MDEV-7264: Assertion `0' failed in subselect_engine::get_identifier()

Switch EXPLAIN JSON from using subselect_engine::get_identifier()
to the number from Item_subselect::unit.

Remove subselect_union_engine::get_identifier() because it was added
only for EXPLAIN JSON code.

mysql-test/r/explain_json.result

@@ -778,3 +778,38 @@ EXPLAIN
   }
 }
 DROP TABLE t1,t2;
+# 
+# MDEV-7264: Assertion `0' failed in subselect_engine::get_identifier() on EXPLAIN JSON
+# 
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (3),(4);
+EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a <> ALL ( SELECT b FROM t2 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t1",
+      "access_type": "ALL",
+      "rows": 2,
+      "filtered": 100,
+      "attached_condition": "(not(<in_optimizer>(t1.a,t1.a in (subquery#2))))"
+    },
+    "subqueries": [
+      {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "t2",
+            "access_type": "ALL",
+            "rows": 2,
+            "filtered": 100
+          }
+        }
+      }
+    ]
+  }
+}
+DROP TABLE t1, t2;

mysql-test/t/explain_json.test

@@ -167,3 +167,15 @@ EXPLAIN FORMAT=JSON SELECT * FROM t1 AS outer_t1 WHERE a <> ALL ( SELECT a FROM
 
 DROP TABLE t1,t2;
 
+--echo # 
+--echo # MDEV-7264: Assertion `0' failed in subselect_engine::get_identifier() on EXPLAIN JSON
+--echo # 
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (3),(4);
+
+EXPLAIN FORMAT=JSON SELECT * FROM t1 WHERE a <> ALL ( SELECT b FROM t2 );
+DROP TABLE t1, t2;
+

sql/item_subselect.cc

@@ -898,10 +898,10 @@ void Item_subselect::print(String *str, enum_query_type query_type)
   if (query_type == QT_EXPLAIN)
   {
     str->append("(subquery#");
-    if (engine)
+    if (unit && unit->first_select())
     {
       char buf[64];
-      ll2str(engine->get_identifier(), buf, 10, 0); 
+      ll2str(unit->first_select()->select_number, buf, 10, 0); 
       str->append(buf);
     }
     else
@@ -3732,10 +3732,6 @@ int subselect_union_engine::exec()
   return res;
 }
 
-int subselect_union_engine::get_identifier()
-{
-  return unit->first_select()->select_number;
-}
 
 /*
   Search for at least one row satisfying select condition

sql/item_subselect.h

@@ -871,7 +871,6 @@ public:
   bool is_executed() const;
   bool no_rows();
   virtual enum_engine_type engine_type() { return UNION_ENGINE; }
-  int get_identifier();
 };