模擬並消除行遷移(Row Migration)

--什麼是行遷移？
A situation in which Oracle Database moves a row from one data block to another data block because the row grows too large to fit in the original block.

注：儘管行遷移與行鏈接是兩個不同的事情，但是在oracle內部，它們被當作一回事。所以當你檢測行遷移與行鏈接時，你應該仔細的分析當前你正在處理的是行遷移還是行鏈接。

--模擬實驗
EODA@PROD1> create table t(x int, y varchar2(50));

Table created.

EODA@PROD1> begin
  2  for i in 1..1000 loop
  3    insert into t values (i,'A');
  4  end loop;
  5  commit;
  6  end;
  7  /

PL/SQL procedure successfully completed.

EODA@PROD1> analyze table t compute statistics;

Table analyzed.

--chain_cnt計算了存在行遷移和行鏈接的行數
EODA@PROD1> select table_name,num_rows,chain_cnt from user_tables where table_name='T';

TABLE_NAME			 NUM_ROWS  CHAIN_CNT
------------------------------ ---------- ----------
T				     1000	   0

EODA@PROD1> update t set y=rpad('A',50,'B');

1000 rows updated.

EODA@PROD1> commit;

Commit complete.

EODA@PROD1> analyze table t compute statistics;

Table analyzed.

EODA@PROD1> select table_name,num_rows,chain_cnt from user_tables where table_name='T';

TABLE_NAME			 NUM_ROWS  CHAIN_CNT
------------------------------ ---------- ----------
T				     1000	 967

--使用腳本創建CHAINED_ROWS表
EODA@PROD1> @?/rdbms/admin/utlchain

Table created.

EODA@PROD1> desc chained_rows
 Name						       Null?	Type
 ----------------------------------------------------- -------- ------------------------------------
 OWNER_NAME							VARCHAR2(30)
 TABLE_NAME							VARCHAR2(30)
 CLUSTER_NAME							VARCHAR2(30)
 PARTITION_NAME 						VARCHAR2(30)
 SUBPARTITION_NAME						VARCHAR2(30)
 HEAD_ROWID							ROWID
 ANALYZE_TIMESTAMP						DATE

EODA@PROD1> analyze table t list chained rows into chained_rows;  --自動分析並存入CHAINED_ROWS表

Table analyzed.

EODA@PROD1> select count(*) from chained_rows;

  COUNT(*)
----------
       967

--通過重新插入解決行遷移
EODA@PROD1> create table tmp as select * from t where rowid in (select head_rowid from chained_rows);

Table created.

EODA@PROD1> delete t where rowid in (select head_rowid from chained_rows);

967 rows deleted.

EODA@PROD1> insert into t select * from tmp;

967 rows created.

EODA@PROD1> commit;

Commit complete.

EODA@PROD1> analyze table t compute statistics;

Table analyzed.

EODA@PROD1> select table_name,num_rows,chain_cnt from user_tables where table_name='T';

TABLE_NAME			 NUM_ROWS  CHAIN_CNT
------------------------------ ---------- ----------
T				     1000	   0


EODA@PROD1> drop table tmp purge;

Table dropped.