SQL_CALC_FOUND_ROWS do not work with UNION ALL

SELECT SQL_CALC_FOUND_ROWS 'foo' UNION ALL SELECT 'bar';

SELECT SQL_CALC_FOUND_ROWS 'foo' UNION ALL SELECT 'bar' LIMIT 0;

SELECT FOUND_ROWS();

SELECT SQL_CALC_FOUND_ROWS 'foo' UNION ALL SELECT 'bar';

foo

foo

bar

SELECT SQL_CALC_FOUND_ROWS 'foo' UNION ALL SELECT 'bar' LIMIT 0;

foo

SELECT FOUND_ROWS();

FOUND_ROWS()

0