Oracle段高水位(HWM, high water mark)问题

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

 

 

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