更新时间：2022-08-16 GMT+08:00
案例：改写SQL消除子查询（案例2）
现象描述
某局点客户反馈如下SQL语句的执行时间超过1天未结束：
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UPDATE calc_empfyc_c_cusr1 t1
SET ln_rec_count =
(
SELECT CASE WHEN current_date - ln_process_date + 1 <= 12 THEN 0 ELSE t2.ln_rec_count END
FROM calc_empfyc_c1_policysend_tmp t2
WHERE t1.ln_branch = t2.ln_branch AND t1.ls_policyno_cusr1 = t2.ls_policyno_cusr1
)
WHERE dsign = '1'
AND flag = '1'
AND EXISTS
(SELECT 1
FROM calc_empfyc_c1_policysend_tmp t2
WHERE t1.ln_branch = t2.ln_branch AND t1.ls_policyno_cusr1 = t2.ls_policyno_cusr1
);
|
对应的执行计划如下：
优化说明
很明显，执行计划中存在SubPlan，并且SubPlan中的运算相当重，即此SubPlan是一个明确的性能瓶颈点。
根据SQL语意等价改写SQL消除SubPlan如下：
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UPDATE calc_empfyc_c_cusr1 t1
SET ln_rec_count = CASE WHEN current_date - ln_process_date + 1 <= 12 THEN 0 ELSE t2.ln_rec_count END
FROM calc_empfyc_c1_policysend_tmp t2
WHERE
t1.dsign = '1' AND t1.flag = '1'
AND t1.ln_branch = t2.ln_branch AND t1.ls_policyno_cusr1 = t2.ls_policyno_cusr1;
|
改写之后SQL语句在50S内执行完成
父主题： 实际调优案例
