为什么要写本篇文章？先来看个案例吧
2012年还是2013具体记不清楚了，某运营商，Oracle10gR2
select count(*) from a left join b on a.id=b.id;a和b上千万行数据，a.id，b.id都是主键，SQL跑12秒。因为b.id是主键，a left join b不管有没有关联上，最终返回的数据始终是a的总行数，所以当时把SQL改写为：
select count(*) from a;最终0.05秒就能返回结果(到了Oracle11g之后，CBO会自动改写上面SQL，去掉b)
我到现在还记得当时优化完这条SQL之后，我得了500块奖励，开发被扣了500块，并且被屌了一顿。
最近几年也做了大量的去O项目，数据库以openGauss系为主，遇到了下面问题：
1.ORDER BY消除问题，
2.DISTINCT/UNIQUE消除问题
3.LEFT JOIN 分析函数，标量子查询，自定义函数消除问题
后来随着数据库内核版本迭代，上面问题基本上都解决了。
现在来测试崖山是否支持上面提到的各种消除技术(其他国产数据库懒得测试了，篇幅不允许)
数据库版本：yashan23.5.1
dept和emp表来自Oracle11g SCOTT示例账户，dept.deptno列是主键，emp.empno列是主键，emp.deptno列是外键，引用dept.deptno
TEST02数据来自DBA_OBJECTS，共8w行数据
TEST01数据来自TEST02，重复512次，共4500W行
TEST02，TEST01不创建索引
1.ORDER BY 消除
select count(*) from (select * from emp order by empno);Oracle
SQL> select count(*) from (select * from emp order by empno);
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 2937609675
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FULL SCAN| PK_EMP | 14 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------yashan
SQL> select count(*) from (select * from emp order by empno);
Execution Plan
----------------------------------------------------------------
SQL hash value: 65278406
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 1| | 34| 2| 0| 0| |
| 1 | AGGREGATE | | | 1| 1| 1( 0)| 32| 2| 0| 0| |
| 2 | VIEW | | | 14| 14| 1( 0)| 30| 15| 0| 0| |
| 3 | INDEX FAST FULL SCAN | PK_EMP | SCOTT | 14| 14| 1( 0)| 27| 15| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
7 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
0 bytes sent via PX
0 block received
24 rows fetched.
Elapsed: 00:00:00.011结论：崖山支持ORDER BY消除
2.单表DISTINCT消除
select distinct * from emp;Oracle
SQL> select distinct * from emp;
14 rows selected.
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 532 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMP | 14 | 532 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------yashan
SQL> select distinct * from emp;
Execution Plan
----------------------------------------------------------------
SQL hash value: 3454160452
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 14| | 10| 15| 0| 0| |
| 1 | TABLE ACCESS FULL | EMP | SCOTT | 14| 14| 1( 0)| 9| 15| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
7 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
14 rows processed
0 bytes sent via PX
0 block received
22 rows fetched.
Elapsed: 00:00:00.004结论：崖山支持单表DISTINCT消除
3.标量子查询消除
select count(*)
from (select a.*,
(select count(*) from test01 b where a.object_id = b.object_id) cnt
from test02 a);Oracle
SQL> select count(*)
2 from (select a.*,
3 (select count(*) from test01 b where a.object_id = b.object_id) cnt
4 from test02 a);
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 322432667
---------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 347 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| TEST02 | 86987 | 347 (1)| 00:00:01 |
---------------------------------------------------------------------yashan
SQL> select count(*)
from (select a.*,
(select count(*) from test01 b where a.object_id = b.object_id) cnt
from test02 a); 2 3 4
Execution Plan
----------------------------------------------------------------
SQL hash value: 232128976
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 1| | 19689| 2| 0| 0| |
| 1 | SUBQUERY | QUERY[1] | | | | | | | | | |
| 2 | AGGREGATE | | | 1| | 1150846( 0)| | | | | |
|* 3 | TABLE ACCESS FULL | TEST01 | SCOTT | 446| | 1150846( 0)| | | | | |
| 4 | AGGREGATE | | | 1| 1|99005259776( 0)| 19688| 2| 0| 0| |
| 5 | VIEW | | | 86987| 86987|99005259776( 0)| 16413| 86988| 0| 0| |
| 6 | TABLE ACCESS FULL | TEST02 | SCOTT | 86987| 86987| 212( 0)| 6623| 86988| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Operation Information (identified by operation id):
---------------------------------------------------
3 - Predicate : filter("A"."OBJECT_ID" = "B"."OBJECT_ID")
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
1315 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
0 bytes sent via PX
0 block received
32 rows fetched.
