sql語句優化總結

 

數據庫優化的幾個原則：

1.盡量避免在列上做運算，這樣會導致索引失敗；

2.使用join是應該用小結果集驅動大結果集，同時把復雜的join查詢拆分成多個query。不然join的越多表，就會導致越多的鎖定和堵塞。

3.注意like模糊查詢的使用，避免使用%%，例如select * from a where name like '%de%';

代替語句：select * from a where name >= 'de' and name < 'df';

4.僅列出需要查詢的字段，不要使用select * from ...，節省內存；

5.使用批量插入語句，節省交互；

insert into a (id ,name)
values(2,'a'),
(3,'s');

6.limit基數比較大時，使用between ... and ...

7.不要使用rand函數隨機獲取記錄；

8.避免使用null ，這就需要在建表時，盡量設置為not null，提升查詢性能；

9，不要使用count(id),而應該是count(*)

10.不要做無謂的排序，盡可能在索引中完成排序；

我們先來看一個sql：

 select
                    ii.product_id, 
                    p.product_name, 
                    count(distinct pim.pallet_id) count_pallet_id, 
                    if(round(sum(itg.quantity),2) > -1 && round(sum(itg.quantity),2) < 0.005, 0, round(sum(itg.quantity),2)) quantity,
                    round(ifnull(sum(itag.locked_quantity), 0.00000),2) locked_quantity,
                    pc.container_unit_code_name,
                    if(round(sum(itg.qoh),2) > -1 && round(sum(itg.qoh),2) < 0.005, 0, round(sum(itg.qoh),2)) qoh,
                    round(ifnull(sum(itag.locked_qoh), 0.00000),2) locked_qoh,
                    p.unit_code,
                    p.unit_code_name
                from (select 
                        it.inventory_item_id item_id, 
                        sum(it.quantity) quantity, 
                        sum(it.real_quantity) qoh 
                    from 
                        ws_inventory_transaction it
                    where 
                        it.enabled = 1 
                    group by 
                        it.inventory_item_id  
                    ) itg 
                    left join (select 
                                    ita.inventory_item_id item_id, 
                                    sum(ita.quantity) locked_quantity, 
                                    sum(ita.real_quantity) locked_qoh 
                               from 
                                    ws_inventory_transaction_action ita
                               where 
                                    1=1 and ita.type in ('locked', 'release') 
                               group by 
                                    ita.inventory_item_id 
                               )itag on itg.item_id = itag.item_id
                    inner join ws_inventory_item ii on itg.item_id = ii.inventory_item_id 
                    inner join ws_pallet_item_mapping pim on ii.inventory_item_id = pim.inventory_item_id  
                    inner join ws_product p on ii.product_id = p.product_id and p.status = 'OK'
                    left join ws_product_container pc on ii.container_id = pc.container_id
//總起來說關聯太多表，設計表時可以多一些冗余字段，減少表之間的關聯查詢；
                where 
                    ii.inventory_type = 'raw_material' and 
                    ii.inventory_status = 'in_stock' and 
                    ii.facility_id = '25' and 
                    datediff(now(),ii.last_updated_time) < 3  //違反了第一個原則
                     and p.product_type = 'goods'
                     and p.product_name like '%果%'   // 違反原則3

                group by 
                    ii.product_id
                having 
                    qoh < 0.005
                order by 
                    qoh desc

上面的sql我們在from 中使用了子查詢，這樣對查詢是非常不利的；

更好的一種做法是下面的語句：

select  
                t.facility_id,
                f.facility_name,
                t.inventory_status,
                wis.inventory_status_name,
                t.inventory_type,
                t.product_type,
                t.product_id, 
                p.product_name,
                t.container_id, 
                t.unit_quantity, 
                p.unit_code,
                p.unit_code_name,
                pc.container_unit_code_name,
                t.secret_key,
                sum(t.quantity) quantity,
                sum(t.real_quantity) real_quantity,
                sum(t.locked_quantity) locked_quantity,
                sum(t.locked_real_quantity) locked_real_quantity
            from ( select 
                        ii.facility_id,
                        ii.inventory_status,
                        ii.inventory_type,
                        ii.product_type,
                        ii.product_id, 
                        ii.container_id, 
                        ii.unit_quantity, 
                        ita.secret_key,
                        ii.quantity quantity,
                        ii.real_quantity real_quantity,
                        sum(ita.quantity) locked_quantity,
                        sum(ita.real_quantity) locked_real_quantity
                    from 
                        ws_inventory_item ii 
                        inner join ws_inventory_transaction_action ita on ii.inventory_item_id = ita.inventory_item_id
                    where 
                        ii.facility_id = '{$facility_id}' and 
                        ii.inventory_status = '{$inventory_status}' and 
                        ii.product_type = '{$product_type}' and 
                        ii.inventory_type = '{$inventory_type}' and
                        ii.locked_real_quantity > 0 and 
                        ita.type in ('locked', 'release') 
                    group by 
                        ii.product_id, ita.secret_key, ii.container_id, ita.inventory_item_id
                    having 
                        locked_real_quantity > 0 
            ) as t
                inner join ws_product p on t.product_id = p.product_id 
                left join ws_facility f on t.facility_id = f.facility_id
                left join ws_inventory_status wis on wis.inventory_status = t.inventory_status
                left join ws_product_container pc on pc.container_id = t.container_id            
            group by 
                t.product_id, t.secret_key, t.container_id

注意：

1、from 語句中一定不要使用子查詢；

2、使用更多的where加以限制，縮小查找范圍；

3、合理利用索引；

4、通過explain查看sql性能；

 

相關優化可以借鑒：http://blog.csdn.net/csh624366188/article/details/8457749