MDEV-27306: SET STATEMENT optimizer_trace=1 Doesn't save the tracebb-10.6-mdev27306

In mysql_execute_command(), move optimizer trace initialization to be
after run_set_statement_if_requested() call.

Unfortunately, mysql_execute_command() code uses "goto error" a lot, and
this means optimizer trace code cannot use RAII objects. Work this around
by:
- Make Opt_trace_start a non-RAII object, add init() method.
- Move the code that writes the top-level object and array into
  Opt_trace_start::init().

1 files changed, 3 insertions, 3 deletions

diff --git a/sql/sql_prepare.cc b/sql/sql_prepare.cc
index 4ead77c225f..67032142591 100644
--- a/sql/sql_prepare.cc
+++ b/sql/sql_prepare.cc
@@ -2437,9 +2437,9 @@ static bool check_prepared_statement(Prepared_statement *stmt)
     For the optimizer trace, this is the symmetric, for statement preparation,
     of what is done at statement execution (in mysql_execute_command()).
   */
-  Opt_trace_start ots(thd, tables, lex->sql_command, &lex->var_list,
-                      thd->query(), thd->query_length(),
-                      thd->variables.character_set_client);
+  Opt_trace_start ots(thd);
+  ots.init(thd, tables, lex->sql_command, &lex->var_list, thd->query(),
+           thd->query_length(), thd->variables.character_set_client);
 
   Json_writer_object trace_command(thd);
   Json_writer_array trace_command_steps(thd, "steps");