(datafusion) 01/01: chore: fix `datafusion-spark` substring (#21963)

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github-merge-queue[bot] pushed a commit to branch gh-readonly-queue/main/pr-21963-948cd09c10604561a0f755eebf8c25b27a79250f
in repository https://gitbox.apache.org/repos/asf/datafusion.git

commit ba038e99c861ac4cb034c7159167c3a91a8ea740
Author: Oleks V <co...@users.noreply.github.com>
AuthorDate: Fri May 1 11:28:40 2026 -0700

    chore: fix `datafusion-spark` substring (#21963)
    
    ## Which issue does this PR close?
    
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    - Closes #.
    
    ## Rationale for this change
    
    Fix negative cases with `substring`, some tests were incorrect
    
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---
 datafusion/spark/src/function/string/substring.rs             |  8 +++++---
 datafusion/sqllogictest/test_files/spark/string/substring.slt | 11 +++++++++--
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/datafusion/spark/src/function/string/substring.rs b/datafusion/spark/src/function/string/substring.rs
index ee6919dc0d..21b2242329 100644
--- a/datafusion/spark/src/function/string/substring.rs
+++ b/datafusion/spark/src/function/string/substring.rs
@@ -159,15 +159,17 @@ fn spark_substring(args: &[ArrayRef]) -> Result<ArrayRef> {
 /// - Zero start: treated as 1
 /// - Negative start: counts from end of string
 ///
-/// Returns the converted 1-based start position for use with `get_true_start_end`.
+/// The result may be `<= 0` when a negative start lands before the string
+/// (e.g. `start=-10` on a 3-char string gives `-6`). Such values are passed
+/// through to `get_true_start_end`, which clamps them and yields an empty
+/// slice — matching Spark's behavior for out-of-range negative positions.
 #[inline]
 fn spark_start_to_datafusion_start(start: i64, len: usize) -> i64 {
     if start >= 0 {
         start.max(1)
     } else {
         let len_i64 = i64::try_from(len).unwrap_or(i64::MAX);
-        let start = start.saturating_add(len_i64).saturating_add(1);
-        start.max(1)
+        start + len_i64 + 1
     }
 }
 
diff --git a/datafusion/sqllogictest/test_files/spark/string/substring.slt b/datafusion/sqllogictest/test_files/spark/string/substring.slt
index 5bf2fdf2fb..9189ce50c2 100644
--- a/datafusion/sqllogictest/test_files/spark/string/substring.slt
+++ b/datafusion/sqllogictest/test_files/spark/string/substring.slt
@@ -146,7 +146,7 @@ ark SQL
 (empty)
 (empty)
 (empty)
-Spa
+(empty)
 NULL
 NULL
 NULL
@@ -197,7 +197,14 @@ ark SQL
 (empty)
 (empty)
 (empty)
-Spa
+(empty)
 NULL
 NULL
 NULL
+
+query T
+SELECT substr(column1, -10, 3)
+FROM VALUES
+('abc'::string)
+----
+(empty)
\ No newline at end of file


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