一、环境
Oracle 11g RAC
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二、测试过程
SQL> show parameter db_file
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_file_multiblock_read_count integer 128
db_file_name_convert string /oracle/oradata/, +DATADG
db_files integer 200
SQL> set timing on
SQL> select count(*) from tt;
COUNT(*)
----------
5524288
Elapsed: 00:00:11.50
SQL> alter system flush buffer_cache;
System altered.
Elapsed: 00:00:00.20
SQL> alter system flush shared_pool;
System altered.
Elapsed: 00:00:00.39
SQL> alter session set db_file_multiblock_read_count=16;
Session altered.
Elapsed: 00:00:00.00
SQL> select count(*) from tt;
COUNT(*)
----------
5524288
Elapsed: 00:00:08.91
SQL> alter system flush buffer_cache;
System altered.
Elapsed: 00:00:00.12
SQL> alter system flush shared_pool;
System altered.
Elapsed: 00:00:00.06
SQL> alter session set db_file_multiblock_read_count=32;
Session altered.
Elapsed: 00:00:00.00
SQL> select count(*) from tt;
COUNT(*)
----------
5524288
Elapsed: 00:00:07.87
SQL> alter system flush buffer_cache;
System altered.
Elapsed: 00:00:00.14
SQL> alter system flush shared_pool;
System altered.
Elapsed: 00:00:00.06
SQL> alter session set db_file_multiblock_read_count=64;
Session altered.
Elapsed: 00:00:00.00
SQL> select count(*) from tt;
COUNT(*)
----------
5524288
Elapsed: 00:00:07.05
SQL> alter system flush buffer_cache;
System altered.
Elapsed: 00:00:00.15
SQL> alter system flush shared_pool;
System altered.
Elapsed: 00:00:00.06
SQL> alter session set db_file_multiblock_read_count=128;
Session altered.
Elapsed: 00:00:00.00
SQL> select count(*) from tt;
COUNT(*)
----------
5524288
Elapsed: 00:00:06.62
SQL>
SQL> 三、小结
1、对于全表扫描来说多块读，增加每次读取的块数，可以提高性能。
2、在OLTP的系统中建议此参数设置为8、16、32
3、在OLAP的系统中建议此参数设置为128最大值