EXPLAIN JSON: Print out the "expensive constant condition" attached to joins.

mysql-test/r/explain_json.result

@@ -551,6 +551,7 @@ EXPLAIN
 {
   "query_block": {
     "select_id": 1,
+    "const_condition": "1",
     "table": {
       "table_name": "t1",
       "access_type": "ALL",
@@ -778,3 +779,54 @@ EXPLAIN
   }
 }
 DROP TABLE t1,t2;
+#
+# Join's constant expression
+#
+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 tbl1.a+10*tbl2.a, 1234 from t0 tbl1, t0 tbl2;
+explain format=json 
+select * from t0 
+where 
+20000 > all (select max(tbl1.a + tbl2.a)
+from t1 tbl1, t1 tbl2 where tbl1.b=tbl2.b);
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "const_condition": "<not>(<in_optimizer>(20000,(<max>(subquery#2) >= 20000)))",
+    "table": {
+      "table_name": "t0",
+      "access_type": "ALL",
+      "rows": 10,
+      "filtered": 100
+    },
+    "subqueries": [
+      {
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "table_name": "tbl1",
+            "access_type": "ALL",
+            "rows": 100,
+            "filtered": 100
+          },
+          "block-nl-join": {
+            "table": {
+              "table_name": "tbl2",
+              "access_type": "ALL",
+              "rows": 100,
+              "filtered": 100
+            },
+            "buffer_type": "flat",
+            "join_type": "BNL",
+            "attached_condition": "(tbl2.b = tbl1.b)"
+          }
+        }
+      }
+    ]
+  }
+}
+drop table t1;
+drop table t0;

mysql-test/t/explain_json.test

@@ -2,7 +2,7 @@
 # EXPLAIN FORMAT=JSON tests. These are tests developed for MariaDB.
 #
 --disable_warnings
-drop table if exists t0,t1;
+drop table if exists t0,t1,t2;
 --enable_warnings
 
 create table t0(a int);
@@ -167,3 +167,21 @@ EXPLAIN FORMAT=JSON SELECT * FROM t1 AS outer_t1 WHERE a <> ALL ( SELECT a FROM
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # Join's constant expression
+--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 tbl1.a+10*tbl2.a, 1234 from t0 tbl1, t0 tbl2;
+
+explain format=json 
+select * from t0 
+where 
+  20000 > all (select max(tbl1.a + tbl2.a)
+               from t1 tbl1, t1 tbl2 where tbl1.b=tbl2.b);
+
+drop table t1;
+drop table t0;
+

sql/sql_explain.cc

@@ -27,6 +27,8 @@ const char * STR_DELETING_ALL_ROWS= "Deleting all rows";
 const char * STR_IMPOSSIBLE_WHERE= "Impossible WHERE";
 const char * STR_NO_ROWS_AFTER_PRUNING= "No matching rows after partition pruning";
 
+static void write_item(Json_writer *writer, Item *item);
+
 Explain_query::Explain_query(THD *thd_arg, MEM_ROOT *root) : 
   mem_root(root), upd_del_plan(NULL),  insert_plan(NULL),
   unions(root), selects(root),  thd(thd_arg), apc_enabled(false),
@@ -732,7 +734,17 @@ void Explain_select::print_explain_json(Explain_query *query,
          Explain_basic_join does not have ORDER/GROUP.
          A: factor out join tab printing loop into a common func.
     */
-    Explain_basic_join::print_explain_json(query, writer, is_analyze);
+    writer->add_member("query_block").start_object();
+    writer->add_member("select_id").add_ll(select_id);
+    
+    if (exec_const_cond)
+    {
+      writer->add_member("const_condition");
+      write_item(writer, exec_const_cond);
+    }
+
+    Explain_basic_join::print_explain_json_interns(query, writer, is_analyze);
+    writer->end_object();
   }
 
 }
@@ -742,10 +754,20 @@ void Explain_basic_join::print_explain_json(Explain_query *query,
                                             Json_writer *writer, 
                                             bool is_analyze)
 {
-  Json_writer_nesting_guard guard(writer);
-
   writer->add_member("query_block").start_object();
   writer->add_member("select_id").add_ll(select_id);
+  
+  print_explain_json_interns(query, writer, is_analyze);
+
+  writer->end_object();
+}
+
+
+void Explain_basic_join::print_explain_json_interns(Explain_query *query, 
+                                                    Json_writer *writer, 
+                                                    bool is_analyze)
+{
+  Json_writer_nesting_guard guard(writer);
   for (uint i=0; i< n_join_tabs; i++)
   {
     if (join_tabs[i]->start_dups_weedout)
@@ -757,7 +779,6 @@ void Explain_basic_join::print_explain_json(Explain_query *query,
       writer->end_object();
   }
   print_explain_json_for_children(query, writer, is_analyze);
-  writer->end_object();
 }
 
 

sql/sql_explain.h

@@ -175,6 +175,9 @@ public:
   void print_explain_json(Explain_query *query, Json_writer *writer, 
                           bool is_analyze);
 
+  void print_explain_json_interns(Explain_query *query, Json_writer *writer,
+                                  bool is_analyze);
+
   /* A flat array of Explain structs for tables. */
   Explain_table_access** join_tabs;
   uint n_join_tabs;
@@ -222,6 +225,9 @@ public:
   */
   const char *message;
   
+  /* Expensive constant condition */
+  Item *exec_const_cond;
+  
   /* Global join attributes. In tabular form, they are printed on the first row */
   bool using_temporary;
   bool using_filesort;

sql/sql_select.cc

@@ -23768,6 +23768,8 @@ int JOIN::save_explain_data_intern(Explain_query *output, bool need_tmp_table,
     if (need_order)
       xpl_sel->using_filesort= true;
 
+    xpl_sel->exec_const_cond= exec_const_cond;
+
     JOIN_TAB* const first_top_tab= first_breadth_first_tab(join, WALK_OPTIMIZATION_TABS);
     JOIN_TAB* prev_bush_root_tab= NULL;