今天幫開發人員優化SQL,發現一個以前不怎麼在意的問題
SELECT bond.store_no FROM bill_order_num_dtl bond WHERE EXISTS ( select 1 from meetorder_staff_dtl b , meetorder_staff a where b.operate_area = bond.store_no and a.seq_no = b.seq_no and a.user_code = 'lym' ) ;
查詢需要2S,bill_order_num_dtl 數據量大概2W多條,meetorder_staff_dtl 50條左右,將EXISTS換爲INNER JOIN後只需0.02S,差了100倍,EXISTS在命中率高的情況下查詢速度較快,像這種需要判斷的表的數據量較大,而條件表的數據量較少時不宜使用。INNER JOIN相對較穩定一些,不會隨命中率的變化而影響性能。改成下面後,只需0.02S
SELECT bond.store_no FROM bill_order_num_dtl bond INNER JOIN meetorder_staff_dtl b on b.operate_area = bond.store_no INNER JOIN meetorder_staff a on a.seq_no = b.seq_no WHERE a.user_code = 'lym'