oracle高效分页性能优化

--分页参数：size = 20 page = 2
--没有order by的查询
-- （推荐使用）嵌套子查询，两次筛选
SELECT *
 FROM (SELECT ROWNUM AS rowno, t.*
 FROM DONORINFO t
 WHERE t.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
 AND TO_DATE ('20060731', 'yyyymmdd')
 AND ROWNUM <= 20*2) table_alias
 WHERE table_alias.rowno > 20*(2-1);        --耗时0.05s
 
-- 一次筛选（数据量大的时候，第一次查询的数据量过大，明显比上面慢，不推荐）
select * from(
SELECT ROWNUM AS rowno, t.*
FROM DONORINFO t
WHERE t.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd') AND TO_DATE ('20060731', 'yyyymmdd')
) r
where r.rowno BETWEEN 20*(2-1)+1 and 20*2;      --耗时0.46s
 
 
--有order by的查询
--嵌套子查询，两次筛选（推荐使用）
SELECT *
FROM (SELECT ROWNUM AS rowno,r.*
 FROM(
 SELECT * FROM DONORINFO t
 WHERE t.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
 AND TO_DATE ('20060731', 'yyyymmdd')
 ORDER BY t.BIRTHDAY desc
 ) r
 where ROWNUM <= 20*2 
 ) table_alias
 WHERE table_alias.rowno > 20*(2-1);                --耗时0.744s
 
-- 一次筛选（数据量大的时候，第一次查询的数据量过大，明显比上面慢，不推荐）
select * from (
SELECT ROWNUM AS rowno,r.*
FROM(
SELECT * FROM DONORINFO t
WHERE t.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
AND TO_DATE ('20060731', 'yyyymmdd')
ORDER BY t.BIRTHDAY desc
) r
----where ROWNUM <= 20; --这里用>查不到数据 =也查不到数据 <= 或者 < 可以查到数据 
----where ROWNUM BETWEEN 20*(2-1)+1 AND 20*2; --查不到数据
----where ROWNUM <=20*2 and ROWNUM > 20*(2-1); --查不到数据
----这是因为查询时，第一条生成的rownum为1,1>20不成立，1=20也不成立，所以这条数据就作废了，依次类推，这样就查不到任何一条数据
) t 
where t.rowno <=20*2 and t.rowno > 20*(2-1); --可以查到数据耗时：3.924s
---- where t.rowno BETWEEN 20*(2-1)+1 AND 20*2; --可以查到数据耗时：3.919s
 
--采用row_number() over 分页函数
select * 
from(select d.*,row_number() over(order by d.BIRTHDAY) as rownumber 
     from DONORINFO d
     WHERE d.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
                             AND TO_DATE ('20060731', 'yyyymmdd')
 ) p 
where p.rownumber BETWEEN 20*(2-1)+1 AND 20*2;  --耗时0.812s
---------------------------------------- 
select * from (
select * 
from(select d.*,row_number() over(order by d.BIRTHDAY) as rownumber 
     from DONORINFO d
     WHERE d.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
                             AND TO_DATE ('20060731', 'yyyymmdd')
 ) p 
where p.rownumber <20*2
) where rownumber > 20*(2-1);    -- 耗时0.813s

从以上探索，我们得知：

（1）ROWNUM

rownum是一个序列，是Oracle数据库从数据文件或缓冲区中读取数据的顺序。它取得第一条记录则rownum值为1，第二条为2，依次类推。

当使用条件查询时，从缓冲区或数据文件中得到的第一条记录的rownum为1，不符合sql语句的条件，会被删除，下条的rownum还是1，所以上限条件必须放在子查询，而下限条件必须放在外层查询。

（2）between and 和>=and <=

这两者查询效率上来说没有区别，between and 最终也是转为>= and <=

（3）Oracle通用分页格式

没有order by

SELECT *
FROM (SELECT ROWNUM AS rowno, t.*
          FROM DONORINFO t
          WHERE t.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
          AND TO_DATE ('20060731', 'yyyymmdd')
          AND ROWNUM <= page*size) table_alias
WHERE table_alias.rowno > (page-1)*size;

有order by

SELECT *
FROM (SELECT ROWNUM AS rowno,r.*
           FROM(SELECT * FROM DONORINFO t
                    WHERE t.BIRTHDAY BETWEEN TO_DATE ('19800101', 'yyyymmdd')
                    AND TO_DATE ('20060731', 'yyyymmdd')
                    ORDER BY t.BIRTHDAY desc
                   ) r
           where ROWNUM <= page*size
          ) table_alias
WHERE table_alias.rowno > (page-1)*size;