Case 2 of SQL performance degradation caused by modifying implicit parameters

The customer sent such an execution time 1.3 Hours of SQL Want to optimize ：

SQL Code ：

INSERT INTO HWDW.TW_CI_SIM_RES (

......

)

SELECT ......

FROM HWODS.TO_CI_I91822 S ----- The big table

,(

SELECT PHONE_NO,TD_PHONE_NO

FROM HWDW.TW_CI_PHONE_TD_INFO----- Watch

WHERE STATIS_DATE = :B1

)T

WHERE S.PHONE_NO = T.TD_PHONE_NO(+);

adopt SQL Monitor The displayed data found that the execution plan was abnormal ： stay Estimate-rows When the valuation is normal ,hash join The left table is made of the big table of , In this case, the relevant parameters are changed .

Sure enough , The inspection found that ,_right_outer_hash_enable The implicit parameter has been changed to false . If you change the parameter back to the default TRUE, expect SQL Performance will improve 10 About times .

explain ：

If set _right_outer_hash_enable=false , Then the external association can only be done by the main table hash join Left table of , Regardless of the size of the table , Large in main table , When the association table is small , The performance will be poor .

This parameter may have been used in a previous version bug, When the customer upgrades, the original set value is still retained , It is recommended to confirm each parameter one by one during upgrade , Maximize database performance .