oracle行迁移和行链接优化

一. 概述、产生原因及影响

row migration：When a row is to be updated and it cannot find the necessary free space in its block,the oracle will move the entie row into a new block and leave a pointer from the orginal block to the new locatio.This process
is called row migration

row chain：When a row is too large to fit into any block,row chaining occurs.In this case,the oracle devide the row into smaller chunks. each chunk is stored in a block along with the necessary poiters to retrive and assemble
the entire row.

行迁移：当一个行上的更新操作(原来的数据存在且没有减少)导致当前的数据不能容纳在当前块，需要进行行迁移。行迁移意味着整行数据都被移动，原始的数据块上仅仅保留的是指向新块的一个地址信息(指针)，并且该行原先空间不再被数据库使用，这些剩余的空间称之为空洞，也是产生表碎片的主要原因，表碎片基本上也是不可避免的，但是可以通过一些策略使其降低到一个可以接受的程度。注意，即使发生了行迁移，发生行迁移的行的rowid还是不会变化，这也是行迁移会引起数据库I/O性能降低的原因。可以认为行迁移是行链接的一种特殊形式，但是起因于行链接有很大不同。

产生：update

行链接：当一行数据太大而不能在一个单数据块容纳时，会产生行链接。例如当使用4kb的oracle数据块大小，而需要插入一行数据是8k,Oracle则需要使用三个数据块分成片来存储。因此，引起行链接的情形通常是，表上行记录的大小超出了数据库oracle块的大小。

产生原因：The row is too large to fit into one data block when it isfirstinserted.

表上使用了Long或Long raw数据类型的时候容易产生行链接(此时行链接通常为不可避免的)。其次表上使用多余255列时oracle会将这些过宽的表分片而产生行链接。

影响：行迁移对索引读产生额外的I/O,对全表扫描没什么影响。行连接则影响索引读和全表扫描。总的来说行迁移和行链接引起性能下降的原因主要是由于多余的I/O造成的。当通过索引访问产生行迁移的行时，数据库必须扫描一个或一个以上的数据块才能检索到该行的数据。

二. 模拟行迁移和行链接

行链接

------------------ 参考tom kyte的例子--------------------------------------------
--创建4k blocksize的表空间
SQL> alter system set db_4k_cache_size=1m scope=both;

System altered.
SQL> create tablespace tbs1 datafile '/u01/app/oracle/oradata/orcl/tbs_1.dbf' size 100m blocksize 4k;

Tablespace created.
--行链接测试
--使用定列宽的char类型来创建行链接测试表
SQL> create table row_chain_demo(
  2  x int primary key,
  3  a char(1000),
  4  b char(1000),
  5  c char(1000),
  6  d char(1000)
  7  ) tablespace tbs1;

Table created.
--插入数据
SQL> insert into row_chain_demo(x,a,b,c,d) values(1,'a','b','c','d');

1 row created.

SQL> commit;

Commit complete
--分析测试表，检查行链接
--首先建chaind_rows相关表
SQL> @/u01/app/oracle/product/11.2.0/rdbms/admin/utlchain.sql

Table created.
--分析表
SQL> analyze table row_chain_demo list chained rows into chained_rows;

Table analyzed.
--查询行链接
SQL> select * from chained_rows where table_name='ROW_CHAIN_DEMO';

OWNER_NAME                     TABLE_NAME
------------------------------ ------------------------------
CLUSTER_NAME                   PARTITION_NAME
------------------------------ ------------------------------
SUBPARTITION_NAME              HEAD_ROWID         ANALYZE_T
------------------------------ ------------------ ---------
SYS                            ROW_CHAIN_DEMO

N/A                            AAAVqhAAHAAAAEGAAA 29-OCT-16
记录下此时的执行计划：
SQL> set autotrace traceonly statistics
SQL> set linesize 100
SQL> select /*+index(ROW_CHAIN_DEMO,x)*/* from ROW_CHAIN_DEMO where x=1;

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
       4815  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
   
4000
       0  sorts (memory)
          0  sorts (disk)
          1  rows processed
--找出row_chain_demo这张表所对应的数据块
SQL> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block# from row_chain_demo;

     FILE#     BLOCK#
---------- ----------
         7        262
--dump该数据块进行分析
SQL> alter system dump datafile 7 block 262;

System altered.

