MDEV-13374: Server crashes in first_linear_tab / st_select_lex::set_explain_type

- Support first_linear_tab() traversal for degenerate joins

mysql-test/r/win.result

@@ -3173,3 +3173,15 @@ Nth_value(i,1) OVER()
 1
 1
 DROP TABLE t1;
+#
+# A regression after MDEV-13351:
+# MDEV-13374 : Server crashes in first_linear_tab / st_select_lex::set_explain_type 
+#              upon UNION with aggregate function	
+#
+CREATE TABLE t1 (i INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(2);
+SELECT i AS fld FROM t1 UNION SELECT COUNT(*) AS fld FROM t1;
+fld
+1
+2
+DROP TABLE t1;

mysql-test/t/win.test

@@ -1954,3 +1954,14 @@ UNION ALL
 ;
 DROP TABLE t1;
 
+--echo #
+--echo # A regression after MDEV-13351:
+--echo # MDEV-13374 : Server crashes in first_linear_tab / st_select_lex::set_explain_type 
+--echo #              upon UNION with aggregate function	
+--echo #
+
+CREATE TABLE t1 (i INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(2);
+SELECT i AS fld FROM t1 UNION SELECT COUNT(*) AS fld FROM t1;
+DROP TABLE t1;
+

sql/sql_select.cc

@@ -8627,7 +8627,7 @@ JOIN_TAB *first_top_level_tab(JOIN *join, enum enum_with_const_tables const_tbls
   JOIN_TAB *tab= join->join_tab;
   if (const_tbls == WITHOUT_CONST_TABLES)
   {
-    if (join->const_tables == join->table_count)
+    if (join->const_tables == join->table_count || !tab)
       return NULL;
     tab += join->const_tables;
   }
@@ -8650,6 +8650,10 @@ JOIN_TAB *first_linear_tab(JOIN *join,
                            enum enum_with_const_tables const_tbls)
 {
   JOIN_TAB *first= join->join_tab;
+
+  if (!first)
+    return NULL;
+
   if (const_tbls == WITHOUT_CONST_TABLES)
     first+= join->const_tables;
 
@@ -8736,7 +8740,7 @@ JOIN_TAB *first_depth_first_tab(JOIN* join)
 {
   JOIN_TAB* tab;
   /* This means we're starting the enumeration */
-  if (join->const_tables == join->top_join_tab_count)
+  if (join->const_tables == join->top_join_tab_count || !join->join_tab)
     return NULL;
 
   tab= join->join_tab + join->const_tables;