sql查询中尽量减少的使用

在数据库中抓到了下面的sql，每小时执行6次，查询条件中大部分是status字段，我推断应该是监控程序，每10分钟运行一次。监控程序一般都是提取异常数据，数据量很少，要不然监控就失去意义了，像这种sql一般是要通过索引迅速定位到数据的。

原始sql：

SQL> select *
2 from tc_xx_trade t
3 where t.xx_status <> 4
4 and t.xx_status <> 5
5 and t.cs_status = 1
6 and t.timeout_type <> 'STOP_TIMEOUT'
7 and t.gmt_xiaoer_inter is not null
8 and t.gmt_xiaoer_inter < sysdate
9 and rownum < 50;

Execution Plan
--------------------------------------------------
SELECT STATEMENT Optimizer=CHOOSE (Cost=9655 Card=49 Bytes=15092)
COUNT (STOPKEY)
TABLE ACCESS (FULL) OF 'TC_xx_TRADE' (Cost=9655 Card
=263124 Bytes=81042192)

Statistics
----------------------------------------------------------
1 recursive calls
1 db block gets
109922 consistent gets
101640 physical reads
120 redo size
5552 bytes sent via SQL*Net to client
656 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
9 rows processed
sql走全表扫描，根据上面的判断我认为cs_status=1的数据量应该很小，走索引会很快。

加上hint：

SQL> select/*+index(t IDX_TC_REF_TRA_GMT_XIAOER)*/ *
2 from tc_xx_trade t
3 where t.xx_status <> 4
4 and t.xx_status <> 5
5 and t.cs_status = 1
6 and t.timeout_type <> 'STOP_TIMEOUT'
7 and t.gmt_xiaoer_inter is not null
8 and t.gmt_xiaoer_inter < sysdate
9 and rownum < 50;

Execution Plan
---------------------------------------------------
SELECT STATEMENT Optimizer=CHOOSE (Cost=113996 Card=49 Bytes =15092)
0 COUNT (STOPKEY)
1 TABLE ACCESS (BY INDEX ROWID) OF 'TC_xx_TRADE' (Cost
=113996 Card=263208 Bytes=81068064)
2 INDEX (RANGE SCAN) OF 'IDX_TC_REF_TRA_GMT_XIAOER' (NON
-UNIQUE) (Cost=32246 Card=273964)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
61745 consistent gets
34586 physical reads
65124 redo size
11553 bytes sent via SQL*Net to client
667 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
28 rows processed
逻辑读还是很大，这表示cs_status=1的数据量应该是很多的，过滤性很差。

监控一般只会取很少的一部分数据，哪个条件具有过滤性呢，不了解业务的情况下我们只能去推测，把xx_status<>转换为 in（）试试看。

<>转化为in：

SQL> select *
2 from tc_xx_trade t
3 where t.xx_status in (1, 2, 3, 6)
4 and t.cs_status = 1
5 and t.timeout_type <> 'STOP_TIMEOUT'
6 and t.gmt_xiaoer_inter is not null
7 and t.gmt_xiaoer_inter < sysdate
8 and rownum < 50;

Execution Plan
----------------------------------------------------------
SELECT STATEMENT Optimizer=CHOOSE (Cost=541550 Card=49 Bytes =15092)
0 COUNT (STOPKEY)
1 INLIST ITERATOR
2 TABLE ACCESS (BY INDEX ROWID) OF 'TC_xx_TRADE' (Co
st=541550 Card=1462696 Bytes=450510368)
3 INDEX (RANGE SCAN) OF 'IDX_TC_REF_TRA_GMT_XIAOER' (N
ON-UNIQUE) (Cost=4475 Card=1462696)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
898 consistent gets
0 physical reads
0 redo size
15366 bytes sent via SQL*Net to client
689 bytes received via SQL*Net from client
5 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
49 rows processed
把<>转化成in字段，效果好了很多，原理我就不多解释了，实际上和用<>不走索引，会去扫描整个表是一样的道理。






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