如題,前端時間羣裏有人問的這個問題,正好有空,就關注了一下。
現狀
dbone=# select * from tb_test;
id | uname | addr
----+-------+------
1 | a |
2 | a |
3 | |
4 | |
5 | |
6 | bb |
7 | |
8 | |
9 | x |
10 | |
(10 rows)
期望結果:
id | uname
----+-------
1 | a
2 | a
3 | a
4 | a
5 | a
6 | bb
7 | bb
8 | bb
9 | x
10 | x
(10 rows)
可以如下:
dbone=# select b.id,
case
when b.uname!='' then b.uname
else (select a.uname from tb_test a where a.uname!='' and a.id<b.id order by a.id desc limit 1)
end
from tb_test b order by b.id;
id | uname
----+-------
1 | a
2 | a
3 | a
4 | a
5 | a
6 | bb
7 | bb
8 | bb
9 | x
10 | x
(10 rows)
也可以用窗口函數,如下:
dbone=# SELECT
dbone-# id, uname, uname_partition, first_value(uname) over (partition by uname_partition order by id)
dbone-# FROM (
dbone(# SELECT
dbone(# id,
dbone(# uname,
dbone(# sum(case when uname is null then 0 else 1 end) over (order by id) as uname_partition
dbone(# FROM tb_test
dbone(# ORDER BY id ASC
dbone(# ) as q;
id | uname | uname_partition | first_value
----+-------+-----------------+-------------
1 | a | 1 | a
2 | a | 2 | a
3 | | 2 | a
4 | | 2 | a
5 | | 2 | a
6 | bb | 3 | bb
7 | | 3 | bb
8 | | 3 | bb
9 | x | 4 | x
10 | | 4 | x
(10 rows)
dbone=# \timing
Timing is on.
dbone=# SELECT
id, uname, uname_partition, first_value(uname) over (partition by uname_partition order by id)
FROM (
SELECT
id,
uname,
sum(case when uname is null then 0 else 1 end) over (order by id) as uname_partition
FROM tb_test
ORDER BY id ASC
) as q;
id | uname | uname_partition | first_value
----+-------+-----------------+-------------
1 | a | 1 | a
2 | a | 2 | a
3 | | 2 | a
4 | | 2 | a
5 | | 2 | a
6 | bb | 3 | bb
7 | | 3 | bb
8 | | 3 | bb
9 | x | 4 | x
10 | | 4 | x
(10 rows)
Time: 0.805 ms
dbone=# select b.id,
case
when b.uname!='' then b.uname
else (select a.uname from tb_test a where a.uname!='' and a.id<b.id order by a.id desc limit 1)
end
from tb_test b order by b.id;
id | uname
----+-------
1 | a
2 | a
3 | a
4 | a
5 | a
6 | bb
7 | bb
8 | bb
9 | x
10 | x
(10 rows)
Time: 0.920 ms
dbone=#
dbone=# explain select b.id,
case
when b.uname!='' then b.uname
else (select a.uname from tb_test a where a.uname!='' and a.id<b.id order by a.id desc limit 1)
end
from tb_test b order by b.id;
QUERY PLAN
------------------------------------------------------------------------------------
Sort (cost=12.97..12.99 rows=10 width=82)
Sort Key: b.id
-> Seq Scan on tb_test b (cost=0.00..12.80 rows=10 width=82)
SubPlan 1
-> Limit (cost=1.16..1.17 rows=1 width=82)
-> Sort (cost=1.16..1.17 rows=3 width=82)
Sort Key: a.id
-> Seq Scan on tb_test a (cost=0.00..1.15 rows=3 width=82)
Filter: (((uname)::text <> ''::text) AND (id < b.id))
(9 rows)
Time: 1.880 ms
dbone=#
dbone=#
dbone=#
dbone=#
dbone=#
dbone=# explain SELECT
id, uname, uname_partition, first_value(uname) over (partition by uname_partition order by id)
FROM (
SELECT
id,
uname,
sum(case when uname is null then 0 else 1 end) over (order by id) as uname_partition
FROM tb_test
ORDER BY id ASC
) as q;
QUERY PLAN
---------------------------------------------------------------------------------------
WindowAgg (cost=1.71..1.91 rows=10 width=90)
-> Sort (cost=1.71..1.73 rows=10 width=90)
Sort Key: q.uname_partition, q.id
-> Subquery Scan on q (cost=1.27..1.54 rows=10 width=90)
-> WindowAgg (cost=1.27..1.44 rows=10 width=82)
-> Sort (cost=1.27..1.29 rows=10 width=82)
Sort Key: tb_test.id
-> Seq Scan on tb_test (cost=0.00..1.10 rows=10 width=82)
(8 rows)
Time: 0.770 ms
這兒只有十條記錄,性能相差不大,如果記錄多還是要用窗口函數來的快
參考
http://stackoverflow.com/questions/18987791/how-do-i-efficiently-select-the-previous-non-null-value
-----------------
轉載請著明出處:
blog.csdn.net/beiigang