ChunkedArray with empty chunks can be constant (#2628)

Author: robert3005

vortex-array/src/arrays/chunked/compute/is_constant.rs

@@ -1,5 +1,6 @@
 use vortex_error::{VortexExpect, VortexResult};
 
+use crate::Array;
 use crate::arrays::{ChunkedArray, ChunkedEncoding};
 use crate::compute::{IsConstantFn, IsConstantOpts, is_constant_opts, scalar_at};
 
@@ -9,7 +10,7 @@ impl IsConstantFn<&ChunkedArray> for ChunkedEncoding {
         array: &ChunkedArray,
         opts: &IsConstantOpts,
     ) -> VortexResult<Option<bool>> {
-        let mut chunks = array.chunks().iter();
+        let mut chunks = array.chunks().iter().skip_while(|c| c.is_empty());
 
         let first_chunk = chunks.next().vortex_expect("Must have at least one value");
 
@@ -20,6 +21,10 @@ impl IsConstantFn<&ChunkedArray> for ChunkedEncoding {
         let first_value = scalar_at(first_chunk, 0)?.into_nullable();
 
         for chunk in chunks {
+            if chunk.is_empty() {
+                continue;
+            }
+
             if !is_constant_opts(chunk, opts)? {
                 return Ok(Some(false));
             }
@@ -32,3 +37,30 @@ impl IsConstantFn<&ChunkedArray> for ChunkedEncoding {
         Ok(Some(true))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use vortex_buffer::{Buffer, buffer};
+    use vortex_dtype::{DType, Nullability, PType};
+
+    use crate::arrays::ChunkedArray;
+    use crate::{Array, IntoArray};
+
+    #[test]
+    fn empty_chunk_is_constant() {
+        let chunked = ChunkedArray::try_new(
+            vec![
+                Buffer::<u8>::empty().into_array(),
+                Buffer::<u8>::empty().into_array(),
+                buffer![255u8, 255].into_array(),
+                Buffer::<u8>::empty().into_array(),
+                buffer![255u8, 255].into_array(),
+            ],
+            DType::Primitive(PType::U8, Nullability::NonNullable),
+        )
+        .unwrap()
+        .into_array();
+
+        assert!(chunked.statistics().compute_is_constant().unwrap());
+    }
+}