Try to guess their size

SELECT * FROM gallery2 WHERE topic_id="976645" UNION SELECT * FROM gallery2 WHERE topic_id="34175" UNION SELECT * FROM gallery2 WHERE topic_id="1725210" UNION SELECT * FROM gallery2 WHERE topic_id="166767" UNION SELECT * FROM gallery2 WHERE topic_id="1392324" UNION SELECT * FROM gallery2 WHERE topic_id="1152082" UNION SELECT * FROM gallery2 WHERE topic_id="81443" UNION SELECT * FROM gallery2 WHERE topic_id="439095" UNION SELECT * FROM gallery2 WHERE topic_id="56469" UNION SELECT * FROM gallery2 WHERE topic_id="915925" UNION SELECT * FROM gallery2 WHERE topic_id="139065" UNION SELECT * FROM gallery2 WHERE topic_id="1020247" UNION SELECT * FROM gallery2 WHERE topic_id="1806047" UNION SELECT * FROM gallery2 WHERE topic_id="1224038" UNION SELECT * FROM gallery2 WHERE topic_id="845844" UNION SELECT * FROM gallery2 WHERE topic_id="1459956"