Re: [I] [Parquet] Add ALP regression coverage for an empty first page [arrow-rs]

["Abdallah-Afifi (via GitHub)" <gi...@apache.org>]

Abdallah-Afifi commented on issue #10644:
URL: https://github.com/apache/arrow-rs/issues/10644#issuecomment-5556130122

   Opened #11004 for this.
   
   While writing the regression test it turned out the empty first page isn't only untested, it's actively harmful. `flush_buffer` caches the first page's preset for the whole column chunk, so the exponent 0 / factor 0 fallback makes every fractional value in every later page an exception. Encoding 3000 values of `i * 0.01`: 3,808 bytes when they are the first page, 31,258 bytes when preceded by an empty one, against 24,000 unencoded.
   
   It's also reachable from the normal write path — the column writer counts levels rather than values, so an all-null first page flushes an empty buffer.
   
   So the PR carries the fix @alamb suggested as a follow-up along with the test. Happy to split them if you'd rather review separately.
   


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