zhuqi-lucas commented on code in PR #16630:
URL: https://github.com/apache/datafusion/pull/16630#discussion_r2179270018
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datafusion/physical-plan/src/sorts/cursor.rs:
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@@ -288,6 +288,64 @@ impl CursorArray for StringViewArray {
}
}
+/// Todo use arrow-rs side api after:
<https://github.com/apache/arrow-rs/pull/7748> released
+/// Builds a 128-bit composite key for an inline value:
+///
+/// - High 96 bits: the inline data in big-endian byte order (for correct
lexicographical sorting).
+/// - Low 32 bits: the length in big-endian byte order, acting as a
tiebreaker so shorter strings
+/// (or those with fewer meaningful bytes) always numerically sort before
longer ones.
+///
+/// This function extracts the length and the 12-byte inline string data from
the raw
+/// little-endian `u128` representation, converts them to big-endian ordering,
and packs them
+/// into a single `u128` value suitable for fast, branchless comparisons.
+///
+/// ### Why include length?
+///
+/// A pure 96-bit content comparison can’t distinguish between two values
whose inline bytes
+/// compare equal—either because one is a true prefix of the other or because
zero-padding
+/// hides extra bytes. By tucking the 32-bit length into the lower bits, a
single `u128` compare
+/// handles both content and length in one go.
+///
+/// Example: comparing "bar" (3 bytes) vs "bar\0" (4 bytes)
+///
+/// | String | Bytes 0–4 (length LE) | Bytes 4–16 (data + padding) |
+/// |------------|-----------------------|---------------------------------|
+/// | `"bar"` | `03 00 00 00` | `62 61 72` + 9 × `00` |
+/// | `"bar\0"`| `04 00 00 00` | `62 61 72 00` + 8 × `00` |
+///
+/// Both inline parts become `62 61 72 00…00`, so they tie on content. The
length field
+/// then differentiates:
+///
+/// ```text
+/// key("bar") = 0x0000000000000000000062617200000003
+/// key("bar\0") = 0x0000000000000000000062617200000004
+/// ⇒ key("bar") < key("bar\0")
+/// ```
+#[inline(always)]
+pub fn inline_key_fast(raw: u128) -> u128 {
Review Comment:
Thank you @alamb for good point, addressed in latest PR!
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