MySQL query took extremely long time to finish and it was in 'Sending data' state
the combination of answer_id and insert_time is unique. I already created that index for the combination



Create this UNIQUE index on destination table (zhihu_answer_meta_reconcile).
Perform the action you need with 2 separate queries:


-- TRUNCATE `zhihu_answer_meta_reconcile`;
INSERT INTO `zhihu_answer_meta_reconcile`
    SELECT * FROM `zhihu_answer_meta_staging`;
INSERT IGNORE INTO `zhihu_answer_meta_reconcile`
    SELECT * FROM `zhihu_answer_meta`;


These queries may be combined into one line. While using from some code check that the inserting function/method allows multi-query in that case.