponds to a page with only null values
// see https://github.com/apache/parquet-format/blob/master/README.md#data-pages
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@mapleFU mapleFU Nov 1, 2025

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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.

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@alamb alamb Nov 3, 2025

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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))

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@mapleFU mapleFU Nov 3, 2025

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Would page without values or some word better here?

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@etseidl etseidl Nov 3, 2025 •
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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.

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@mapleFU mapleFU Nov 4, 2025

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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

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@alamb alamb Nov 5, 2025

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I agree a page with num_rows == 0 doesn't seem like it should happen

However, my understanding of this comment is that it is trying to say that there are no non-null values

I don't think I fully understand your comment @mapleFU -- if you think what is written is imprecise, could you perhaps suggest some alternate wording?

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@mapleFU mapleFU Nov 6, 2025

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However, my understanding of this comment is that it is trying to say that there are no non-null values

Current comment is ok for me, I wonder would:

 // decompressed size of zero corresponds to a page with no non-null values

Be better, since this also cover the case that num_rows == 0 => num_values == 0 && num_nulls == 0

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@alamb alamb Nov 6, 2025

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got it - that makes sense. thank you

if decompressed_size > 0 {
let compressed = &buffer[offset..];
decompressor.decompress(compressed, &mut decompressed, Some(decompressed_size))?;
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