I wrote a InnoDB wrapper class and implemented virtual int info(uint) function in sql/handler.h.

int innodb_wra::info(uint flag)

{

	error = innodb_handler->info(flag);

	DBUG_RETURN(error);

}

But Index Merge Optimization was not executed. So, I added a few line below.

int innodb_wra::info(uint flag)

{

	error = innodb_handler->info(flag);

	// if (stats.records < 2)   <---- tried but index merge method was not executed

		// stats.records= 2;

	stats = innodb_handler->stats;   // so I copied all stats

	DBUG_RETURN(error);

}

so I was able to see index merge optimizer plan in EXPLAIN output.

But I got this error, after virtual handler *clone(const char *name, MEM_ROOT *mem_root) funtion is called.

Out of memory; check if mysqld or some other process uses all available memory; if not, you may have to use 'ulimit' to allow mysqld to use more memory or you can add more swap space

I set ullimit and MariaDB system variables. But the result was the same.

I guess my_malloc() failed in handler *handler::clone(const char *name, MEM_ROOT *mem_root) function.