diff --git a/mysql-test/main/merge.result b/mysql-test/main/merge.result
index 0e9a2cb9896ab..f98c0f4d28472 100644
--- a/mysql-test/main/merge.result
+++ b/mysql-test/main/merge.result
@@ -3970,3 +3970,96 @@ DROP TABLE t1;
 #
 # End of 10.11 tests
 #
+#
+# MDEV-30088 Assertion `cond_selectivity <= 1.0' failed in get_range_limit_read_cost
+#
+CREATE TABLE t1 (a TIMESTAMP, KEY(a)) ENGINE=MRG_MyISAM;
+explain SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+a	COUNT(*)
+DROP TABLE t1;
+#
+# MDEV-30525: Assertion `ranges > 0' fails in IO_AND_CPU_COST handler::keyread_time
+#
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE t2 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE tm (a INT, KEY(a)) ENGINE=MRG_MyISAM UNION=(t1,t2);
+SELECT DISTINCT a FROM tm WHERE a > 50;
+a
+DROP TABLE tm, t1, t2;
+# Testcase 2:
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE t2 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE tm (a INT, KEY(a)) ENGINE=MERGE UNION = (t1, t2) INSERT_METHOD=FIRST;
+ANALYZE TABLE tm PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.tm	analyze	status	Engine-independent statistics collected
+test.tm	analyze	note	The storage engine for the table doesn't support analyze
+SELECT DISTINCT a FROM (SELECT * FROM tm WHERE a iS NOT NULL) AS sq;
+a
+DROP TABLE tm, t1, t2;
+#
+# MDEV-30568 Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost
+#
+CREATE TABLE t1 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE t2 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE tm (f INT, KEY(f)) ENGINE=MERGE UNION = (t1, t2);
+SELECT DISTINCT f FROM tm WHERE f IN (47, 126, 97, 48, 73, 0);
+f
+DROP TABLE tm, t1, t2;
+#
+# MDEV-30786 SIGFPE in cost_group_min_max on EXP
+#
+SET use_stat_tables='preferably';
+CREATE TABLE t1 (a INT,b INT,KEY i1 (a),KEY i2 (b)) ENGINE=MRG_MyISAM;
+ANALYZE LOCAL TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	note	The storage engine for the table doesn't support analyze
+EXPLAIN SELECT DISTINCT a FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	i1	5	NULL	1	Using index for group-by
+drop table t1;
+set use_stat_tables=default;
+#
+# End of 11.0 tests
+#
+#
+# MDEV-29174: UPDATE of view that uses MERGE table
+#
+CREATE TABLE t1 (a int) ENGINE=MERGE;
+CREATE VIEW v1 AS SELECT a FROM t1;
+UPDATE v1 SET a=0;
+DROP VIEW v1;
+DROP TABLE t1;
+# End of 11.1 tests
+#
+# MDEV-38474 Double free or corruption, ASAN heap-use-after-free in st_join_table::cleanup
+#
+# Test case 1, fails on 10.11+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+# Inserts are optional, fails with and without data
+INSERT INTO t1 VALUES (1),(2);
+INSERT INTO t2 VALUES (3),(4);
+INSERT INTO t3 VALUES (5),(6);
+EXPLAIN SELECT * FROM t1 WHERE a IN (SELECT b FROM t2 WHERE a IN ((SELECT c FROM t3 WHERE FALSE HAVING c < 0)));
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	MATERIALIZED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+DROP TABLE t1, t2, t3;
+# Test case 2, fails on 11.4 but not on 10.11
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+CREATE TABLE t4 (d INT PRIMARY KEY);
+SET SQL_SAFE_UPDATES=1;
+UPDATE t1 STRAIGHT_JOIN t2 SET a = 89 WHERE 9 IN (SELECT c FROM t3 WHERE c IN (SELECT MAX(d) FROM t4));
+ERROR HY000: You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
+DROP TABLE t1, t2, t3, t4;
+#
+# End of 11.4 tests
+#
diff --git a/mysql-test/main/merge.test b/mysql-test/main/merge.test
index e66b3ea1dc44e..16a5a6853a4c7 100644
--- a/mysql-test/main/merge.test
+++ b/mysql-test/main/merge.test
@@ -2905,3 +2905,98 @@ DROP TABLE t1;
 --echo #
 --echo # End of 10.11 tests
 --echo #
+
+--echo #
+--echo # MDEV-30088 Assertion `cond_selectivity <= 1.0' failed in get_range_limit_read_cost
+--echo #
+
+CREATE TABLE t1 (a TIMESTAMP, KEY(a)) ENGINE=MRG_MyISAM;
+explain SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+DROP TABLE t1;
+
+--echo #
+--echo # MDEV-30525: Assertion `ranges > 0' fails in IO_AND_CPU_COST handler::keyread_time
+--echo #
