MDEV-9652: EXPLAIN FORMAT=JSON should show outer_ref_cond

Show outer_ref_condition in EXPLAIN FORMAT=JSON output.

mysql-test/r/explain_json.result

@@ -1543,3 +1543,43 @@ ANALYZE
 set optimizer_switch=@tmp_optimizer_switch;
 set join_cache_level=@tmp_join_cache_level;
 drop table t1,t2,t3,t4;
+#
+# MDEV-9652: EXPLAIN FORMAT=JSON should show outer_ref_cond
+#
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (a int, b int);
+insert into t1 select a,a from t0;
+explain format=json 
+select a, (select max(a) from t1 where t0.a<5 and t1.b<t0.a) from t0;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t0",
+      "access_type": "ALL",
+      "rows": 10,
+      "filtered": 100
+    },
+    "subqueries": [
+      {
+        "expression_cache": {
+          "state": "uninitialized",
+          "query_block": {
+            "select_id": 2,
+            "outer_ref_condition": "(t0.a < 5)",
+            "table": {
+              "table_name": "t1",
+              "access_type": "ALL",
+              "rows": 10,
+              "filtered": 100,
+              "attached_condition": "((t0.a < 5) and (t1.b < t0.a))"
+            }
+          }
+        }
+      }
+    ]
+  }
+}
+drop table t0,t1;

mysql-test/t/explain_json.test

@@ -394,5 +394,15 @@ set join_cache_level=@tmp_join_cache_level;
 
 drop table t1,t2,t3,t4;
 
+--echo #
+--echo # MDEV-9652: EXPLAIN FORMAT=JSON should show outer_ref_cond
+--echo #
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (a int, b int);
+insert into t1 select a,a from t0;
 
+explain format=json 
+select a, (select max(a) from t1 where t0.a<5 and t1.b<t0.a) from t0;
+drop table t0,t1;
 

sql/sql_explain.cc

@@ -862,6 +862,12 @@ void Explain_select::print_explain_json(Explain_query *query,
       writer->add_member("const_condition");
       write_item(writer, exec_const_cond);
     }
+    if (outer_ref_cond)
+    {
+      writer->add_member("outer_ref_condition");
+      write_item(writer, outer_ref_cond);
+    }
+
     /* we do not print HAVING which always evaluates to TRUE */
     if (having || (having_value == Item::COND_FALSE))
     {

sql/sql_explain.h

@@ -232,9 +232,10 @@ class Explain_select : public Explain_basic_join
 
   /* Expensive constant condition */
   Item *exec_const_cond;
+  Item *outer_ref_cond;
 
   /* HAVING condition */
-  COND *having;
+  Item *having;
   Item::cond_result having_value;
 
   /* Global join attributes. In tabular form, they are printed on the first row */

sql/sql_select.cc

@@ -24252,6 +24252,7 @@ int JOIN::save_explain_data_intern(Explain_query *output, bool need_tmp_table,
       xpl_sel->using_filesort= true;
 
     xpl_sel->exec_const_cond= exec_const_cond;
+    xpl_sel->outer_ref_cond= outer_ref_cond;
     if (tmp_having)
       xpl_sel->having= tmp_having;
     else