sollhui opened a new pull request, #69716:
URL: https://github.com/apache/doris/pull/69716

   ### What problem does this PR solve?
   
   Issue Number: None
   
   Related PR: #69333
   
   Problem Summary:
   
   After CSV ingestion switched to FileScannerV2, `DelimitedTextReader` opened 
remote files directly through `FileFactory`, bypassing the sequential prefetch 
used by the original CSV reader. S3 reads therefore block the scanner instead 
of overlapping remote I/O with CSV parsing.
   
   Restore `DelegateReader::create_file_reader(SEQUENTIAL)` in the shared 
CSV/Text reader, passing the shared IOContext and the adjusted split range. 
Unknown-size first splits use an end sentinel that the prefetch reader clamps 
to the physical file size. Keep tracing outside the prefetch layer and preserve 
the existing stream-load pipe path.
   
   ### Release note
   
   Restore asynchronous S3 prefetch for FileScannerV2 CSV and text reads.
   
   ### Check List (For Author)
   
   - Test
       - [x] Unit Test added: 
`CsvV2ReaderTest.S3PrefetchPreservesSplitRowsAndReadStatistics` covers 
whole-file, mid-row split and unknown-size reads; checks row values, IO 
statistics and background-thread S3 reads.
       - Local runtime execution is **blocked**, not passed: `./run-be-ut.sh 
--run --filter='CsvV2ReaderTest.*' -j 8` stops during JDK initialization with 
`Failed setting boot class path`. Reproduced outside the sandbox. Runtime 
validation is pending CI.
       - Passed repository clang-format 16 formatting/check scripts, 
`build-support/check-build-hygiene.sh`, and `git diff --check`.
       - No post-fix throughput or same-host benchmark result is claimed.
   
   - Behavior changed:
       - [x] Yes. Eligible S3 CSV/Text readers use the existing sequential 
prefetch implementation. No row parsing, schema or stream-pipe semantics change.
   
   - Does this need documentation?
       - [x] No.
   
   ### Check List (For Reviewer who merge this PR)
   
   - [ ] Confirm the release note
   - [ ] Confirm test cases
   - [ ] Confirm document
   - [ ] Add branch pick label
   


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