latch free等待问题之Result Cache:RC Latch优化案例

问题背景：

客户反馈数据库运行缓慢，DB （12c）

问题解决

1> 查看相关时间段的AWR报告

 

查看DB的整体负载情况看出数据库已经比较繁忙

AWR报告中存在大量异常sql（不属于业务模块）

例如

 1 select chainviewi0_.id                as id1_112_,
 2        chainviewi0_.actId             as actId2_112_,
 3        chainviewi0_.activityName      as activityName3_112_,
 4        chainviewi0_.approved          as approved4_112_,
 5        chainviewi0_.approvedDate      as approvedDate5_112_,
 6        chainviewi0_.barcode           as barcode6_112_,
 7        chainviewi0_.chainSku_id       as chainSku_id7_112_,
 8        chainviewi0_.channelCrowdCode  as channelCrowdCode8_112_,
 9        chainviewi0_.conditionType     as conditionType9_112_,
10        chainviewi0_.coverBefore       as coverBefore10_112_,
11        chainviewi0_.dailyLimit        as dailyLimit11_112_,
12        chainviewi0_.description       as description12_112_,
13        chainviewi0_.discountType      as discountType13_112_,
14        chainviewi0_.effective         as effective14_112_,
15        chainviewi0_.endDate           as endDate15_112_,
16        chainviewi0_.endTime           as endTime16_112_,
17        chainviewi0_.itemDiscount_id   as itemDiscount_id17_112_,
18        chainviewi0_.management_id     as management_id18_112_,
19        chainviewi0_.matnr             as matnr19_112_,
20        chainviewi0_.memberLimit       as memberLimit20_112_,
21        chainviewi0_.merchantCrowdCode as merchantCrowdCode21_112_,
22        chainvi                        ewi0_.oldPrice as oldPrice22_112_,
23        chainviewi0_.onditionNum       as onditionNum23_112_,
24        chainviewi0_.outActId          as outActId24_112_,
25        chainviewi0_.poi               as poi25_112_,
26        chainviewi0_.price             as price26_112_,
27        chainviewi0_.priorityValue     as priorityValue27_112_,
28        chainviewi0_.skuCode           as skuCode28_112_,
29        chainviewi0_.skuId             as skuId29_112_,
30        chainviewi0_.source            as source30_112_,
31        chainviewi0_.startDate         as startDate31_112_,
32        chainviewi0_.startTime         as startTime32_112_,
33        chainviewi0_.storeCode         as storeCode33_112_,
34        chainviewi0_.totalLimit        as totalLimit34_112_,
35        chainviewi0_.upc               as upc35_112_,
36        chainviewi0_.userDailyLimit    as userDailyLimit36_112_,
37        chainviewi0_.userLimit         as userLimit37_112_,
38        chainviewi0_.zkfd              as zkfd38_112_,
39        chainviewi0_.zkl               as zkl39_112_
40   from dbusrchain.view_chain_ItemDiscountItem chainviewi0_
41  where chainviewi0_.poi = :1
42    and chainviewi0_.chainSku_id = :2
43    and chainviewi0_.approved = :3
44    and chainviewi0_.effective = :4
45  order by chainviewi0_.source       desc nulls last,
46           chainviewi0_.approvedDate desc nulls last,
47           chainviewi0_.id           desc
48 
49 
50 根据ADDM，latch ID 559
51 
52 SQL> select * from v$latchname where latch#=559;
53     LATCH# NAME                                                             DISPLAY_NAME                                                    D
54 
55 ---------- ---------------------------------------------------------------- -----------------------------------------------------------------
56 
57        559 Result Cache: RC Latch                                           Result Cache: RC Latch       

latch free的真正原因是RC Latch

查询mos发现

The cause of this issue is automatic dynamic statistics which is enabled by default in 12c automatically decides whether dynamic statistics are useful and which statistics level to use for all SQL statements.

It collects dynamic statistics when the optimizer deems it necessary.

When Automatic Dynamic Sampling is used for the SQL statements, it can decide, based upon these statistics, that a better response time could be achieved by using the result cache for those queries.

This can cause heavy usage of the result cache leading to the contention on latch free for “Result Cache: RC Latch”.

也就是说,12c在自动采样有改进，而且默认使用result cache特性，从而引起该问题，即使你设置了 RESULT_CACHE_MODE = MANUAL，依旧会有大量动态采样引起 Result Cache: RC Latch，

彻底解决给问题就是通过隐含参数禁止Automatic Dynamic Statistics使用result cache

可以通过调整隐含参数规避

1 alter system set "_optimizer_ads_use_result_cache" = FALSE；

设置该优化参数之后效果

业务反馈已经运行正常