10g中引入了对索引的shrink功能，索引shrink操作会扫描索引的页块，并且通过归并当前存在的数据将先前已删除记录的空间重新利用；很多书籍亦或者MOS的Note中都会提及SHRINK命令与早期版本中就存在的COALESCE(合并)命令具有完全相同的功能，或者说2者是完全等价的-" alter index shrink space is equivalent to coalesce"，事实是这样的吗？
SQL> conn maclean/maclean
Connected.
/* 测试使用版本10.2.0.4 * /
SQL> select * from v$VERSION;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production
/* 建立测试用表YOUYUS，高度为3 */
SQL> drop table YOUYUS;
Table dropped.
SQL> create table YOUYUS as select rownum t1,rpad('A',20,'B') t2 from dual connect by level<=999999;
Table created.
SQL> create index ind_youyus on youyus(t1,t2) nologging;
Index created.
SQL> analyze index IND_YOUYUS validate structure;
Index analyzed.
/*
大家因该很熟悉 analyze index .. validate structure 命令 ，实际上该命令存在一个兄弟:
analyze index IND_YOUYUS validate structure online，
加上online子句后validate structure可以在线操作，但该命令不会填充index_stats临时视图
*/
SQL> set linesize 200;
SQL> set linesize 200;
SQL> select height,
2 blocks,
3 lf_blks,
4 lf_rows_len,
5 lf_blk_len,
6 br_blks,
7 br_rows,
8 br_rows_len,
9 br_blk_len,
10 btree_space,
11 used_space,
12 pct_used
13 from index_stats;
HEIGHT BLOCKS LF_BLKS LF_ROWS_LEN LF_BLK_LEN BR_BLKS BR_ROWS BR_ROWS_LEN BR_BLK_LEN BTREE_SPACE USED_SPACE PCT_USED
---------- ---------- ---------- ----------- ---------- ---------- ---------- ----------- ---------- ----------- ---------- ----------
3 5376 5154 36979767 7996 9 5153 61784 8028 41283636 37041551 90
/* 可以看到IND_YOUYUS索引的基本结构，在初始状态下其block总数为5376，其中页块共5154 */
/* 我们在表上执行删除操作，均匀删除三分之一的数据 */
SQL> delete YOUYUS where mod(t1,3)=1;
333333 rows deleted.
SQL> commit;
Commit complete.
SQL> conn maclean/maclean
Connected.
SQL> select vs.name, ms.value
2 from v$mystat ms, v$sysstat vs
3 where vs.statistic# = ms.statistic#
4 and vs.name in ('redo size','consistent gets');
NAME VALUE
---------------------------------------------------------------- ----------
consistent gets 45
redo size 0
SQL> alter index ind_youyus coalesce;
Index altered.
SQL> select vs.name, ms.value
2 from v$mystat ms, v$sysstat vs
3 where vs.statistic# = ms.statistic#
4 and vs.name in ('redo size','consistent gets');
NAME VALUE
---------------------------------------------------------------- ----------
consistent gets 788
redo size 70649500
/* coalesce 操作产生了大约67MB的redo数据 */
SQL> analyze index IND_YOUYUS validate structure;
Index analyzed.
SQL> set linesize 200;
SQL> select height,
2 blocks,
3 lf_blks,
4 lf_rows_len,
5 lf_blk_len,
6 br_blks,
7 br_rows,
8 br_rows_len,
9 br_blk_len,
10 btree_space,
11 used_space,
12 pct_used
13 from index_stats;
HEIGHT BLOCKS LF_BLKS LF_ROWS_LEN LF_BLK_LEN BR_BLKS BR_ROWS BR_ROWS_LEN BR_BLK_LEN BTREE_SPACE USED_SPACE PCT_USED
---------- ---------- ---------- ----------- ---------- ---------- ---------- ----------- ---------- ----------- ---------- ----------
3 5376 3439 24653178 7996 9 3438 41188 8028 27570496 24694366 90
/* 可以看到执行coalesce(合并)操作后页块数量下降到3439，同时coalesc命令并不释放索引上的多余空间，
但索引结构实际占用的空间BTREE_SPACE下降到了27570496 bytes */
/* 清理测试现场 */
SQL> drop table YOUYUS;
Table dropped.
SQL> create table YOUYUS as select rownum t1,rpad('A',20,'B') t2 from dual connect by level<=999999;
Table created.
SQL> create index ind_youyus on youyus(t1,t2) nologging;
Index created.
