领导让我帮忙支持下其他项目的SQL优化工作，呦西，是收集案例的好时机。😍
下面SQL都是在不能远程的情况下，按照原SQL的逻辑等价改写完成否发给现场同事验证。
案例一
慢SQL，4.32秒：
SELECT MY_.*, RM
FROM (SELECT ROWNUM RM, V_.*
FROM (SELECT *
FROM (select count(1) processidnum,
t.processid,
t.proc_name_ as procname
FROM tkdkdkdk t
WHERE 1 = 1
and (t.ASSIGNEE_ = 'server' or exists(select 1
FROM pepepep p
WHERE p.task_ = t.ID_
and (p.agent_userid_ = 'server' or
(substr(p.groupid_, 6) in
(select role_code
FROM upupupup
WHERE user_code = 'server') or
p.userid_ = 'server'))))
GROUP BY t.processid, t.proc_name_)) V_
WHERE ROWNUM <= 100000) MY_
WHERE RM >= 1;慢SQL执行计划：
改写优化，445ms：
SELECT *
FROM (SELECT *
FROM (SELECT a.*,
rownum rn
FROM (SELECT count(1) processidnum,
t.processid,
t.proc_name_ AS procname
FROM tkdkdkdk t
LEFT JOIN
(SELECT distinct p.task_
FROM pepepep p
LEFT JOIN
(SELECT role_code
FROM upupupup
WHERE user_code = 'server'
GROUP BY role_code) tsu
ON (substr(p.groupid_, 6) = tsu.role_code)
WHERE (p.agent_userid_ = 'server'
OR (tsu.role_code is NOT null
OR p.userid_ = 'server'))) x
ON t.ID_ = x.task_
WHERE 1 = 1
AND (t.ASSIGNEE_ = 'server'
OR x.task_ is NOT NULL)
GROUP BY t.processid, t.proc_name_) a)
WHERE rownum <= 100000)
WHERE rn >= 1; 改写优化后执行计划：
优化思路：
** 1、原SQL有很多子查询，可能会导致计划走NL，改成JOIN后让CBO自动判断是否走HASH还是NL。**
** 2、换了个标准的分页框架。**
案例二
慢SQL，2.6秒：
SELECT MY_.*, RM
FROM (SELECT ROWNUM RM, V_.*
FROM (SELECT *
FROM (select t.*, t.org_code || '-' || t.org_name as codename
FROM (select tc.*
FROM tgtgtgtg tc
start with TC.ORG_ID = '6000001'
connect by prior ORG_ID = tc.parent_id) t
WHERE org_level <= 3
ORDER BY CASE
WHEN ',' || nvl(null, 'fingard') || ',' like '%,' || ORG_ID || ',%' THEN CASE
WHEN length(nvl(org_order, '')) = '9'
then org_order || ''
else '1' || org_code end
when length(nvl(org_order, '')) = '9' then '99999999' || org_order || ''
else '999999991' || org_code end)) V_
WHERE ROWNUM <= 10) MY_;
WHERE RM >= 1;
改写优化一，3.4秒：
SELECT MY_.*, RM
FROM (SELECT ROWNUM RM, V_.*
FROM (SELECT *
FROM (select a.*, a.org_code || '-' || a.org_name as codename
FROM (WITH t(
lv,
codename,
ORG_ID,
parent_id,
org_order,
org_code,
org_name,
org_level
) AS (SELECT 1 as lv,
tc.org_code || '-' || tc.org_name AS codename,
tc.org_name,
tc.ORG_ID,
tc.parent_id,
tc.org_order,
tc.org_code,
tc.org_level
FROM tgtgtgtg tc
WHERE tc.ORG_ID = '6000001'
UNION ALL
SELECT t.lv + 1,
e.org_code || '-' || e.org_name AS codename,
e.org_name,
e.ORG_ID,
e.parent_id,
e.org_order,
e.org_code,
e.org_level
FROM tgtgtgtg e
INNER JOIN t ON t.ORG_ID = e.parent_id)
SELECT *
FROM t) a
WHERE a.org_level <= 3
ORDER BY CASE
WHEN ',' || nvl(null, 'fingard') || ',' like '%,' || ORG_ID || ',%' THEN CASE
WHEN length(nvl(org_order, '')) = '9'
then org_order || ''
else '1' || org_code end
when length(nvl(org_order, '')) = '9' then '99999999' || org_order || ''
else '999999991' || org_code end)) V_
WHERE ROWNUM <= 10) MY_;
WHERE RM >= 1;
使用CTE递归改写方案在PostgreSQL上是个通用的做法，也能取得比较好的性能效果。
但是在OB上反而效果更差点，NL算子性能不够强，使用NESTED-LOOP JOIN 性能反而没有NESTED-LOOP CONNECT BY 算子好。****
OB研发在NESTED-LOOP JOIN算子上还有继续优化的空间。
改写优化二，1.5秒：
既然使用NL性能不够理想的情况下，就要想办法使用HASH来优化SQL整体的执行效率。
将自动递归的方式改成手动。
1、首先需要知道数据整体的层级有多少。
SELECT DISTINCT lv
FROM (SELECT level lv
FROM tgtgtgtg tc
START WITH TC.ORG_ID = '6000001'
CONNECT BY PRIOR ORG_ID = tc.parent_id) t; 
2、了解到整体的数据是13层，然后使用self join 将不同层级的数据关联起来。
SELECT *
FROM (SELECT *
FROM (SELECT a.*, rownum rn
FROM (SELECT x.*
FROM (WITH tgtgtgtg AS
(SELECT org_code, org_name, org_id, parent_id, org_order, org_level
FROM tgtgtgtg)
SELECT 1 AS lv,
v1.org_code || '-' || v1.org_name AS codename,
v1.ORG_ID,
v1.parent_id,
v1.org_order,
v1.org_code,
v1.org_level
FROM tgtgtgtg v1
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 2 AS lv,
v2.org_code || '-' || v2.org_name AS codename,
v2.ORG_ID,
v2.parent_id,
v2.org_order,
v2.org_code,
