Rich-T-kid commented on code in PR #11173:
URL: https://github.com/apache/arrow-rs/pull/11173#discussion_r4077848785
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parquet/src/arrow/arrow_writer/mod.rs:
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@@ -1145,21 +1145,11 @@ impl ArrowColumnWriter {
let non_null = levels.non_null_indices();
match array.as_any_dictionary_opt() {
Some(dict) => {
- // For dictionary arrays, hash the integer keys rather
than the actual values.
- // Key cardinality equals value cardinality, so
distinct-value counting stays
- // correct while avoiding the cost of hashing
arbitrary-length values.
- let keys = dict.keys();
- let key_data = keys.to_data();
- let offset = key_data.offset();
- let width = arrow_key_byte_width(keys.data_type());
- if width > 0 {
- let buffer = key_data.buffers()[0].as_slice();
- // Only visit non-null rows to avoid counting nulls as
a distinct value.
- for &row in non_null {
- let pos = (offset + row) * width;
- seen.insert(hash_bytes(&buffer[pos..pos + width]));
- }
- }
+ // Hash values, not key indices: the same key index can
map to different values across batches, causing undercounting.
Review Comment:
updated this. thanks @etseidl 🫡
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