Question

我需要根据另一个表记录更新一个表记录。

我试过

update      currencies
set         priority_order= t2.priority_order
from        currencies t1
inner join  currencies1 t2
on          t1.id = t2.id

但是给出错误（MySQL和SQL Server的查询工作相同）。

然后我在下面尝试了：

update      currencies
set         priority_order= (select 
                            priority_order 
                            from currencies1 
                            where 
                            currencies.id=currencies1.id
                           )

它工作但很慢，我也需要为一些大桌子做这件事。

有什么想法吗？

Answer 1

在Postgres中，这看起来像是：

update currencies t1
    set priority_order = t2.priority_order
    from currencies1 t2
    where t1.id = t2.id;

Answer 2

UPDATE currencies dst
   SET priority_order = src.priority_order
  FROM currencies src
 WHERE dst.id = src.id
   -- Suppress updates if the value does not actually change
   -- This will avoid creation of row-versions
   -- which will need to be cleaned up afterwards, by (auto)vacuum.
AND  dst.priority_order IS DISTINCT FROM src.priority_order
        ;

测试（10K行，在缓存预热后），对源和目标使用相同的表进行更新：

CREATE TABLE
INSERT 0 10000
VACUUM
Timing is on.
cache warming:
UPDATE 0
Time: 16,410 ms
zero-rows-touched:
UPDATE 0
Time: 8,520 ms
all-rows-touched:
UPDATE 10000
Time: 84,375 ms

通常情况下，您很少会看到没有行影响的情况，也不会影响所有行的情况。但是只触摸了50％的行，查询的速度仍然是原来的两倍。（加上查询后减少的真空工作）

Postgres中的批量更新

2 个答案: