fix: return 0.0 compression ratio to prevent automatic selection

VarBinScheme is a format conversion utility, not a compression scheme.
VarBinView is generally more efficient than VarBin for most workloads,
especially with small strings that can be inlined (≤12 bytes).

Return 0.0 from expected_compression_ratio to indicate this scheme should
not be automatically selected by the compressor. It can still be used
explicitly when VarBin format is specifically needed.

This fixes test failures where VarBin was being selected and causing
incompatibilities with Arrow operations that expect consistent types.

Signed-off-by: Joe Isaacs <[email protected]>
Co-Authored-By: Claude <[email protected]>

Author: joseph-isaacs

vortex-btrblocks/src/string.rs

@@ -184,31 +184,16 @@ impl Scheme for VarBinScheme {
 
     fn expected_compression_ratio(
         &self,
-        stats: &Self::StatsType,
+        _stats: &Self::StatsType,
         _is_sample: bool,
         _allowed_cascading: usize,
         _excludes: &[StringCode],
     ) -> VortexResult<f64> {
-        if stats.src.is_empty() {
-            return Ok(1.0);
-        }
-
-        let src = stats.source();
-
-        // Calculate VarBinView size using nbytes()
-        let varbinview_size = src.as_ref().nbytes();
-
-        let string_bytes = src.buffers().iter().map(|b| b.len() as u64).sum::<u64>();
-
-        // Determine offset type size based on total string bytes
-        // Arrow/Vortex uses i32 offsets if total size < u32::MAX, otherwise i64
-        let offset_type_size = if string_bytes < u32::MAX as u64 { 4 } else { 8 };
-        let offset_bytes = (src.len() as u64 + 1) * offset_type_size;
-
-        let varbin_size = string_bytes + offset_bytes;
-        assert!(varbin_size > 0, "cannot be empty");
-
-        Ok(varbinview_size as f64 / varbin_size as f64)
+        // VarBinScheme doesn't provide compression - it's a format conversion.
+        // VarBinView is generally more efficient than VarBin for most workloads
+        // (especially with small strings that can be inlined).
+        // Return 0.0 to indicate this scheme should not be selected by the compressor.
+        Ok(0.0)
     }
 
     fn compress(