MDEV-11975: SQLCOM_PREPARE of EXPLAIN & ANALYZE statement do not return correct metadata info

Added metadate info after prepare EXPLAIN/ANALYZE.

sql/sql_class.cc

@@ -2405,18 +2405,31 @@ CHANGED_TABLE_LIST* THD::changed_table_dup(const char *key, long key_length)
 }
 
 
-int THD::send_explain_fields(select_result *result, uint8 explain_flags, bool is_analyze)
+void THD::prepare_explain_fields(select_result *result,
+                                 List<Item> *field_list,
+                                 uint8 explain_flags,
+                                 bool is_analyze)
 {
-  List<Item> field_list;
   if (lex->explain_json)
-    make_explain_json_field_list(field_list, is_analyze);
+    make_explain_json_field_list(*field_list, is_analyze);
   else
-    make_explain_field_list(field_list, explain_flags, is_analyze);
+    make_explain_field_list(*field_list, explain_flags, is_analyze);
+
+  result->prepare(*field_list, NULL);
+}
 
-  result->prepare(field_list, NULL);
-  return (result->send_result_set_metadata(field_list,
-                                           Protocol::SEND_NUM_ROWS | 
-                                           Protocol::SEND_EOF));
+
+int THD::send_explain_fields(select_result *result,
+                             uint8 explain_flags,
+                             bool is_analyze)
+{
+  List<Item> field_list;
+  int rc;
+  prepare_explain_fields(result, &field_list, explain_flags, is_analyze);
+  rc= result->send_result_set_metadata(field_list,
+                                       Protocol::SEND_NUM_ROWS |
+                                       Protocol::SEND_EOF);
+  return(rc);
 }
 
 

sql/sql_class.h

@@ -3331,6 +3331,8 @@ class THD :public Statement,
   void add_changed_table(TABLE *table);
   void add_changed_table(const char *key, long key_length);
   CHANGED_TABLE_LIST * changed_table_dup(const char *key, long key_length);
+  void prepare_explain_fields(select_result *result, List<Item> *field_list,
+                              uint8 explain_flags, bool is_analyze);
   int send_explain_fields(select_result *result, uint8 explain_flags,
                           bool is_analyze);
   void make_explain_field_list(List<Item> &field_list, uint8 explain_flags,

sql/sql_prepare.cc

@@ -2475,8 +2475,28 @@ static bool check_prepared_statement(Prepared_statement *stmt)
     break;
   }
   if (res == 0)
-    DBUG_RETURN(stmt->is_sql_prepare() ?
-                FALSE : (send_prep_stmt(stmt, 0) || thd->protocol->flush()));
+  {
+    if (!stmt->is_sql_prepare())
+    {
+       if (lex->describe || lex->analyze_stmt)
+       {
+         if (!lex->result &&
+             !(lex->result= new (stmt->mem_root) select_send(thd)))
+              DBUG_RETURN(TRUE);
+         List<Item> field_list;
+         thd->prepare_explain_fields(lex->result, &field_list,
+                                     lex->describe, lex->analyze_stmt);
+         res= send_prep_stmt(stmt, lex->result->field_count(field_list)) ||
+              lex->result->send_result_set_metadata(field_list,
+                                                    Protocol::SEND_EOF);
+       }
+       else
+         res= send_prep_stmt(stmt, 0);
+       if (!res)
+         thd->protocol->flush();
+    }
+    DBUG_RETURN(FALSE);
+  }
 error:
   DBUG_RETURN(TRUE);
 }