v2.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 3 AS lv,
v3.org_code || '-' || v3.org_name AS codename,
v3.ORG_ID,
v3.parent_id,
v3.org_order,
v3.org_code,
v3.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 4 AS lv,
v4.org_code || '-' || v4.org_name AS codename,
v4.ORG_ID,
v4.parent_id,
v4.org_order,
v4.org_code,
v4.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 5 AS lv,
v5.org_code || '-' || v5.org_name AS codename,
v5.ORG_ID,
v5.parent_id,
v5.org_order,
v5.org_code,
v5.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 6 AS lv,
v6.org_code || '-' || v6.org_name AS codename,
v6.ORG_ID,
v6.parent_id,
v6.org_order,
v6.org_code,
v6.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 7 AS lv,
v7.org_code || '-' || v7.org_name AS codename,
v7.ORG_ID,
v7.parent_id,
v7.org_order,
v7.org_code,
v7.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 8 AS lv,
v8.org_code || '-' || v8.org_name AS codename,
v8.ORG_ID,
v8.parent_id,
v8.org_order,
v8.org_code,
v8.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
JOIN tgtgtgtg v8 ON v7.ORG_ID = v8.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 9 AS lv,
v9.org_code || '-' || v9.org_name AS codename,
v9.ORG_ID,
v9.parent_id,
v9.org_order,
v9.org_code,
v9.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
JOIN tgtgtgtg v8 ON v7.ORG_ID = v8.parent_id
JOIN tgtgtgtg v9 ON v8.ORG_ID = v9.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 10 AS lv,
v10.org_code || '-' || v10.org_name AS codename,
v10.ORG_ID,
v10.parent_id,
v10.org_order,
v10.org_code,
v10.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
JOIN tgtgtgtg v8 ON v7.ORG_ID = v8.parent_id
JOIN tgtgtgtg v9 ON v8.ORG_ID = v9.parent_id
JOIN tgtgtgtg v10 ON v9.ORG_ID = v10.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 11 AS lv,
v11.org_code || '-' || v11.org_name AS codename,
v11.ORG_ID,
v11.parent_id,
v11.org_order,
v11.org_code,
v11.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
JOIN tgtgtgtg v8 ON v7.ORG_ID = v8.parent_id
JOIN tgtgtgtg v9 ON v8.ORG_ID = v9.parent_id
JOIN tgtgtgtg v10 ON v9.ORG_ID = v10.parent_id
JOIN tgtgtgtg v11 ON v10.ORG_ID = v11.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 12 AS lv,
v12.org_code || '-' || v12.org_name AS codename,
v12.ORG_ID,
v12.parent_id,
v12.org_order,
v12.org_code,
v12.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
JOIN tgtgtgtg v8 ON v7.ORG_ID = v8.parent_id
JOIN tgtgtgtg v9 ON v8.ORG_ID = v9.parent_id
JOIN tgtgtgtg v10 ON v9.ORG_ID = v10.parent_id
JOIN tgtgtgtg v11 ON v10.ORG_ID = v11.parent_id
JOIN tgtgtgtg v12 ON v11.ORG_ID = v12.parent_id
WHERE v1.ORG_ID = '6000001'
UNION ALL
SELECT 13 AS lv,
v13.org_code || '-' || v13.org_name AS codename,
v13.ORG_ID,
v13.parent_id,
v13.org_order,
v13.org_code,
v13.org_level
FROM tgtgtgtg v1
JOIN tgtgtgtg v2 ON v1.ORG_ID = v2.parent_id
JOIN tgtgtgtg v3 ON v2.ORG_ID = v3.parent_id
JOIN tgtgtgtg v4 ON v3.ORG_ID = v4.parent_id
JOIN tgtgtgtg v5 ON v4.ORG_ID = v5.parent_id
JOIN tgtgtgtg v6 ON v5.ORG_ID = v6.parent_id
JOIN tgtgtgtg v7 ON v6.ORG_ID = v7.parent_id
JOIN tgtgtgtg v8 ON v7.ORG_ID = v8.parent_id
JOIN tgtgtgtg v9 ON v8.ORG_ID = v9.parent_id
JOIN tgtgtgtg v10 ON v9.ORG_ID = v10.parent_id
JOIN tgtgtgtg v11 ON v10.ORG_ID = v11.parent_id
JOIN tgtgtgtg v12 ON v11.ORG_ID = v12.parent_id
JOIN tgtgtgtg v13 ON v12.ORG_ID = v13.parent_id
WHERE v1.ORG_ID = '6000001') x
WHERE org_level <= 3
ORDER BY CASE
WHEN ',' || NVL(NULL, 'fingard') || ',' LIKE '%,' || ORG_ID || ',%' THEN
CASE
WHEN LENGTH(NVL(org_order, '')) = '9' THEN
org_order || ''
ELSE '1' || org_code
END
WHEN LENGTH(NVL(org_order, '')) = '9' THEN
'99999999' || org_order || ''
ELSE '999999991' || org_code END ) a)
WHERE rownum <= 10)
WHERE rn >= 1;
现场同学差集比较，确认改写后的SQL是等价的，执行时间从2.6秒降低到1.5秒能跑出结果。
原来18行的SQL改成了250多行后才优化了1秒的执行时间，实在没其他办法了，希望OB产研后续能CBO算子继续优化下。😂😂😂