Oracle对表做全表扫描的时候 ，会扫描完HWM以下的数据块。如果某个表delete(delete操作不会降低高水位)了大量数据，那么这时对表做全表扫描就会做很多无用功，扫描了一大堆数据块，最后发现块里面居然没有数据。
通常，在对表做了大批量delete操作之后，就应该马上降低表的高水位，可以使用shrink 命令或者alter table table_name move降低表的高水位。在降低表的高水位之后，表上面的索引会失效，因为表的rowid更改了，这个时候需要rebuild索引。
如何求出段的高水位？其实很简单，首先对表收集统计信息，然后查询DBA_TABLES的blocks，以及empty_blocks字段，blocks表示已经用了多少个blocks,empty_blocks表示从来没有使用过的blocks。那么blocks就表示段的高水位。
可以使用下面的语句查看表到底用了多少个blocks
select count( distinct dbms_rowid.rowid_block_number(rowid)) from table_name;
然后再对比dba_tables表中的blocks列，如果求出的blocks数与dba_tables相差在10左右，那么表示这个表不需要shrink,用上面的脚本求出的blocks数没计算段头，位图管理块。如果相差很大，那么表示这个表需要shrink了，不过这样做比较麻烦，不是吗？
其实还可以监控表的DML操作，根据监控的结果，我们就可以判断哪些表需要降低高水位。在Oracle10g中DBA_TAB_MODIFICATIONS这个视图记录了自上次收集统计信息以来表的DML操作信息。当我们再次对表收集统计信息，该视图的记录就会被清空。
对于非分区表，可以使用下面脚本初步检测哪些表需要降低高水位
exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO ;
col table format a35
select a.owner || '.' || a.table_name "Table",a.num_rows,a.avg_row_len,b.inserts,b.deletes,a.num_rows+nvl(b.inserts,0)-nvl(b.deletes,0) total_rows,
round(a.avg_row_len*(a.num_rows+nvl(b.inserts,0)-nvl(b.deletes,0))/1024/1024,4) "Calculate_Sizle(Mb)",c.bytes/1024/1024 "Segment_Size(Mb)"
from dba_tables a left join all_tab_modifications b
on a.owner=b.table_owner and a.table_name=b.table_name inner join dba_segments c on a.owner=c.owner and a.table_name=c.segment_name
where a.last_analyzed is not null and a.partitioned='NO' and b.deletes>100 and a.owner not like ‘%SYS%' and c.bytes/1024/1024>100
and (a.avg_row_len*(a.num_rows+nvl(b.inserts,0)-nvl(b.deletes,0)))/c.bytes<0.5 order by b.deletes desc;
上面的脚本查询收集过统计信息的非分区表，并且delete超过100，段大小超过100M，num_rows*avg_row_len/段大小小于0.5的表。这里解释一下为什么要用num_rows*avg_row_len，因为只看insert,delete是不够的。比如先对表insert,再delete，这个时候需要shrink表，但是如果先delete,再insert就不需要shrink表了，所以我引入了num_rows*avg_row_len比上段大小作为参考依据。
注意，使用上面的脚本查询出来total_rows可能是负数，那表明你对表收集统计信息有问题。
你可以适当修改上面的脚本,比如设置deletes>0，段大小超过200M,比值小于0.3等等。利用上面脚本查询出可疑的表之后，可以最开始讲的方法检查是否要shrink表，你也可以用Segment Advisor来检查是否需要shrink这个表。Segment Advisor使用方法：
SQL> variable task_id number;
SQL> begin
2 declare
3 object_id number;
4 name varchar2(100);
5 task_desc varchar2(100);
6 begin
7 name := 'test';
8 task_desc := 'Segment Advisor TEST';
9 dbms_advisor.create_task(advisor_name => 'Segment Advisor',
10 task_id => :task_id,
11 task_name => name,
12 task_desc => task_desc
13 );
14 dbms_advisor.create_object(task_name => name,
15 object_type => 'TABLE',
16 attr1 => 'ROBINSON',
17 attr2 => 'TEST',
18 attr3 => NULL,
19 attr4 => NULL,
20 attr5 => NULL,
21 object_id =>object_id
22 );
23 dbms_advisor.set_task_parameter(task_name => name,
24 parameter => 'recommend_all',
25 value => 'TRUE'
26 );
27 dbms_advisor.execute_task(task_name => name);
28 end;
29 end;
30 /
PL/SQL procedure successfully completed
task_id
---------
560
SQL> col task_name format a8
SQL> col segname format a8
SQL> col partition format a8
SQL> col type format a8
SQL> col message format a100
SQL> select af.task_name, ao.attr2 segname, ao.attr3 partition, ao.type, af.message
2 from dba_advisor_findings af, dba_advisor_objects ao
3 where ao.task_id = af.task_id
4 and ao.object_id = af.object_id
5 and ao.owner = 'ROBINSON';
TASK_NAM SEGNAME PARTITIO TYPE MESSAGE
-------- -------- -------- -------- ----------------------------------------------------------------------------------------------------
test TEST TABLE Enable row movement of the table ROBINSON.TEST and perform shrink, estimated savings is 28451785 bytes.
