don't overwrite skip_rows in slice_pushdown opotimization (#2556)

polars/polars-lazy/src/frame/mod.rs

@@ -938,7 +938,7 @@ impl LazyFrame {
         self.slice(-1, 1)
     }
 
-    /// Get the n last rows
+    /// Get the last `n` rows
     pub fn tail(self, n: u32) -> LazyFrame {
         let neg_tail = -(n as i64);
         self.slice(neg_tail, n)

polars/polars-lazy/src/logical_plan/optimizer/slice_pushdown.rs

@@ -155,7 +155,7 @@ impl SlicePushDown {
                 aggregate,
             }, Some(state)) if state.offset >= 0 && predicate.is_none() => {
                 let mut options = options;
-                options.skip_rows = state.offset as usize;
+                options.skip_rows += state.offset as usize;
                 options.n_rows = Some(state.len as usize);
 
                 let lp = CsvScan {

polars/polars-lazy/src/tests/io.rs

@@ -316,3 +316,15 @@ fn test_slice_filter() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn skip_rows_and_slice() -> Result<()> {
+    let out = LazyCsvReader::new(FOODS_CSV.to_string())
+        .with_skip_rows(4)
+        .finish()?
+        .limit(1)
+        .collect()?;
+    assert_eq!(out.column("fruit")?.get(0), AnyValue::Utf8("seafood"));
+    assert_eq!(out.shape(), (1, 4));
+    Ok(())
+}

polars/polars-lazy/src/tests/projection_queries.rs

@@ -35,6 +35,7 @@ fn test_join_suffix_and_drop() -> Result<()> {
 }
 
 #[test]
+#[cfg(feature = "cross_join")]
 fn test_cross_join_pd() -> Result<()> {
     let food = df![
         "name"=> ["Omelette", "Fried Egg"],