fix: Only rewrite numeric ineq joins (#19083)

crates/polars-plan/src/plans/conversion/join.rs

@@ -316,8 +316,25 @@ fn resolve_join_where(
         )?;
 
         if let Some(ie_op_) = to_inequality_operator(&op) {
-            // We already have an IEjoin or an Inner join, push to remaining
-            if ie_op.len() >= 2 || !eq_right_on.is_empty() {
+            fn is_numeric(e: &Expr, schema: &Schema) -> bool {
+                expr_to_leaf_column_names_iter(e).any(|name| {
+                    if let Some(dt) = schema.get(name.as_str()) {
+                        dt.to_physical().is_numeric()
+                    } else {
+                        false
+                    }
+                })
+            }
+
+            // We fallback to remaining if:
+            // - we already have an IEjoin or Inner join
+            // - we already have an Inner join
+            // - data is not numeric (our iejoin doesn't yet implement that)
+            if ie_op.len() >= 2
+                || !eq_right_on.is_empty()
+                || !is_numeric(&left, &schema_left)
+                || !is_numeric(&right, &schema_right)
+            {
                 remaining_preds.push(to_binary_post_join(left, op, right, &schema_right, &suffix))
             } else {
                 ie_left_on.push(left);

crates/polars-plan/src/plans/optimizer/collapse_joins.rs

@@ -5,7 +5,7 @@
 
 use std::sync::Arc;
 
-use polars_core::schema::SchemaRef;
+use polars_core::schema::*;
 #[cfg(feature = "iejoin")]
 use polars_ops::frame::{IEJoinOptions, InequalityOperator};
 use polars_ops::frame::{JoinCoalesce, JoinType};
@@ -304,8 +304,28 @@ pub fn optimize(root: Node, lp_arena: &mut Arena<IR>, expr_arena: &mut Arena<AEx
                         } else {
                             #[cfg(feature = "iejoin")]
                             if let Some(ie_op_) = to_inequality_operator(&op) {
-                                // We already have an IEjoin or an Inner join, push to remaining
-                                if ie_op.len() >= 2 || !eq_left_on.is_empty() {
+                                fn is_numeric(
+                                    node: Node,
+                                    expr_arena: &Arena<AExpr>,
+                                    schema: &Schema,
+                                ) -> bool {
+                                    aexpr_to_leaf_names_iter(node, expr_arena).any(|name| {
+                                        if let Some(dt) = schema.get(name.as_str()) {
+                                            dt.to_physical().is_numeric()
+                                        } else {
+                                            false
+                                        }
+                                    })
+                                }
+
+                                // We fallback to remaining if:
+                                // - we already have an IEjoin or Inner join
+                                // - we already have an Inner join
+                                // - data is not numeric (our iejoin doesn't yet implement that)
+                                if ie_op.len() >= 2
+                                    || !eq_left_on.is_empty()
+                                    || !is_numeric(left, expr_arena, left_schema)
+                                {
                                     remaining_predicates.push(node);
                                 } else {
                                     ie_left_on.push(ExprIR::from_node(left, expr_arena));

py-polars/tests/unit/operations/test_inequality_join.py

@@ -575,3 +575,22 @@ def test_raise_invalid_predicate() -> None:
 
     with pytest.raises(pl.exceptions.InvalidOperationError):
         left.join_where(right, pl.col.index >= pl.col.a).collect()
+
+
+def test_join_on_strings() -> None:
+    df = pl.LazyFrame(
+        {
+            "a": ["a", "b", "c"],
+            "b": ["b", "b", "b"],
+        }
+    )
+
+    q = df.join_where(df, pl.col("a").ge(pl.col("a_right")))
+
+    assert "CROSS JOIN" in q.explain()
+    assert q.collect().to_dict(as_series=False) == {
+        "a": ["a", "b", "b", "c", "c", "c"],
+        "b": ["b", "b", "b", "b", "b", "b"],
+        "a_right": ["a", "a", "b", "a", "b", "c"],
+        "b_right": ["b", "b", "b", "b", "b", "b"],
+    }