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.

1 files changed, 35 insertions, 0 deletions

diff --git a/mysql-test/r/explain_json.result b/mysql-test/r/explain_json.result
index 0824132de05..d0a0b8f5be3 100644
--- a/mysql-test/r/explain_json.result
+++ b/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;