MDEV-28881 Server crash in Dep_analysis_context::create_table_value

SELECT_LEX::first_select()->join is NULL for degenerate derived tables
which are known to have just one row and so were already materialized
by the optimizer.
This commit adds a check for this.

mysql-test/main/table_elim.result

@@ -985,3 +985,21 @@ drop table t1, t11, t12, t13, t2;
 #
 # End of MDEV-26278: Table elimination does not work across derived tables
 #
+#
+# MDEV-28881: Server crashes in Dep_analysis_context::create_table_value/
+#                                check_func_dependency
+#
+CREATE TABLE t1 (a1 int, a2 int);
+INSERT INTO t1 VALUES (0,276),(5,277),(NULL,278);
+CREATE TABLE t2 ( a1 int, a2 int, KEY a2 (a2));
+INSERT INTO t2 VALUES (11,NULL),(185,0);
+SELECT t1.* FROM t1 LEFT JOIN
+( SELECT * FROM (SELECT t2.a1 AS a1, min(t2.a2) AS a2 FROM t2
+WHERE t2.a2 <> NULL
+GROUP BY t2.a1) dt
+) dt2 ON dt2.a2 = t1.a2;
+a1	a2
+0	276
+5	277
+NULL	278
+DROP TABLE t1, t2;

mysql-test/main/table_elim.test

@@ -759,3 +759,22 @@ drop table t1, t11, t12, t13, t2;
 --echo #
 --echo # End of MDEV-26278: Table elimination does not work across derived tables
 --echo #
+
+--echo #
+--echo # MDEV-28881: Server crashes in Dep_analysis_context::create_table_value/
+--echo #                                check_func_dependency
+--echo #
+
+CREATE TABLE t1 (a1 int, a2 int);
+INSERT INTO t1 VALUES (0,276),(5,277),(NULL,278);
+
+CREATE TABLE t2 ( a1 int, a2 int, KEY a2 (a2));
+INSERT INTO t2 VALUES (11,NULL),(185,0);
+
+SELECT t1.* FROM t1 LEFT JOIN
+ ( SELECT * FROM (SELECT t2.a1 AS a1, min(t2.a2) AS a2 FROM t2
+    WHERE t2.a2 <> NULL
+    GROUP BY t2.a1) dt
+ ) dt2 ON dt2.a2 = t1.a2;
+
+DROP TABLE t1, t2;
\ No newline at end of file

sql/opt_table_elimination.cc

@@ -1720,9 +1720,14 @@ void Dep_analysis_context::create_unique_pseudo_key_if_needed(
 
   /*
     GROUP BY expression is considered as a unique pseudo-key
-    for the derived table. Add this pseudo key as a dependency
+    for the derived table. Add this pseudo key as a dependency.
+
+    first_select->join is NULL for degenerate derived tables
+    which are known to have just one row and so were already materialized
+    by the optimizer, check this here
   */
-  if (first_select && first_select->group_list.elements > 0)
+  if (first_select && first_select->join &&
+      first_select->group_list.elements > 0)
   {
     bool valid= true;
     std::set<field_index_t> exposed_fields_indexes;