一：序列的nocache导致的log file sync现象
发现数据库大量的log file sync等待，awr报告的top10 wait time如下
数据库中也是大量的查询操作，少量的dml操作，初步判断是查询大量的nocache的序列导致大量的commit 导致log file sync
Cache就是Oracle每次向Sequence进行请求时，分配出的独立数字数量。例如，当我们使用<seq_name>.nextval获取一个独立值时，Oracle需要将sequence对象的数据字典信息更新。如果我们设置cache为10，那么第一次请求nextval的时候，就更新数据字典信息增加10，取出的10个号放在Oracle服务器的缓存中。
在以后每次请求nextval的时候，Oracle就从服务器缓存中去获取序列值。而不需要更新数据字典信息。只有在分配到缓存的10个数字都已经分配完，或者因为缓存刷新操作剩余数字被清理的情况下，才会再次调用sequence分配机制，再次分出cache个数字。
查询库里cache小于1000的序列
select 'alter sequence '||sequence_owner||'.'||sequence_name||' cache 1000; ',t.* from dba_sequences t where sequence_owner in (
'XXX'
) and cache_size<1000 order by max_value
修改序列的cache
altersequence XXX.x_SEQ_ALARM_ID cache1000;
二：实验 nocache序列导致产生的redo size 大小
实验如下：
--创建nocache的序列
SQL> create sequence seq_nocache minvalue 1 maxvalue 999 start with 2 increment by 1 nocache;
Sequence created.
--查询nocache的序列，有 628的redo size出现
SQL> set autotrace traceonly statistics;
SQL> select seq_nocache.nextval from dual;
Statistics
----------------------------------------------------------
6 recursive calls
3 db block gets
3 consistent gets
0 physical reads
628 redosize
525 bytes sent via SQL*Net toclient
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> SET AUTOTRACE OFF;
三：实验 cache序列导致产生的redo size 大小
--创建cache的序列(cache 为5，会在第1次取出5条记录 产生redo size，之后会再第6次查询时取出5条记录，产生redosize)
SQL> create sequence seq_cache minvalue 1 maxvalue 999 start with 2 increment by 1 cache 5;
Sequence created.
--第一次查询cache的序列，有 676的redo size出现
SQL> set autotrace traceonly statistics;
SQL> select seq_cache.nextval from dual;
Statistics
----------------------------------------------------------
3 recursive calls
3 db block gets
3 consistent gets
0 physical reads
676 redo size
525 bytes sent via SQL*Net toclient
523 bytes received via SQL*Netfrom client
2 SQL*Net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> SET AUTOTRACE OFF;
--第二次查询cache的序列，redo size为0 了，不产生redosize了
SQL> set autotrace traceonly statistics;
SQL> select seq_cache.nextval from dual;
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redosize
525 bytes sent via SQL*Net toclient
523 bytes received via SQL*Netfrom client
2 SQL*Net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> SET AUTOTRACE OFF;
--直到第6次查询才再次出现 redosize
SQL> set autotrace traceonly statistics;
SQL> select seq_cache.nextval from dual;
Statistics
----------------------------------------------------------
1 recursive calls
3 db block gets
1 consistent gets
0 physical reads
652 redo size
525 bytes sent via SQL*Net toclient
523 bytes received via SQL*Netfrom client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> SET AUTOTRACE OFF;