Elapsed: 00:00:00.024结论：崖山支持标量子查询消除(标量子查询部分A-TIME为空，说明没执行)，执行计划中应该不显示标量子查询，建议崖山后续修复此问题。还好查看执行计划用的是A-TIME方式，如果用EXPLAIN看执行计划会误认为崖山不支持标量子查询消除
4.自定义函数消除
CREATE OR REPLACE FUNCTION f_get_cnt_by_owner(p_owner varchar) RETURN int AS
v_cnt int;
BEGIN
IF 1=1 THEN ---加了这里
select count(*) into v_cnt from test01 where owner = p_owner;
END IF; ---加了这里
RETURN v_cnt;
END;
/Oracle
SQL> select count(*)
2 from (select a.*,
3 f_get_cnt_by_owner(owner) cnt
4 from test02 a);
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 322432667
---------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
---------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 347 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| TEST02 | 86987 | 347 (1)| 00:00:01 |
---------------------------------------------------------------------yashan
SQL> select count(*)
from (select a.*,
f_get_cnt_by_owner(owner) cnt
from test02 a); 2 3 4
Execution Plan
----------------------------------------------------------------
SQL hash value: 2742543714
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 1| | 3280| 2| 0| 0| |
| 1 | TABLE ACCESS FULL (AGGR PUSHED)| TEST02 | SCOTT | 1| 1| 212( 0)| 3277| 2| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
1315 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
0 bytes sent via PX
0 block received
22 rows fetched.
Elapsed: 00:00:00.008结论：崖山支持自定义函数消除，不然不可能只跑0.008秒
5.INNER JOIN DISTINCT消除
select count(*) from (select distinct * from emp e inner join dept d on e.deptno=d.deptno);
select count(*) from (select unique * from emp e inner join dept d on e.deptno=d.deptno); Oracle
SQL> select count(*) from (select distinct * from emp e inner join dept d on e.deptno=d.deptno);
Elapsed: 00:00:00.02
Execution Plan
----------------------------------------------------------
Plan hash value: 680252336
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX FULL SCAN| IDX_EMP_DEPTNO | 14 | 42 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("E"."DEPTNO" IS NOT NULL)yashan
SQL> select count(*) from (select distinct * from emp e inner join dept d on e.deptno=d.deptno);
Execution Plan
----------------------------------------------------------------
SQL hash value: 1267998346
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 1| | 133| 2| 0| 0| |
| 1 | AGGREGATE | | | 1| 1| 1( 0)| 133| 2| 0| 0| |
| 2 | VIEW | | | 14| 14| 1( 0)| 131| 15| 0| 0| |
| 3 | SORT DISTINCT | | | 14| 14| 1( 0)| 123| 15| 0| 0| |
| 4 | NESTED LOOPS INNER | | | 14| 14| 1( 0)| 101| 15| 0| 0| |
| 5 | TABLE ACCESS FULL | EMP | SCOTT | 14| 14| 1( 0)| 10| 15| 0| 0| |
| 6 | TABLE ACCESS BY INDEX ROWID| DEPT | SCOTT | 1| | 1( 0)| | | | | |
|* 7 | INDEX UNIQUE SCAN | PK_DEPT | SCOTT | 1| 6| 1( 0)| 23| 12| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Operation Information (identified by operation id):
---------------------------------------------------
3 - Distinct Expression: ("E"."EMPNO"[OPTMZ-3], "D"."LOC"[OPTMZ-2], "D"."DNAME"[OPTMZ-2])
4 - Execution : NDV ENABLE : 1 NDV Affected Rows : 11 NDV Match Rows : 8 NDV Mem Blocks : 6 NDV Mem Key Size : 27 NDV Mem Value Size : 77 NDV Cost : 0 No NDV Cost : 0
Nest Loop Join NDV Expression: ("E"."DEPTNO"[OPTMZ-3][OPTMZ-0])
7 - Predicate : access("D"."DEPTNO" = "E"."DEPTNO"[OPTMZ-3][OPTMZ-2])
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
19 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
0 bytes sent via PX
0 block received
36 rows fetched.
Elapsed: 00:00:00.003
SQL> select count(*) from (select unique * from emp e inner join dept d on e.deptno=d.deptno);
[1:30]YAS-04213 expression expected
Elapsed: 00:00:00.000结论：崖山不支持INNER JOIN DISTINCT消除，另外崖山还不支持SELECT UNIQUE 语法
6.LEFT JOIN 消除
select e.empno,e.ename from emp e left join dept d on e.deptno=d.deptno;Oracle
SQL> select e.empno,e.ename from emp e left join dept d on e.deptno=d.deptno;
14 rows selected.
Elapsed: 00:00:00.00
Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 140 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMP | 14 | 140 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------yashan
SQL> select e.empno,e.ename from emp e left join dept d on e.deptno=d.deptno;
Execution Plan
----------------------------------------------------------------
SQL hash value: 2062330261
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 14| | 2447| 15| 0| 0| |
| 1 | NESTED LOOPS LEFT OUTER | | | 14| 14| 1( 0)| 2436| 15| 0| 0| |
| 2 | TABLE ACCESS FULL | EMP | SCOTT | 14| 14| 1( 0)| 46| 15| 0| 0| |
|* 3 | INDEX UNIQUE SCAN | PK_DEPT | SCOTT | 1| 6| 1( 0)| 38| 12| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Operation Information (identified by operation id):
---------------------------------------------------
1 - Execution : NDV ENABLE : 1 NDV Affected Rows : 11 NDV Match Rows : 8 NDV Mem Blocks : 6 NDV Mem Key Size : 27 NDV Mem Value Size : 27 NDV Cost : 0 No NDV Cost : 0
Nest Loop Join NDV Expression: ("E"."DEPTNO"[OPTMZ-3][OPTMZ-0])
3 - Predicate : access("D"."DEPTNO" = "E"."DEPTNO"[OPTMZ-3][OPTMZ-2])
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
13 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
14 rows processed
0 bytes sent via PX
0 block received
31 rows fetched.
Elapsed: 00:00:00.006结论：崖山不支持LEFT JOIN消除
后面不用测试了，Oracle都支持，崖山都不支持
7.LEFT JOIN DISTINCT/UNIQUE消除
select count(*) from (select distinct * from emp e left join dept d on e.deptno=d.deptno);8.LEFT JOIN 分析函数消除
select count(*)
from (select e.*, row_number() over(partition by e.deptno order by d.loc) rn
from emp e
left join dept d
on e.deptno = d.deptno);...等等等，还有很多类似的变种SQL就不一一列举了...
9.LEFT JOIN GROUP BY消除
虽然Oracle支持很多消除技术，但是Oracle目前还不支持我认为很重要的LEFT JOIN GROUP BY消除
比如下面SQL
select count(*)
from test02 a
left join (select object_id, count(*) from test01 group by object_id) b
on a.object_id = b.object_id;PG18
postgres=# explain analyze select count(*)
postgres-# from test02 a
postgres-# left join (select object_id, count(*) from test01 group by object_id) b
postgres-# on a.object_id = b.object_id;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Aggregate (cost=2528.34..2528.35 rows=1 width=8) (actual time=7.282..7.283 rows=1.00 loops=1)
Buffers: shared hit=1441
-> Seq Scan on test02 a (cost=0.00..2310.87 rows=86987 width=0) (actual time=0.027..4.281 rows=86987.00 loops=1)
Buffers: shared hit=1441
Planning Time: 0.066 ms
Execution Time: 7.303 ms
(6 rows)
Time: 7.618 msOracle
SQL> select count(*)
2 from test02 a
3 left join (select object_id, count(*) from test01 group by object_id) b
4 on a.object_id = b.object_id;
Elapsed: 00:00:13.38
Execution Plan
----------------------------------------------------------
Plan hash value: 3748704499
