fix: Turn off cse if cache node found (#15554)

crates/polars-plan/src/logical_plan/optimizer/mod.rs

@@ -113,21 +113,24 @@ pub fn optimize(
     }
 
     #[cfg(feature = "cse")]
-    let _cse_plan_changed =
-        if comm_subplan_elim && members.has_joins_or_unions && members.has_duplicate_scans() {
-            if verbose {
-                eprintln!("found multiple sources; run comm_subplan_elim")
-            }
-            let (lp, changed, cid2c) = cse::elim_cmn_subplans(lp_top, lp_arena, expr_arena);
-
-            prune_unused_caches(lp_arena, cid2c);
-
-            lp_top = lp;
-            members.has_cache |= changed;
-            changed
-        } else {
-            false
-        };
+    let _cse_plan_changed = if comm_subplan_elim
+        && members.has_joins_or_unions
+        && members.has_duplicate_scans()
+        && !members.has_cache
+    {
+        if verbose {
+            eprintln!("found multiple sources; run comm_subplan_elim")
+        }
+        let (lp, changed, cid2c) = cse::elim_cmn_subplans(lp_top, lp_arena, expr_arena);
+
+        prune_unused_caches(lp_arena, cid2c);
+
+        lp_top = lp;
+        members.has_cache |= changed;
+        changed
+    } else {
+        false
+    };
     #[cfg(not(feature = "cse"))]
     let _cse_plan_changed = false;
 

py-polars/polars/lazyframe/frame.py

@@ -2371,7 +2371,11 @@ def lazy(self) -> Self:
         return self
 
     def cache(self) -> Self:
-        """Cache the result once the execution of the physical plan hits this node."""
+        """
+        Cache the result once the execution of the physical plan hits this node.
+
+        It is not recommended using this as the optimizer likely can do a better job.
+        """
         return self._from_pyldf(self._ldf.cache())
 
     def cast(

py-polars/tests/unit/test_cse.py

@@ -658,3 +658,12 @@ def test_cse_15536() -> None:
             data.filter(pl.lit(True) & (pl.col("a") == 7) | (pl.col("a") == 8)),
         ]
     ).collect()["a"].to_list() == [6, 9, 7, 8]
+
+
+def test_cse_15548() -> None:
+    ldf = pl.LazyFrame({"a": [1, 2, 3]})
+    ldf2 = ldf.filter(pl.col("a") == 1).cache()
+    ldf3 = pl.concat([ldf, ldf2])
+
+    assert len(ldf3.collect(comm_subplan_elim=False)) == 4
+    assert len(ldf3.collect(comm_subplan_elim=True)) == 4