EXPLAIN FORMAT=JSON: Full scan on NULL key (join case)

mysql-test/r/explain_json.result

@@ -719,3 +719,62 @@ EXPLAIN
 }
 drop table t1;
 drop table t0;
+#
+# MDEV-7265: "Full scan on NULL key", the join case
+#
+CREATE TABLE t1 (a INT, KEY(a));
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (3),(4);
+EXPLAIN FORMAT=JSON SELECT * FROM t1 AS outer_t1 WHERE a <> ALL ( SELECT a FROM t1, t2 WHERE b <> outer_t1.a );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "outer_t1",
+      "access_type": "index",
+      "key": "a",
+      "key_length": "5",
+      "used_key_parts": ["a"],
+      "rows": 2,
+      "filtered": 100,
+      "attached_condition": "(not(<in_optimizer>(outer_t1.a,<exists>(subquery#2))))",
+      "using_index": true
+    },
+    "subqueries": [
+      {
+        "query_block": {
+          "select_id": 2,
+          "full-scan-on-null_key": {
+            "table": {
+              "table_name": "t1",
+              "access_type": "ref_or_null",
+              "possible_keys": ["a"],
+              "key": "a",
+              "key_length": "5",
+              "used_key_parts": ["a"],
+              "ref": ["func"],
+              "rows": 2,
+              "filtered": 100,
+              "attached_condition": "trigcond(((<cache>(outer_t1.a) = t1.a) or isnull(t1.a)))",
+              "using_index": true
+            }
+          },
+          "block-nl-join": {
+            "table": {
+              "table_name": "t2",
+              "access_type": "ALL",
+              "rows": 2,
+              "filtered": 100
+            },
+            "buffer_type": "flat",
+            "join_type": "BNL",
+            "attached_condition": "((t2.b <> outer_t1.a) and trigcond(((<cache>(outer_t1.a) = t1.a) or isnull(t1.a))))"
+          }
+        }
+      }
+    ]
+  }
+}
+DROP TABLE t1,t2;

mysql-test/t/explain_json.test

@@ -153,3 +153,17 @@ select * from t1 tbl1, t1 tbl2 where tbl2.a < tbl1.b;
 drop table t1;
 drop table t0;
 
+--echo #
+--echo # MDEV-7265: "Full scan on NULL key", the join case
+--echo #
+
+CREATE TABLE t1 (a INT, KEY(a));
+INSERT INTO t1 VALUES (1),(2);
+
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (3),(4);
+
+EXPLAIN FORMAT=JSON SELECT * FROM t1 AS outer_t1 WHERE a <> ALL ( SELECT a FROM t1, t2 WHERE b <> outer_t1.a );
+
+DROP TABLE t1,t2;
+

sql/sql_explain.cc

@@ -1139,6 +1139,8 @@ void Explain_table_access::tag_to_json(Json_writer *writer, enum explain_extra_t
     case ET_START_TEMPORARY:
     case ET_END_TEMPORARY:
       /* Handled as "duplicates_removal: { ... } */
+    case ET_FULL_SCAN_ON_NULL_KEY:
+      /* Handled in full_scan_on_null_key */
       break;
     case ET_FIRST_MATCH:
       writer->add_member("first_match").add_str(firstmatch_table_name.c_ptr());
@@ -1188,6 +1190,9 @@ void Explain_table_access::print_explain_json(Explain_query *query,
     add_json_keyset(writer, "keys", &possible_keys);
   }
 
+  if (full_scan_on_null_key)
+    writer->add_member("full-scan-on-null_key").start_object();
+
   writer->add_member("table").start_object();
 
   writer->add_member("table_name").add_str(table_name);
@@ -1289,6 +1294,9 @@ void Explain_table_access::print_explain_json(Explain_query *query,
     tag_to_json(writer, extra_tags.at(i));
   }
   
+  if (full_scan_on_null_key)
+    writer->end_object(); //"full-scan-on-null_key"
+
   if (range_checked_fer)
     writer->end_object(); // "range-checked-for-each-record"
 

sql/sql_explain.h

@@ -562,6 +562,7 @@ public:
     non_merged_sjm_number(0),
     extra_tags(root),
     range_checked_fer(NULL),
+    full_scan_on_null_key(false),
     start_dups_weedout(false),
     end_dups_weedout(false),
     where_cond(NULL),
@@ -635,6 +636,8 @@ public:
   /* Non-NULL values means this tab uses "range checked for each record" */
   Explain_range_checked_fer *range_checked_fer;
  
+  bool full_scan_on_null_key;
+
   // valid with ET_USING_JOIN_BUFFER
   EXPLAIN_BKA_TYPE bka_type;
 

sql/sql_select.cc

@@ -23687,6 +23687,7 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta, table_map prefix_tab
       if (tab->ref.cond_guards[part])
       {
         eta->push_extra(ET_FULL_SCAN_ON_NULL_KEY);
+        eta->full_scan_on_null_key= true;
         break;
       }
     }