From 7aa6edba9f5a345eb3fdbdadd907c7df1397ef5b Mon Sep 17 00:00:00 2001
From: Simon Lin <simonlin.rqmmw@slmail.me>
Date: Tue, 28 May 2024 18:56:34 +1000
Subject: [PATCH] fix: Crash selecting columns after non-coalesced join

---
 .../optimizer/projection_pushdown/joins.rs    |  3 +--
 py-polars/tests/unit/test_projections.py      | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/joins.rs b/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/joins.rs
index 0a663e9e9195..a30b674f8e39 100644
--- a/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/joins.rs
+++ b/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/joins.rs
@@ -258,8 +258,7 @@ pub(super) fn process_join(
             already_added_local_to_local_projected.insert(local_name);
         }
         // In full outer joins both columns remain. So `add_local=true` also for the right table
-        let add_local = matches!(options.args.how, JoinType::Full)
-            && !options.args.coalesce.coalesce(&options.args.how);
+        let add_local = !options.args.coalesce.coalesce(&options.args.how);
         for e in &right_on {
             // In case of full outer joins we also add the columns.
             // But before we do that we must check if the column wasn't already added by the lhs.
diff --git a/py-polars/tests/unit/test_projections.py b/py-polars/tests/unit/test_projections.py
index 976e05fb3e51..02b029a46a98 100644
--- a/py-polars/tests/unit/test_projections.py
+++ b/py-polars/tests/unit/test_projections.py
@@ -1,4 +1,7 @@
+from typing import Literal
+
 import numpy as np
+import pytest
 
 import polars as pl
 from polars.testing import assert_frame_equal
@@ -434,3 +437,19 @@ def test_double_projection_pushdown_15895() -> None:
         "C": [[0]],
         "A": [[1]],
     }
+
+
+@pytest.mark.parametrize("join_type", ["inner", "left", "full"])
+def test_non_coalesce_join_projection_pushdown_16515(
+    join_type: Literal["inner", "left", "full"],
+) -> None:
+    left = pl.LazyFrame({"x": 1})
+    right = pl.LazyFrame({"y": 1})
+
+    assert (
+        left.join(right, how=join_type, left_on="x", right_on="y", coalesce=False)
+        .select("y")
+        .collect()
+        .item()
+        == 1
+    )