2010YOUY01 opened a new pull request, #15610:
URL: https://github.com/apache/datafusion/pull/15610
## Which issue does this PR close?
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- Closes #14692
## Rationale for this change
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### Background for memory-limited sort execution
See figures in
https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/sorts/sort.rs
### Current limitation for reading back spill files
Let's say the memory buffer can only hold 10 batches, but there are 900
files spilled, the current implementation will try to merge 900 files at once,
and fail the query.
However, this scenario is possible to continue if it can only merge 10 files
at a time, re-spill, until there are only less than 10 spill files in total,
and finally read them back and merge them as the output.
### High-level approach of this PR
Added one configuration option for max spill merge degree (haven't
implemented now, in POC it's a hard-coded `const MAX_SPILL_MERGE_DEGREE` for
simplicity)
At the final stage of external sort, there are initial spill files to merge,
perform multi-pass read-merge-respill operation, the number of merged spill
file in the next pass is decided by the closet power of `MAX_SPILL_MERGE_DEGREE`
Example:
Initial spill files to merge: 900
max merge degree: 10
pass1: merge 900 files to 100 (closet power of 10) files
pass2: 100 -> 10
pass3: 10 -> final single output
Inside each pass the number of files to merge in each step is split as even
as possible, while is always <= max merge degree, see details in the
implementation.
## What changes are included in this PR?
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Updated the `sort.rs` for multi-pass reading spill. The entry point for the
described logic is function `merge_spilled_files_multi_pass()`
## Are these changes tested?
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To be done, I think it's adding varying `max_spill_merge_degree` to the sort
fuzzer.
## Are there any user-facing changes?
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No
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