SQL> select count(*) from tm m LEFT join td d on m.x+0=d.x+0;

Select Expression
    -> Aggregate
        ->  Nested Loop Join (outer)
            -> Table "M" Full Scan
            -> Filter
                -> Table "D" Full Scan

SQL> select count(*) from (select m.* from tm m join td d on m.x+0=d.x+0 union distinct select m.* from tm m);

Select Expression
    -> Aggregate
        -> Sort (unique)
            -> Union
                -> Filter
                    -> Hash Join (inner)
                        -> Table "D" Full Scan
                        -> Record Buffer
                            -> Table "M" Full Scan
                -> Table "M" Full Scan

                COUNT
=====================
                  253

Current memory = 2450751584
Delta memory = 4480
Max memory = 2453993504
Elapsed time= 8.28 sec
Cpu = 0.00 sec
Buffers = 524288
Reads = 15572
Writes = 0
Fetches = 2556261

double OPT_getRelationCardinality(thread_db* tdbb, jrd_rel* relation, const Format* format)
{
	SET_TDBB(tdbb);

	if (relation->isVirtual())
	{
		// Just a dumb estimation
		 return (double) 100; 
	}

	if (relation->rel_file)
	{
		return EXT_cardinality(tdbb, relation);
	}

	MET_post_existence(tdbb, relation);
	const double cardinality =  DPM_cardinality(tdbb, relation, format); 
	MET_release_existence(tdbb, relation);
	return cardinality;
}

double DPM_cardinality(thread_db* tdbb, jrd_rel* relation, const Format* format)
{
. . .
	return (double) dataPages * (dbb->dbb_page_size - DPG_SIZE) /
		(minRecordSize + (format->fmt_length * 0.5));
}