SQL> delete YOUYUS where mod(t1,3)=1;
333333 rows deleted.
SQL> commit;
Commit complete.
SQL> conn maclean/maclean
Connected.
SQL> select vs.name, ms.value
2 from v$mystat ms, v$sysstat vs
3 where vs.statistic# = ms.statistic#
4 and vs.name in ('redo size','consistent gets');
NAME VALUE
---------------------------------------------------------------- ----------
consistent gets 45
redo size 0
SQL> alter index ind_youyus shrink space;
Index altered.
SQL> select vs.name, ms.value
2 from v$mystat ms, v$sysstat vs
3 where vs.statistic# = ms.statistic#
4 and vs.name in ('redo size','consistent gets');
NAME VALUE
---------------------------------------------------------------- ----------
consistent gets 2951
redo size 90963340
/* SHRINK SPACE操作产生了86MB的redo数据，多出coalesce时的28% */
SQL> analyze index IND_YOUYUS validate structure;
Index analyzed.
SQL> set linesize 200;
SQL> select height,
2 blocks,
3 lf_blks,
4 lf_rows_len,
5 lf_blk_len,
6 br_blks,
7 br_rows,
8 br_rows_len,
9 br_blk_len,
10 btree_space,
11 used_space,
12 pct_used
13 from index_stats;
HEIGHT BLOCKS LF_BLKS LF_ROWS_LEN LF_BLK_LEN BR_BLKS BR_ROWS BR_ROWS_LEN BR_BLK_LEN BTREE_SPACE USED_SPACE PCT_USED
---------- ---------- ---------- ----------- ---------- ---------- ---------- ----------- ---------- ----------- ---------- ----------
3 3520 3439 24653178 7996 9 3438 41188 8028 27570496 24694366 90
/* 索引结构与coalesce命令维护后相同，但shrink space操作释放了索引上的空闲空间 */
/* 再次清理测试现场 */
SQL> drop table YOUYUS;
Table dropped.
SQL> create table YOUYUS as select rownum t1,rpad('A',20,'B') t2 from dual connect by level<=999999;
Table created.
SQL> create index ind_youyus on youyus(t1,t2) nologging;
Index created.
SQL> delete YOUYUS where mod(t1,3)=1;
333333 rows deleted.
SQL> commit;
Commit complete.
SQL> conn maclean/maclean
Connected.
SQL> select vs.name, ms.value
2 from v$mystat ms, v$sysstat vs
3 where vs.statistic# = ms.statistic#
4 and vs.name in ('redo size','consistent gets');
NAME VALUE
---------------------------------------------------------------- ----------
consistent gets 45
redo size 0
SQL> alter index ind_youyus shrink space compact;
Index altered.
SQL> select vs.name, ms.value
2 from v$mystat ms, v$sysstat vs
3 where vs.statistic# = ms.statistic#
4 and vs.name in ('redo size','consistent gets');
NAME VALUE
---------------------------------------------------------------- ----------
consistent gets 3208
redo size 90915424
SQL> analyze index IND_YOUYUS validate structure;
Index analyzed.
SQL> set linesize 200;
SQL> select height,
2 blocks,
3 lf_blks,
4 lf_rows_len,
5 lf_blk_len,
6 br_blks,
7 br_rows,
8 br_rows_len,
9 br_blk_len,
10 btree_space,
11 used_space,
12 pct_used
13 from index_stats;
HEIGHT BLOCKS LF_BLKS LF_ROWS_LEN LF_BLK_LEN BR_BLKS BR_ROWS BR_ROWS_LEN BR_BLK_LEN BTREE_SPACE USED_SPACE PCT_USED
---------- ---------- ---------- ----------- ---------- ---------- ---------- ----------- ---------- ----------- ---------- ----------
3 5376 3439 24653178 7996 9 3438 41188 8028 27570496 24694366 90
/* shrink space compact 起到了和coalesce完全相同的作用，但其产生的redo仍要多于coalesce于28% */
coalesce与shrink space命令对比重建索引(rebuild index)有一个显著的优点:不会导致索引降级。从以上测试可以看到coalesce与shrink space compact功能完全相同；在OLTP环境中，大多数情况下我们并不希望回收索引上的空闲空间，那么coalesce或者shrink space compact(not shrink space)可以成为我们很好的选择，虽然实际操作过程中2者消耗的资源有不少差别。
并不是说coalesce就一定会消耗更少的资源，这需要在您的实际环境中具体测试，合适的才是最好的！
如有错误，欢迎指正
