CREATE TEMPORARY TABLE tmp SELECT id FROM comments WHERE user_id = 123;

SELECT COUNT(*) FROM tmp;

SELECT column_names FROM comments c JOIN tmp t ON c.id = t.id;

SELECT column_names FROM comments c JOIN cache_table ct ON c.id = ct.id LIMIT X;

resultSet = SELECT id FROM comments WHERE user_id = 123;

if (resultSet.length < 1000)
    just select all data (Use already selected id's for select)
else
    copy ID's to cache

SELECT column_names FROM
(SELECT id FROM comments WHERE user_id = 123 LIMIT 100) c
LEFT JOIN cache_table ct ON c.id = ct.id;

SELECT column_names FROM
(SELECT id FROM comments WHERE user_id = 123 LIMIT 0,100) c
LEFT JOIN cache_table ct ON c.id = ct.id;