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 18 | | 222K (1)| 00:00:09 |
| 1 | SORT AGGREGATE | | 1 | 18 | | | |
|* 2 | HASH JOIN OUTER | | 86987 | 1529K| | 222K (1)| 00:00:09 |
| 3 | TABLE ACCESS FULL | TEST02 | 86987 | 424K| | 347 (1)| 00:00:01 |
| 4 | VIEW | | 88136 | 1118K| | 221K (1)| 00:00:09 |
| 5 | HASH GROUP BY | | 88136 | 430K| 511M| 221K (1)| 00:00:09 |
| 6 | TABLE ACCESS FULL| TEST01 | 44M| 212M| | 172K (1)| 00:00:07 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."OBJECT_ID"="B"."OBJECT_ID"(+))yashan
SQL> select count(*)
from test02 a
left join (select object_id, count(*) from test01 group by object_id) b
on a.object_id = b.object_id; 2 3 4
Execution Plan
----------------------------------------------------------------
SQL hash value: 2609380292
Optimizer: ADOPT_C
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| Id | Operation type | Name | Owner | E - Rows | A - Rows | Cost(%CPU) | A - Time | Loops | Memory | Disk | Partition info |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
| 0 | SELECT STATEMENT | | | | 1| | 8506448| 2| 0| 0| |
| 1 | AGGREGATE | | | 1| 1| 1149511( 0)| 8506444| 2| 0| 0| |
|* 2 | HASH JOIN LEFT OUTER | | | 86987| 86987| 1149510( 0)| 8503307| 86988| 0| 0| |
| 3 | TABLE ACCESS FULL | TEST02 | SCOTT | 86987| 86987| 212( 0)| 4907| 86988| 0| 0| |
| 4 | VIEW | | | 86987| 86987| 1149278( 0)| 8464424| 86988| 0| 0| |
| 5 | HASH GROUP | | | 86987| 86987| 1149278( 0)| 8454903| 86988| 0| 0| |
| 6 | TABLE ACCESS FULL | TEST01 | SCOTT | 44537344| 44537344| 1145820( 0)| 2346533| 44537345| 0| 0| |
+----+--------------------------------+----------------------+------------+----------+----------+-------------+----------+----------+----------+----------+--------------------------------+
Operation Information (identified by operation id):
---------------------------------------------------
2 - Execution : Algorithm : 1 PartCount : 1 BuildCount : 86987 BuildDistinct : 87208 BuildSize : 3827428 IsCacheProbe : 0
Predicate : access("A"."OBJECT_ID"[OPTMZ-3] = "TEST01"."OBJECT_ID"[OPTMZ-2])
5 - Execution : PartCount : 1 RowSetCount : 1 TotalChunks : 125 EstimateDistinct : 86987 HllEstimateDistinct : 87208 RealDistinct : 86987 TotalRows : 44537344 MatRows : 86987 SwapCount : 0 Dop : 1 Is2PhaseAggr : 1 CombineTime : 1
Group Expression: ("TEST01"."OBJECT_ID"[OPTMZ-1])
Statistics
----------------------------------------------------------------------------------------------------
0 physical reads
651026 db block gets
0 consistent gets
0 redo size
0 recursive calls
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
0 bytes sent via PX
0 block received
35 rows fetched.
Elapsed: 00:00:08.511结论：PG(其他国产数据库没测)支持LEFT JOIN GROUP BY消除，Oracle，崖山不支持LEFT JOIN GROUP BY消除
Oracle优化器目前还不支持LEFT JOIN GROUP BY消除功能，本人遇到过N次相关案例，最终都是通过人工改写SQL手动消除达到优化目的
举个例子吧(限于SQL原始文本过于复杂，VIEW过于复杂，就不拿出来分享了，见谅)：
比如select ...,复杂view一个列 from a left join 复杂view，复杂view里面一堆left join group by left join group by ...
SQL只取复杂view一个列(来自主表)，但是复杂view后面有一大堆left join group by，因为Oracle没有left join group by消除功能，所以SQL跑得慢
最终把后面一大堆left join group by干掉后秒杀
结束语：
从内核研发角度来说，上面提到的各种消除技术实现并不难，可能是崖山之前没遇到类似场景，也可能是遇到了类似场景，研发主要精力不在这，相信崖山后面很快能实现上述功能。
PS：还有些消除技术本文没有列举，见谅