--以下为trace文件的内容
data_block_dump,data header at 0x7f28ca793264
===============
tsiz: 0xf98
hsiz: 0x14
pbl: 0x7f28ca793264
     76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0xba1
avsp=0xb8d
tosp=0xb8d
0xe:pti[0]      nrow=1  offs=0
0x12:pri[0]     offs=0xba1
block_row_dump:
tab 0, row 0, @0xba1
tl: 1015 fb: --H-F lb: 0x1  cc: 2
--正常的行记录为--H-FL--,而这里为只有F(fisrt)而没有L(last)，说明在这个数据块中只有行的开始，而没有行的结束，同样cc为2说明这个块中只包含了表的两个列 */
nrid:  0x01c00105.0
-- nrid表示数据块的下一个指针，即其他列数据存放的数据块地址
--通过bbed分析
BBED> p kdbr
sb2 kdbr[0]                                 @118      2977

BBED> p *kdbr[0]   
rowdata[0]
----------
ub1 rowdata[0]                              @3077     0x28

BBED> x /rnccccccc
rowdata[0]                                  @3077    
----------
flag@3077: 0x28 (KDRHFF, KDRHFH)
lock@3078: 0x01
cols@3079:    2    --从这也可以看出数据块中只有表的两个列
nrid@3080:0x01c00105.0

col    0[2] @3086: 1
col 1[1000] @3089: a                                         
--消除行链接
--创建blocksize为32的表空间，并将测试表移动到该表空间
SQL> alter system set db_32k_cache_size=1m scope=both;

System altered.

SQL> create tablespace tbs2 datafile '/u01/app/oracle/oradata/orcl/tbs2.dbf' size 100m blocksize 32k;

Tablespace created.

SQL> alter table row_chain_demo move tablespace tbs2;

Table altered.
SQL> select index_name from dba_indexes where table_name='ROW_CHAIN_DEMO';

INDEX_NAME
------------------------------
SYS_C0011489
SQL> alter index SYS_C0011489 rebuild;

Index altered.
--检查行链接
SQL> delete from chained_rows;

1 row deleted.

SQL> commit;

Commit complete.
SQL> analyze table ROW_CHAIN_DEMO list chained rows into chained_rows;

Table analyzed.

SQL> select * from chained_rows where table_name='CHAIN_ROW_DEMO';

no rows selected
--查看消除行链接后的执行计划状态
SQL> select /*+index(ROW_CHAIN_DEMO,x)*/* from ROW_CHAIN_DEMO where x=1;

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
       4668  bytes sent via SQL*Net to client
        512  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)http://write.blog.csdn.net/postedit/52963389
          1  rows processed

--可以看出只发生了两次一致性读

行迁移

--创建测试表，保证表的字段能够大于blocksize
SQL> create table row_mig_demo(
  2  x int primary key,
  3  a char(1000),
  4  b char(1000),
  5  c char(1000),
  6  d char(1000)
  7  ) tablespace tbs1;

Table created.
--插入数据，只插入一个字段的值
SQL> insert into row_mig_demo(x) values(1);

1 row created.

SQL> commit;

Commit complete.
--分析表，查看是否有行迁移情况(此时必然没有)
SQL> analyze table row_mig_demo list chained rows into chained_rows;

Table analyzed.

