[GitHub] [arrow-datafusion] xudong963 commented on a change in pull request #1687: Make `SortPreservingMergeStream` stable on input stream order

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xudong963 commented on a change in pull request #1687:
URL: https://github.com/apache/arrow-datafusion/pull/1687#discussion_r794425301



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File path: datafusion/src/physical_plan/sorts/sort_preserving_merge.rs
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@@ -58,6 +58,27 @@ use crate::physical_plan::{
 /// provided each partition of the input plan is sorted with respect to
 /// these sort expressions, this operator will yield a single partition
 /// that is also sorted with respect to them
+///
+/// ```text
+/// ┌─────────────────────────┐
+/// │ ┌───┬───┬───┬───┐       │
+/// │ │ A │ B │ C │ D │ ...   │──┐
+/// │ └───┴───┴───┴───┘       │  │
+/// └─────────────────────────┘  │  ┌───────────────────┐    ┌───────────────────────────────┐
+///   Stream 1                   │  │                   │    │ ┌───┬───╦═══╦───┬───╦═══╗     │
+///                              ├─▶│SortPreservingMerge│───▶│ │ A │ B ║ B ║ C │ D ║ E ║ ... │
+///                              │  │                   │    │ └───┴─▲─╩═══╩───┴───╩═══╝     │
+/// ┌─────────────────────────┐  │  └───────────────────┘    └─┬─────┴───────────────────────┘
+/// │ ╔═══╦═══╗               │  │
+/// │ ║ B ║ E ║     ...       │──┘                             │
+/// │ ╚═══╩═══╝               │              Note Stable Sort: the merged stream
+/// └─────────────────────────┘                places equal rows from stream 1

Review comment:
       @liukun4515 It also surprised me in a previous issue 😂




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