数据库优化查询更新操作/查询最大值

1- 在数据查询时面对大批量数据，如何关联查询。这个对查询效率影响很大。这里有两种方法提高查询效率。更新大批量数据时通过rowid查找

在单表查询时

可以增加并行度

select /*+ parallel(tcustomerinfo 8) */

c_custno

from tcustomerinfo ti

多表查询

select /*+ all_rows leading(hm) use_hash(h) no_merge(h) */

h.rowid ,hm.d_opendate

from (

select c_custno, d_opendate

from(

select /*+ parallel(TACCOINFO 8) */

c_custno,

d_opendate,

row_number() over(partition by c_custno order by d_opendate desc nulls last) rn

from TACCOINFO

where c_custtype = ''1'' )

where rn=1

) hm ,TORGEXINFO h

where hm.c_custno =h.c_custno order by h.rowid

2- 大数据量更新时选择rowid查找更新，这样可以直接找到物理地址。形式如下

declare



type refcursor is ref cursor;

v_cursor refcursor;

type array_rowid is table of rowid;

type array_value is table of varchar2(20);

a_rowid array_rowid := array_rowid();

a_value array_value := array_value();



begin



open v_cursor for 'select h.rowid,hm.d_lastconfirmdate from (

select c_custno,to_char(to_date(d_cdate),''yyyy-mm-dd'') d_lastconfirmdate from(

select /*+ parallel(tconfirm 8) */

c_custno,d_cdate,row_number() over(partition by c_custno order by d_cdate desc nulls last) rn from tconfirm

where c_businflag in (''02'', ''39'')

and c_custtype = ''1''

)

where rn=1) hm ,tperexp h where hm.c_custno =h.c_custno order by h.rowid';

loop

fetch v_cursor bulk collect into a_rowid,a_value limit 5000;

if a_rowid.count() > 0 then

forall i in a_rowid.first .. a_rowid.last

update tperexp set d_lastconfirmdate =a_value(i) where rowid = a_rowid(i);

end if;

exit when v_cursor%notfound;

end loop;

close v_cursor;

end;

/

3- 查询最大值如下查询方式要比直接使用max效率高

select c_custno, d_opendate

from(

select /*+ parallel(TACCOINFO 8) */

c_custno,

d_opendate,

row_number() over(partition by c_custno order by d_opendate desc nulls last) rn

from TACCOINFO

where c_custtype = ''1'' )

where rn=1