SQL> select * from chained_rows where table_name='ROW_MIG_DEMO';

no rows selected
--查看测试表所在的数据块
SQL> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block# from row_mig_demo;

     FILE#     BLOCK#
---------- ----------
         7        277
--dump数据块查看
SQL> alter system dump datafile 7 block 277;

System altered.
--trace文件内容
data_block_dump,data header at 0x7fa1ea9e1864
===============
tsiz: 0xf98
hsiz: 0x14
pbl: 0x7fa1ea9e1864
     76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0xf8c
avsp=0xf7b
tosp=0xf7b
0xe:pti[0]      nrow=1  offs=0
0x12:pri[0]     offs=0xf8c
block_row_dump:
tab 0, row 0, @0xf8c
tl: 6 fb: --H-FL-- lb: 0x1  cc: 1  --FL:说明此时数据行的头和尾都在block内，cc:1,即只有一个字段的数据
col  0: [ 2]  c1 02
end_of_block_dump
--通过bbed查看block数据层内容

BBED> p kdbr
sb2 kdbr[0]                                 @118      3980

BBED> p *kdbr[0]
rowdata[0]
----------
ub1 rowdata[0]                              @4080     0x2c

BBED> x /rnccccccc
rowdata[0]                                  @4080    
----------
flag@4080: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@4081: 0x01
cols@4082:    1

col    0[2] @4083: 1
--此时的执行计划状态
SQL> select * from ROW_MIG_DEMO where x=1;

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        639  bytes sent via SQL*Net to client
        512  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
--update测试表，是表的行数据大于blocksize
SQL> update row_mig_demo set a='a',b='b',c='c',d='d' where x=1;

1 row updated.

SQL> commit;

Commit complete.
SQL> delete from chained_rows;

0 rows deleted.

SQL> commit;

Commit complete.

SQL> analyze table row_mig_demo list chained rows into chained_rows;

Table analyzed.

SQL> select * from chained_rows where table_name='ROW_MIG_DEMO';

OWNER_NAME                     TABLE_NAME
------------------------------ ------------------------------
CLUSTER_NAME                   PARTITION_NAME
------------------------------ ------------------------------
SUBPARTITION_NAME              HEAD_ROWID         ANALYZE_T
------------------------------ ------------------ ---------
SYS                            ROW_MIG_DEMO

N/A                            AAAVqrAAHAAAAEVAAA 29-OCT-16
--可以看到已经产生了行迁移
--dump数据块查看
SQL> select dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block# from row_mig_demo;

     FILE#     BLOCK#
---------- ----------
         7        277  --表所在数据块地址并未改变
SQL> alter system dump datafile 7 block 277;

System altered.

--trace文件内容
data_block_dump,data header at 0x7f28ca793264
===============
tsiz: 0xf98
hsiz: 0x14
pbl: 0x7f28ca793264
     76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0xba1
avsp=0xb8d
tosp=0xb8d
0xe:pti[0]      nrow=1  offs=0
0x12:pri[0]     offs=0xba1
block_row_dump:
tab 0, row 0, @0xba1
tl: 1015 fb: --H-F--- lb: 0x1  cc: 2
nrid:  0x01c00105.0
col  0: [ 2]  c1 02
col  1: [1000]
--bbed查看
BBED> p kdbr
sb2 kdbr[0]                                 @118      2965

BBED> p *kdbr[0]
rowdata[0]
----------
ub1 rowdata[0]                              @3065     0x28

BBED> x /rnccccccc
rowdata[0]                                  @3065    
----------
flag@3065: 0x28 (KDRHFF, KDRHFH)
lock@3066: 0x02
cols@3067:    2
nrid@3068:0x01c00116.0

col    0[2] @3074: 1
col 1[1000] @3077: a
--消除行迁移
SQL> create table row_mig_demp1 as select * from row_mig_demo;

Table created.

三. 优化

行迁移的优化：

a. CTAS(create table as select)将发生行迁移的数据进行重新整理

--将行迁移的数据汇总到临时表中

SQL> create table row_mig_demo_temp asselect * from row_mig_demo where rowid in (select head_rowid from chained_rowswhere table_name='ROW_MIG_DEMO');

--删除表中发生行迁移的数据

SQL> delete from row_mig_demo where rowidin (select head_rowid from chained_rows where table_name='ROW_MIG_DEMO');

--将临时表中发生行迁移的数据插回到原表

SQL> insert into row_mig_demo select *from row_mig_demo_temp;

--删除临时表

drop table row_mig_demo_temp purge;

另外，对于行链接的优化已经在实验中说明