我们来看看该sort_area_size参数对创建索引时排序的具体影响:
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production
/* 测试使用版本10.2.0.4 */
SQL> archive log list;
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination /s01/arch
Oldest online log sequence 27
Current log sequence 34
/* 为了不受影响我们采用非归档模式 */
SQL> conn maclean/maclean
Connected.
SQL> alter session set workarea_size_policy=MANUAL;
Session altered.
/* 只有使用手动PGA管理时sort_area_size等参数才生效 */
SQL> alter session set db_file_multiblock_read_count=128;
Session altered.
/* 加大多块读参数帮助我们节约时间 */
SQL> alter session set "_sort_multiblock_read_count"=128;
Session altered.
/* 10g中sort_multiblock_read_count成为隐式参数，我们尝试手动固定它 */
SQL> set timing on;
SQL> alter session set events '10032 trace name context forever ,level 10';
Session altered.
Elapsed: 00:00:00.00
/* 在session级别设置10032事件，该事件帮助输出排序相关的统计信息*/
SQL> drop index ind_youyus;
alter session set sort_area_size=1048576;
alter session set sort_area_size=1048576;
/* 注意10g目前存在一个bug，sort_area_size需要手动设置2次方能生效，否则无效！ */
create index ind_youyus on youyus(t1,t2) nologging;
Index dropped.
Elapsed: 00:00:00.07
SQL>
Session altered.
Elapsed: 00:00:00.00
SQL>
Session altered.
Elapsed: 00:00:00.00
SQL>
Index created.
Elapsed: 00:00:35.70
/* 以下为对应创建索引排序的10032 trace * /
---- Sort Parameters ------------------------------
sort_area_size 1048576
sort_area_retained_size 1048576
sort_multiblock_read_count 29
max intermediate merge width 2
*** 2010-09-09 21:15:52.703
---- Sort Statistics ------------------------------
Initial runs 1
Input records 10000001
Output records 10000001
Disk blocks 1st pass 58690
Total disk blocks used 58692
Total number of comparisons performed 10104798
Comparisons performed by in-memory sort 10098798
Comparisons while searching for key in-memory 6000
Temp segments allocated 1
Extents allocated 459
Uses version 2 sort
Does not use asynchronous IO
---- Run Directory Statistics ----
Run directory block reads (buffer cache) 2
Block pins (for run directory) 1
Block repins (for run directory) 1
---- Direct Write Statistics -----
Write slot size 49152
Write slots used during in-memory sort 2
Number of direct writes 10011
Num blocks written (with direct write) 58690
Block pins (for sort records) 58690
Cached block repins (for sort records) 667
---- Direct Read Statistics ------
Size of read slots for output 524288
Number of read slots for output 2
Number of direct sync reads 58627
Number of blocks read synchronously 58690
---- End of Sort Statistics -----------------------
/* 可以看到这里实际的sort_multiblock_read_count是29，而非我们设置的128，而max intermediate merge width最大中间运行合并宽度为2；我们知道实际的SMRC有公式MIN(ROUND(SORT_AREA_SIZE/(2*2*BLOCK_SIZE))-1, _sort_multiblock_read_count)=MIN(32-1,128)=31，而此处的实际值为29； merge width = ROUND(SORT_AREA_SIZE/(2*SMRC*BLOCK_SIZE))-1= round( 1048576/(2*29*8k))-1= round(2.2)-1=2-1=1；看起来实际的公式有所修正。*/
SQL> drop index ind_youyus;
alter session set sort_area_size=524288000;
alter session set sort_area_size=524288000;
create index ind_youyus on youyus(t1,t2) nologging;
Index dropped.
Elapsed: 00:00:00.04
SQL>
Session altered.
Elapsed: 00:00:00.00
SQL>
Session altered.
Elapsed: 00:00:00.00
SQL>
Index created.
Elapsed: 00:00:36.82
---- Sort Parameters ------------------------------
sort_area_size 524288000
sort_area_retained_size 524288000
sort_multiblock_read_count 128
max intermediate merge width 225
*** 2010-09-09 21:32:06.517
---- Sort Statistics ------------------------------
Initial runs 2
Number of merges 1
Input records 10000001
Output records 10000001
Disk blocks 1st pass 58690
Total disk blocks used 58692
Total number of comparisons performed 17571986
Comparisons performed by in-memory sort 10098438
Comparisons performed during merge 7473532
Comparisons while searching for key in-memory 16
Temp segments allocated 1
Extents allocated 459
Uses version 2 sort
Does not use asynchronous IO
---- Run Directory Statistics ----
Run directory block reads (buffer cache) 3
Block pins (for run directory) 1
Block repins (for run directory) 2
---- Direct Write Statistics -----
Write slot size 1048576
Write slots used during in-memory sort 50
Number of direct writes 460
Num blocks written (with direct write) 58690
Block pins (for sort records) 58690
Cached block repins (for sort records) 1
---- Direct Read Statistics ------
Size of read slots for output 1048576
Number of read slots for output 500
Number of direct sync reads 58563
Number of blocks read synchronously 58690
---- End of Sort Statistics -----------------------
/* 10g中引入了新的排序算法， 排序初始化运行完成后，会保存初始数据集的键值到内存中，在进行数据集进行合并时，会根据键值来选择数据集。从trace文件中可以看到这样的统计信息:Comparisons while searching for key in-memory 16;*/
/* 可以看到write slot的大小也随sort_area_size变化，sort_area_size增大的同时Number of direct writes由10011次下降到460次,此外read slots的总大小(524288*2=1048576=sort_area_size，1048576*500=sort_area_size) */
/* 在合并merge阶段，因为读取缓存足够大，因此合并次数下降到1 */
to be continued ..............