下面是一个生产环境数据库，可以看到有很多表都需要shrink,我就不贴出具体的处理步骤了。
SQL> select a.owner || '.' || a.table_name "Table",a.num_rows,a.avg_row_len,b.inserts,b.deletes,a.num_rows+nvl(b.inserts,0)-nvl(b.deletes,0) total_rows,
2 round(a.avg_row_len*(a.num_rows+nvl(b.inserts,0)-nvl(b.deletes,0))/1024/1024,4) "Calculate_Sizle(Mb)",c.bytes/1024/1024 "Segment_Size(Mb)"
3 from dba_tables a left join all_tab_modifications b
4 on a.owner=b.table_owner and a.table_name=b.table_name inner join dba_segments c on a.owner=c.owner and a.table_name=c.segment_name
5 where a.last_analyzed is not null and a.partitioned='NO' and b.deletes>100 and c.bytes/1024/1024>100 and a.owner not like '%SYS%'
6 and (a.avg_row_len*(a.num_rows+nvl(b.inserts,0)-nvl(b.deletes,0)))/c.bytes<0.5 order by b.deletes desc;
Table NUM_ROWS AVG_ROW_LEN INSERTS DELETES TOTAL_ROWS Calculate_Sizle(Mb) Segment_Size(Mb)
------------------------------ ---------- ----------- ---------- ---------- ---------- ------------------- ----------------
IDWSU14.OMCR4_SHIP_HIST_SRC_V 39370237 77 805667850 884712377 -39674290 -2913.3991 1088
IDWSU12.SH08_STJP7T_7001_VIEW 39761320 133 229568455 268620133 709642 90.0101 6164
IDWSU14.SH30_DP_SSFV_PROD_REPL 4871971 11 231242749 230333428 5781292 60.6482 132
C_2_AA
IDWSU14.SH30_DP_SSFV_PROD_REPL 4868627 11 231242749 230333428 5777948 60.6131 130.125
C_2_NEA
IDWSU14.SH30_DP_SSFV_PROD_REPL 4877610 11 231242749 230333428 5786931 60.7073 130.125
C_2_GC
IDWSU11.YMCR4_SHIP_HIST_SRC_V 22753157 76 203507302 226332317 -71858 -5.2082 688
IDWSU13.SH30_DP_SSFV_PROD_REPL 5638000 11 204891145 204449473 6079672 63.7783 136
C_2_WE
IDWSU13.SH30_V_CUST_SUBCTRY_67 4232552 35 165554103 167511636 2275019 75.937 452
7
IDWSU14.OMCR7_SHIP_HIST_SRC_V 40399302 77 123348208 163744322 3188 0.2341 1136
IDWSU14.SH30_V_CUST_SUBCTRY_67 4232198 35 156844906 156528506 4548598 151.8258 432
7
ADWU.GDF_SHPMT_EXTRACT_MV 0 0 142410224 142410224 0 0 13884
IDWSU10.SYMPH_CHECK_RDS_LOG 24632789 84 90571770 111067817 4136742 331.3888 2143.5
IDWSU13.OMCR8_SHIP_HIST_SRC_V 9606113 72 86326358 95986530 -54059 -3.7119 316
IDWSU11.SH28_GKC_DP3606_PR9005 11369578 19 47034509 53631501 4772586 86.4784 356
_1
IDWSU11.MCR_MV_SRCE_PLANT_BW 1384608 31 47708177 47676037 1416748 41.8846 316
