Add a unit tests.

Signed-off-by: Johannes Kalmbach <[email protected]>

test/EngineTest.cpp

@@ -278,6 +278,18 @@ TEST(OptionalJoin, specialOptionalJoinTwoColumns) {
     IdTable expectedResult = makeIdTableFromVector(
         {{4, 1, 2, U}, {2, 1, 3, 3}, {1, 1, 4, U}, {2, 2, 2, 4}, {1, 3, 1, 1}});
 
+    testOptionalJoin(a, b, jcls, expectedResult);
+  }
+  {
+    // Test a corner case that previously contained a bug.
+    IdTable a{makeIdTableFromVector({{4, U, 2}})};
+    IdTable b{makeIdTableFromVector({{3, 3, 1}})};
+    // Join a and b on the column pairs 1,2 and 2,1 (entries from columns 1 of
+    // a have to equal those of column 2 of b and vice versa).
+    JoinColumns jcls{{1, 2}, {2, 1}};
+
+    IdTable expectedResult = makeIdTableFromVector({{4, U, 2, U}});
+
     testOptionalJoin(a, b, jcls, expectedResult);
   }
 }