arraysize 对性能的影响
arraysize 和 fetch size 参数都是客户段的一个参数，需要在客户段来设置，arraysize 是在sqlplus 中设置的，如果通过程序去连数据库，那么这个参数就是fetch size 。Arraysize specifies how manyrows SQL*Plus will fetch in a call. The number n can be between 1 and 5000。arraysize定义了一次返回到客户端的行数，当扫描了arraysize 行后，停止扫描，返回数据，然后继续扫描。arraysize 默认是15行，如果按照15行扫描一次，那么每次扫描要多次扫描一个数据块，一个数据块也可能就会重复扫描多次。增加arraysize可以减少逻辑读。
SQL> createtable test(a1 number(10));
SQL> begin
2 for i in 1 .. 1000 loop
3 insert into test values(i);
4 end loop;
5 commit;
6 end;
7 /
SQL> execdbms_stats.gather_table_stats(user,'test');
SQL> showarraysize
arraysize 15
SQL> setautotrace trace stat
SQL> select *from test;
已选择1000行。
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
74 consistent gets
0 physical reads
0 redo size
17903 bytes sent via SQL*Net to client
1111 bytes received via SQL*Net from client
68 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed
SQL> setarraysize 100
SQL> select *from test;
已选择1000行。
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
17 consistent gets
0 physical reads
0 redo size
10493 bytes sent via SQL*Net to client
484 bytes received via SQL*Net from client
11 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed
SQL> setarraysize 500
SQL> select *from test;
已选择1000行。
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
9 consistent gets
0 physical reads
0 redo size
9453 bytes sent via SQL*Net to client
396 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed
SQL> setarraysize 1000
SQL> select *from test;
已选择1000行。
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
8 consistent gets
0 physical reads
0 redo size
9323 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed