oralce个人备忘

1.压缩存储数据时，create table 加上COMPRESS选项，向表插入数据时，要排序插入，这样相同数据就存放一次，节约空间提高I/O效率。弊端，经常UPDATE的话会影响DML性能

2.表字段的unused和查看

alter table t1 set unused column column_name

select obj#,col#,name from sys.col$
where obj# = 'select object_id from dba_objects where object_name = 'T1';
alter table t1 drop unused columns;

3.使用UTL_FILE包导出表为文本，适用大表导出

declare
v_file utl_file.file_type;
begin
v_file:=utl_file.fopen('/u01','output.txt','w');
utl_file.putf(v_file,'output date: %s\n',sysdate);
utl_file.new_line(v_file);
for i in(select * from scott.t1) loop
utl_file.putf(v_file,'%s,%s,%s\n',i.ename,i.empno,i.sal);
end loop;
utl_file.fclose(v_file);
end;
/

 4. v$transaction 跟 v$session 使用v$transaction.addr = v$session.taddr关联

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