Make SortPreservingMergeStream stable on input stream order
#1687
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alamb
commented
Jan 26, 2022
| Ok(Ordering::Equal) | ||
| // Break ties using stream_idx to ensure a predictable | ||
| // ordering of rows when comparing equal streams. | ||
| Ok(self.stream_idx.cmp(&other.stream_idx)) |
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This is the only code change. The rest is comments and tests
| // Expect the data to be sorted first by "batch_number" (because | ||
| // that was the order it was fed in, even though only "value" | ||
| // is in the sort key) | ||
| assert_batches_eq!( |
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Example test output before this fix:
[
"+--------------+-------+",
"| batch_number | value |",
"+--------------+-------+",
"| 0 | A |",
"| 2 | A |",
"| 6 | A |",
"| 9 | A |",
"| 5 | A |",
"| 1 | A |",
"| 4 | A |",
"| 3 | A |",
"| 8 | A |",
"| 7 | A |",
"| 0 | B |",
"| 6 | B |",
"| 8 | B |",
"| 7 | B |",
"| 4 | B |",
"| 2 | B |",
"| 1 | B |",
"| 3 | B |",
"| 9 | B |",
"| 5 | B |",
"+--------------+-------+",
]
yjshen
approved these changes
Jan 27, 2022
| /// | ||
| /// Input Streams Output stream | ||
| /// (sorted) (sorted) | ||
| /// ``` |
houqp
approved these changes
Jan 27, 2022
liukun4515
reviewed
Jan 28, 2022
| /// │ ╔═══╦═══╗ │ │ | ||
| /// │ ║ B ║ E ║ ... │──┘ │ | ||
| /// │ ╚═══╩═══╝ │ Note Stable Sort: the merged stream | ||
| /// └─────────────────────────┘ places equal rows from stream 1 |
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wow!!
Can I know what tool you used to create this diagram?
@alamb
It's clearly for every review.
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😄 -- I used https://monodraw.helftone.com/ which some of the other engineers at InfluxData use -- it is quite good though not free :(
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@liukun4515 It also surprised me in a previous issue 😂
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Which issue does this PR close?
Closes #1686
Rationale for this change
I would like to have a predicatable output order for rows that have equal sorting keys
What changes are included in this PR?
SortKeyCursorto usestream_indexif it is otherwise equalAre there any user-facing changes?
(more) sorting output (?)