MDEV-30007 SIGSEGV in st_select_lex_unit::is_derived_eliminated, runtime...

MDEV-30007 SIGSEGV in st_select_lex_unit::is_derived_eliminated, runtime error: member access within null pointer of type 'struct TABLE' in st_select_lex_unit::is_derived_eliminated()

Take into account case of a degenerate subquery (like SELECT 1).

mysql-test/main/table_elim.result

@@ -1003,3 +1003,23 @@ a1	a2
 5	277
 NULL	278
 DROP TABLE t1, t2;
+#
+# MDEV-30007: SIGSEGV in st_select_lex_unit::is_derived_eliminated,
+# runtime error: member access within null pointer of type
+# 'struct TABLE' in st_select_lex_unit::is_derived_eliminated()
+#
+CREATE VIEW v AS SELECT 1 AS a;
+SELECT ROUND ((SELECT 1 FROM v)) FROM v GROUP BY ROUND ((SELECT 1 FROM v));
+ROUND ((SELECT 1 FROM v))
+1
+EXPLAIN
+SELECT ROUND ((SELECT 1 FROM v)) FROM v GROUP BY ROUND ((SELECT 1 FROM v));
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived4>	system	NULL	NULL	NULL	NULL	1	
+4	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+2	SUBQUERY	<derived5>	system	NULL	NULL	NULL	NULL	1	
+5	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+DROP VIEW v;
+#
+# End of 10.10 tests
+#

mysql-test/main/table_elim.test

@@ -784,3 +784,18 @@ SELECT t1.* FROM t1 LEFT JOIN
  ) dt2 ON dt2.a2 = t1.a2;
 
 DROP TABLE t1, t2;
+
+--echo #
+--echo # MDEV-30007: SIGSEGV in st_select_lex_unit::is_derived_eliminated,
+--echo # runtime error: member access within null pointer of type
+--echo # 'struct TABLE' in st_select_lex_unit::is_derived_eliminated()
+--echo #
+CREATE VIEW v AS SELECT 1 AS a;
+SELECT ROUND ((SELECT 1 FROM v)) FROM v GROUP BY ROUND ((SELECT 1 FROM v));
+EXPLAIN
+SELECT ROUND ((SELECT 1 FROM v)) FROM v GROUP BY ROUND ((SELECT 1 FROM v));
+DROP VIEW v;
+
+--echo #
+--echo # End of 10.10 tests
+--echo #

sql/sql_lex.cc

@@ -11924,9 +11924,18 @@ bool SELECT_LEX_UNIT::explainable() const
                false;
 }
 
+/*
+  Determines whether the derived table was eliminated during
+  the call of eliminate_tables(JOIN *) made at the optimization stage
+  or completely optimized out (for such degenerate statements like
+  "SELECT 1", for example)
+*/
+
 bool SELECT_LEX_UNIT::is_derived_eliminated() const
 {
   if (!derived)
     return false;
+  if (!derived->table)
+    return true;
   return derived->table->map & outer_select()->join->eliminated_tables;
 }