ADWU_OPTIMA_AP10.OPT_FUND_DIM_ 43513659 220 34516 43643704 -95529 -20.0428 9580
ERR
IDWSU11.SH30_DP_SSFV_PROD_REPL 6148402 11 37392566 37279353 6261615 65.687 140
C_1
ADWGU_IVC_RSTMT.IVC_EFRTN_RULE 17280240 67 19963487 35209550 2034177 129.9761 2797.375
_PLC
IDWSU11.SH28_GKC_DP3602_PR9005 4741685 19 23071830 26059892 1753623 31.7753 192
_1
IDWSU12.SH08_STJP2D_MVIEW 4664224 33 20742310 25330983 75551 2.3777 224
Table NUM_ROWS AVG_ROW_LEN INSERTS DELETES TOTAL_ROWS Calculate_Sizle(Mb) Segment_Size(Mb)
------------------------------ ---------- ----------- ---------- ---------- ---------- ------------------- ----------------
IDWSU11.SH30_V_CUST_SUBCTRY_67 4508160 35 20378604 22611745 2275019 75.937 460
7
ADWGU_IVC_RSTMT.IVC_RSTMT_RULE 17218047 60 4784752 20724823 1277976 73.1264 1080
_AS_PLC
IDWSU12.SH30_V_CUST_SUBCTRY_67 4232552 35 10624250 12719507 2137295 71.3399 300
7
IDWSU11.SH30_DP_SSFV_PROD_REPL 6056749 11 12122875 12110928 6068696 63.6632 140
C_1_LA
IDWSU12.SH30_V_PROD_BOM_DENORM 1987829 54 6009188 7987214 9803 0.5048 204
_FAC
IDWSU29.R3_EQSCN_ATTR_RPT_FACT 2298703 143 1436116 3148585 586234 79.9479 353
ADWGU_TFF2.TF_DPSHP_PULL_SFADS 1734816 88 1742583 1743434 1733965 145.5201 6120
ADWGU_TFF2.TF_DPSHP_UNOFL_PULL 1738361 88 1721664 1740119 1719906 144.3403 4440
_SFADS
IDWSU29.R3_EQSCN_ATTR_FACT 610243 165 1155068 1475080 290231 45.6697 107
IDWSU13.DSHB_MCR_GLB3_WE_MC_FC 141849 57 1426032 1427202 140679 7.6472 166.375
T
IDWSU14.SH30_PRCSS_EXCTN_LOG 107340 133 1602797 1295252 414885 52.6235 106.25
ADWU_GPOS.GPOS_DD_WMUS_WHSE_TF 3109445 115 2995416 894588 5210273 571.4239 11537.375
ADS
ADWU_GPOS.GPOS_CD_ERR 31250000 229 1305866 816661 31739205 6931.57 30428
IDWSU14.DSHB_MCR_GLB3_AS_MC_FC 69719 58 535419 539333 65805 3.6399 133.5
T
IDWSU14.DSHB_MCR_GLB3_AS_MC500 49550 58 397994 397727 49817 2.7555 224
9_FCT
ADWU.GEO_705_EFSR_DAY_FDIM 793 276 1526 1622 697 0.1835 496
ADWU.EFSR_PRMTN_TYPE_LKP 198 15 452 582 68 0.001 396
ADWU.EFSR_PRMTN_TYPE_SLKP 198 15 452 582 68 0.001 396
ADWU.CUST_GRP_EFSR_DAY_FDIM 221 20 442 442 221 0.0042 496
ADWU.CUST_656_EFSR_DAY_FDIM 142 644 296 286 152 0.0934 524
IDWSU14.MCR_PLANT_DERIV 1909908 51 42088 193 1951803 94.9306 4069.875
41 rows selected