例如,在查询 SELECT * FROM people WHERE country_id=5 and city_id=6 order by last_online desc limit 10 时,如果在(country_id,city_id)上有索引,在(country_id,last_online)上也有索引,那么即使会导致文件排序,也可能会选择第一个索引。 要解决这个问题,要么扩展你的索引,这样 MySQL 优化器就不必在更...
2、not in 优化, 如果not in 的指标范围非常大的话,这个效率很差。 举个例子 select customer_id ,first_name ,last_name ,email from customer where customer_id not in (select customer_id from payment); 1. 2. 3. 4. 每个customer_id都要到payment中查询一遍, 数据量大时很慢。 优化后 ---> ...
select actor.first_name , actor.last_name, count(*) from film_actor inner join actor using(actor_id) group by actor.first_name, actor.last_name 1. 2. 3. 优化后的SQL: select actor.first_name , actor.last_name, count(*) from film_actor inner join actor using(actor_id) group by f...
Note:While sorting in ascending order, NULL values always come up first, and while sorting in descending order, NULL value rows come up in the last order. Different Methods to use ORDER BY Let us look at the different methods to use ORDER BY Keyword in MySQL. Sorting by Single Column Fir...
When unparsing LogicalPlans back to SQL statements for the MySQL dialect, any plan that includes a Sort node will produce a SQL statement that looks like: SELECT "ta"."j1_id" FROM "j1" AS "ta" ORDER BY "ta"."j1_id" ASC NULLS LAST LIMIT 10 ^---^ The NULLS LAST causes issues...
Posted by developer: Below is the analysis and conclusion of the current behaviour. Issue: MySQL 8.0 is slower than 5.7 for the given customer scenario. Query: SELECT * FROM test1 WHERE EXISTS (SELECT 1 FROM test2 WHERE x2=x1) AND id > 3 ORDER BY id ASC LIMIT 5; Scenario: Creates ta...
不加ORDER BY t.CREATED_Date DESC查询需要2秒加上后,查询需要15秒如果单独查询一个rd_pro_inventory_temp表的话,加不加都是几毫米而已,这是为什么呀? SELECT t.LAST_UPD_BY, t.LAST_UPD_DATE, CASE WHEN t.ADD9 = 0 THEN det.item_code ELSE NULL END AS LOC_CODE, CASE WHEN t.ADD9 = 1 THEN...
SELECT * FROM Table WHERE Col1 ORDER BY total/value It's like this... 0 0 0 0 5 4.5 3 1 and I want the results to be like this... 5 4.5 3 1 0 0 0 0 Is there a way to sort results with the nulls last? Any help would be appreciated, thanks. ...
t.forum_id ORDER BY t.topic_last_post_time DESC LIMIT 7 And I get a query plan of: id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t index NULL last_post_time 4 NULL 7 1 SIMPLE f eq_ref PRIMARY PRIMARY 3 t.forum_id 1 Which is obviously much more...
mysql> select tv.fromClientID as clientID,tv.trackDate,p.realname,p.lastSeen,p.clientName, p.image_serverID,p.age,p.maritalStatus,p.imageID,p.imageCategoryID from trackviews tv left join mym p on (tv.fromClientID=p.clientID) where tv.clientID=32 ORDER BY trackdate ASC limit 5; ...