mysql - 在 MySQL 中缓存行排名？

Question

我有一个要排序的列，定期更新排名(每天)。我目前在代码中使用它

get all rows from table order by column

rank = 1
foreach row in table
   update row's rank to rank
   rank++

这对 MySQL 中的每一行进行更新。有没有更有效的方法来做到这一点？

Answer 1

使用连接更新:

set @rank := 0;

update tbl a join
  (select id, @rank := @rank + 1 as new_rank from tbl order by col) b
  on a.id = b.id set a.rank = b.new_rank;

如果期望有很多行，您将通过对索引表进行连接来获得最佳性能，例如:

set @rank := 0;

create temporary table tmp (id int primary key, rank int)
  select id, @rank := @rank + 1 as rank from tbl order by col;

update tbl join tmp on tbl.id = tmp.id set tbl.rank = tmp.rank;

最后，您可以通过完全跳过更新步骤并换入新表(并非总是可行)来使其更快:

set @rank := 0;

create table new_tbl (id int primary key, rank int, col char(10),
  col2 char(20)) select id, @rank := @rank + 1 as rank, col, col2
  from tbl order by col;

drop table tbl;
rename table new_tbl to tbl;