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE t2 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE tm (a INT, KEY(a)) ENGINE=MRG_MyISAM UNION=(t1,t2);
+SELECT DISTINCT a FROM tm WHERE a > 50;
+DROP TABLE tm, t1, t2;
+
+--echo # Testcase 2:
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE t2 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE tm (a INT, KEY(a)) ENGINE=MERGE UNION = (t1, t2) INSERT_METHOD=FIRST;
+ANALYZE TABLE tm PERSISTENT FOR ALL;
+SELECT DISTINCT a FROM (SELECT * FROM tm WHERE a iS NOT NULL) AS sq;
+DROP TABLE tm, t1, t2;
+
+--echo #
+--echo # MDEV-30568 Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost
+--echo #
+CREATE TABLE t1 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE t2 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE tm (f INT, KEY(f)) ENGINE=MERGE UNION = (t1, t2);
+SELECT DISTINCT f FROM tm WHERE f IN (47, 126, 97, 48, 73, 0);
+DROP TABLE tm, t1, t2;
+
+--echo #
+--echo # MDEV-30786 SIGFPE in cost_group_min_max on EXP
+--echo #
+SET use_stat_tables='preferably';
+CREATE TABLE t1 (a INT,b INT,KEY i1 (a),KEY i2 (b)) ENGINE=MRG_MyISAM;
+ANALYZE LOCAL TABLE t1;
+EXPLAIN SELECT DISTINCT a FROM t1;
+drop table t1;
+set use_stat_tables=default;
+
+--echo #
+--echo # End of 11.0 tests
+--echo #
+--echo #
+--echo # MDEV-29174: UPDATE of view that uses MERGE table
+--echo #
+
+CREATE TABLE t1 (a int) ENGINE=MERGE;
+CREATE VIEW v1 AS SELECT a FROM t1;
+UPDATE v1 SET a=0;
+DROP VIEW v1;
+DROP TABLE t1;
+
+--echo # End of 11.1 tests
+
+--echo #
+--echo # MDEV-38474 Double free or corruption, ASAN heap-use-after-free in st_join_table::cleanup
+--echo #
+
+--echo # Test case 1, fails on 10.11+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+
+--echo # Inserts are optional, fails with and without data
+INSERT INTO t1 VALUES (1),(2);
+INSERT INTO t2 VALUES (3),(4);
+INSERT INTO t3 VALUES (5),(6);
+
+EXPLAIN SELECT * FROM t1 WHERE a IN (SELECT b FROM t2 WHERE a IN ((SELECT c FROM t3 WHERE FALSE HAVING c < 0)));
+
+DROP TABLE t1, t2, t3;
+
+--echo # Test case 2, fails on 11.4 but not on 10.11
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+CREATE TABLE t4 (d INT PRIMARY KEY);
+
+SET SQL_SAFE_UPDATES=1;
+--error ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE
+UPDATE t1 STRAIGHT_JOIN t2 SET a = 89 WHERE 9 IN (SELECT c FROM t3 WHERE c IN (SELECT MAX(d) FROM t4));
+
+DROP TABLE t1, t2, t3, t4;
+
+--echo #
+--echo # End of 11.4 tests
+--echo #
+
diff --git a/sql/sql_lex.cc b/sql/sql_lex.cc
index 43615249ecb90..670c719555ae0 100644
--- a/sql/sql_lex.cc
+++ b/sql/sql_lex.cc
@@ -2975,7 +2975,7 @@ void st_select_lex_node::init_query_common()
   into the front of the stranded_clean_list:
     before: root -> B -> A
      after: root -> this -> B -> A
-  During cleanup, the stranded units are cleaned in FIFO order.
+  During cleanup, the stranded units are cleaned in LIFO order (parent-first).
  */
 void st_select_lex_unit::remember_my_cleanup()
 {
@@ -2994,11 +2994,16 @@ void st_select_lex_unit::remember_my_cleanup()
 
 void st_select_lex_unit::cleanup_stranded_units()
 {
-  if (!stranded_clean_list)
-    return;
-
-  stranded_clean_list->cleanup();
+  st_select_lex_unit *cur= stranded_clean_list;
   stranded_clean_list= nullptr;
+
+  while (cur)
+  {
+    st_select_lex_unit *next= cur->stranded_clean_list;
+    cur->stranded_clean_list= nullptr;
+    cur->cleanup();
+    cur= next;
+  }
 }
 
 
diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index dda95677332c2..0d87c830adf38 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -2597,8 +2597,6 @@ bool st_select_lex_unit::exec_recursive()
 
 bool st_select_lex_unit::cleanup()
 {
-  cleanup_stranded_units();
-
   bool error= 0;
   DBUG_ENTER("st_select_lex_unit::cleanup");
 
@@ -2684,6 +2682,12 @@ bool st_select_lex_unit::cleanup()
     }
   }
 
+  /*
+    Cleanup stranded units only after this unit has completed its own
+    cleanup, ensuring a parent-first (LIFO) cleanup order for merged tables.
+  */
+  cleanup_stranded_units();
+
   DBUG_RETURN(error);
 }