MDEV-27206: [ERROR] Duplicated key: cause, Assertion `is_uniq_key' failed with optimizer trace

mysql-test/main/explain_json.result

@@ -2000,3 +2000,16 @@ EXPLAIN
   }
 }
 DROP TABLE t1;
+#
+# MDEV-27206: [ERROR] Duplicated key: cause, Assertion `is_uniq_key' failed with optimizer trace
+#
+CREATE TABLE t1 (a INT) ENGINE=MyISAM;
+CREATE TABLE t2 (pk TIME, b INT, primary key (pk), key (b)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES
+('00:13:33',0),('00:13:34',1),('00:13:35',2),('00:13:36',3),
+('00:13:37',4),('00:13:38',5),('00:13:39',6),('00:13:40',7),
+('00:13:41',8),('00:13:42',9);
+SET optimizer_trace = 'enabled=on';
+SELECT * FROM t1 WHERE a IN ( SELECT b FROM t2 INNER JOIN t1 ON (a = pk) );
+a
+DROP TABLE t1, t2;

mysql-test/main/explain_json.test

@@ -429,3 +429,17 @@ explain FORMAT=JSON
 SELECT * FROM t1 t0 
 WHERE t0.a IN (SELECT t2.a FROM t1 t2 WHERE t0.a IN (SELECT t3.a FROM t1 t3));
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-27206: [ERROR] Duplicated key: cause, Assertion `is_uniq_key' failed with optimizer trace
+--echo #
+
+CREATE TABLE t1 (a INT) ENGINE=MyISAM;
+CREATE TABLE t2 (pk TIME, b INT, primary key (pk), key (b)) ENGINE=MyISAM;
+INSERT INTO t2 VALUES
+  ('00:13:33',0),('00:13:34',1),('00:13:35',2),('00:13:36',3),
+  ('00:13:37',4),('00:13:38',5),('00:13:39',6),('00:13:40',7),
+  ('00:13:41',8),('00:13:42',9);
+SET optimizer_trace = 'enabled=on';
+SELECT * FROM t1 WHERE a IN ( SELECT b FROM t2 INNER JOIN t1 ON (a = pk) );
+DROP TABLE t1, t2;
\ No newline at end of file

sql/sql_select.cc

@@ -7891,7 +7891,7 @@ best_access_path(JOIN      *join,
                 /* quick_range couldn't use key! */
                 records= (double) s->records/rec;
                 trace_access_idx.add("used_range_estimates", false)
-                                .add("cause", "not available");
+                                .add("reason", "not available");
               }
             }
             else