MDEV-10372: EXPLAIN fixes for recursive CTEs, including FORMAT=JSON

- Tabular EXPLAIN now prints "RECURSIVE UNION".
- There is a basic implementation of EXPLAIN FORMAT=JSON.
- it produces "recursive_union" JSON struct
- No other details or ANALYZE support, yet.

mysql-test/t/cte_recursive.test

@@ -977,5 +977,106 @@ as
 select * from ancestors;
 
 
+--echo #
+--echo # EXPLAIN FORMAT=JSON on a query where one recursive CTE uses another:
+--echo #
+explain 
+with recursive 
+prev_gen
+as
+(
+  select folks.*
+    from folks, prev_gen
+      where folks.id=prev_gen.father or folks.id=prev_gen.mother
+  union
+  select * 
+    from folks
+      where name='Me'
+),
+ancestors
+as
+(
+  select *
+    from folks
+      where name='Me'
+  union 
+  select *
+    from ancestors
+  union
+    select *
+     from prev_gen
+)
+select ancestors.name, ancestors.dob from ancestors;
+
+explain FORMAT=JSON
+with recursive 
+prev_gen
+as
+(
+  select folks.*
+    from folks, prev_gen
+      where folks.id=prev_gen.father or folks.id=prev_gen.mother
+  union
+  select * 
+    from folks
+      where name='Me'
+),
+ancestors
+as
+(
+  select *
+    from folks
+      where name='Me2'
+  union 
+  select *
+    from ancestors where id < 234
+  union
+    select *
+     from prev_gen where id < 345
+)
+select ancestors.name, ancestors.dob from ancestors;
+
+--echo #
+explain format=json
+with recursive
+ancestor_couples(h_id, h_name, h_dob, h_father, h_mother,
+                 w_id, w_name, w_dob, w_father, w_mother)
+as
+(
+  select h.*, w.*
+    from folks h, folks w,  coupled_ancestors a
+      where a.father = h.id AND a.mother = w.id
+  union 
+  select h.*, w.*
+    from folks v, folks h, folks w
+      where v.name = 'Me' and
+            (v.father = h.id AND v.mother= w.id)
+),
+coupled_ancestors (id, name, dob, father, mother)
+as
+(
+  select h_id, h_name, h_dob, h_father, h_mother
+    from ancestor_couples
+  union all
+  select w_id, w_name, w_dob, w_father, w_mother
+    from ancestor_couples
+)
+select h_name, h_dob, w_name, w_dob
+  from ancestor_couples;
+
+
 drop table folks;
 
+--echo #
+--echo # MDEV-10372: [bb-10.2-mdev9864 tree] EXPLAIN with recursive CTE enters endless recursion
+--echo #
+create table t1(a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+ 
+explain format=json 
+with recursive T as (select a from t1 union select a+10 from T where a < 1000) 
+select  * from T;
+
+drop table t1;
+
+

sql/sql_explain.cc

@@ -547,7 +547,12 @@ void Explain_union::print_explain_json(Explain_query *query,
   bool started_object= print_explain_json_cache(writer, is_analyze);
 
   writer->add_member("query_block").start_object();
+  
+  if (is_recursive_cte)
+    writer->add_member("recursive_union").start_object();
+  else
     writer->add_member("union_result").start_object();
+
   // using_temporary_table
   make_union_table_name(table_name_buffer);
   writer->add_member("table_name").add_str(table_name_buffer);

sql/sql_explain.h

@@ -327,6 +327,7 @@ class Explain_union : public Explain_node
 public:
   Explain_union(MEM_ROOT *root, bool is_analyze) : 
     Explain_node(root),
+    is_recursive_cte(false),
     fake_select_lex_explain(root, is_analyze)
   {}
 
@@ -362,6 +363,7 @@ class Explain_union : public Explain_node
   const char *fake_select_type;
   bool using_filesort;
   bool using_tmp;
+  bool is_recursive_cte;
   
   /*
     Explain data structure for "fake_select_lex" (i.e. for the degenerate

sql/sql_lex.cc

@@ -4368,7 +4368,26 @@ void st_select_lex::set_explain_type(bool on_the_fly)
         type= is_uncacheable ? "UNCACHEABLE UNION": "UNION";
         if (this == master_unit()->fake_select_lex)
           type= "UNION RESULT";
-
+        /*
+          join below may be =NULL when this functions is called at an early
+          stage. It will be later called again and we will set the correct
+          value.
+        */
+        if (join)
+        {
+          bool uses_cte= false;
+          for (JOIN_TAB *tab= first_explain_order_tab(join); tab;
+               tab= next_explain_order_tab(join, tab))
+          {
+            if (tab->table->pos_in_table_list->with)
+            {
+              uses_cte= true;
+              break;
+            }
+          }
+          if (uses_cte)
+            type= "RECURSIVE UNION";
+        }
       }
     }
   }
@@ -4683,6 +4702,8 @@ int st_select_lex_unit::save_union_explain(Explain_query *output)
     new (output->mem_root) Explain_union(output->mem_root, 
                                          thd->lex->analyze_stmt);
 
+  if (with_element && with_element->is_recursive)
+    eu->is_recursive_cte= true;
  
   if (derived)
     eu->connection_type= Explain_node::EXPLAIN_NODE_DERIVED;

sql/sql_select.cc

@@ -24186,6 +24186,7 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta,
     In case this is a derived table, here we remember the number of 
     subselect that used to produce it.
   */
+  if (!(table_list && table_list->is_with_table_recursive_reference()))
     eta->derived_select_number= table->derived_select_number;
 
   /* The same for non-merged semi-joins */

sql/sql_select.h

@@ -2278,4 +2278,8 @@ class Pushdown_query: public Sql_alloc
 bool test_if_order_compatible(SQL_I_List<ORDER> &a, SQL_I_List<ORDER> &b);
 int test_if_group_changed(List<Cached_item> &list);
 int create_sort_index(THD *thd, JOIN *join, JOIN_TAB *tab, Filesort *fsort);
+
+JOIN_TAB *first_explain_order_tab(JOIN* join);
+JOIN_TAB *next_explain_order_tab(JOIN* join, JOIN_TAB* tab);
+
 #endif /* SQL_SELECT_INCLUDED */

sql/sql_union.cc

@@ -1166,7 +1166,7 @@ bool st_select_lex_unit::exec()
 }
 
 
-
+// One step of recursive execution
 bool st_select_lex_unit::exec_recursive(bool is_driving_recursive)
 {
   st_select_lex *lex_select_save= thd->lex->current_select;