Postgresql如何在时间戳中绕半微秒？

Question

我的印象是PostgreSQL在时间戳上将半微秒的时间缩短到最接近的微秒。 E.g：

public static int getMaxRow()
{
   return maxrow;
}
public static int getMaxCol()
{
   return maxcol;
}

然后我发现：

> select '2000-01-01T00:00:00.0000585Z'::timestamptz;
          timestamptz          
-------------------------------
 2000-01-01 01:00:00.000058+01
(1 row)

> select '2000-01-01T00:00:00.0000575Z'::timestamptz;
          timestamptz          

-------------------------------
 2000-01-01 01:00:00.000058+01
(1 row)

有人知道Postgresql用于时间戳的舍入算法吗？

有关您的信息，这是我正在运行的Postgresql的版本：

> select '2000-01-01T00:00:00.5024585Z'::timestamptz;
          timestamptz          
-------------------------------
 2000-01-01 01:00:00.502459+01
(1 row)

Answer 1

All the PostgreSQL time types的分辨率为微秒，小数点后六位。 舍入到最近的偶数微秒不会是微秒分辨率。

它的行为与我round half-up一致，这是通常的回合方式。 ＆gt; = 0.5向上舍入，否则向下舍入。

0.5024585四舍五入到小数点后6位舍入到0.502459因为第7位是5。

test=# select '2000-01-01T00:00:00.5024585Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502459
(1 row)

0.5024584999999向下舍入到0.502458，因为第7位是4。

test=# select '2000-01-01T00:00:00.5024584999999Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502458
(1 row)

<击>

没关系，上面似乎是异常的。单步执行＆＃39; 2000-01-01T00：00：00.5024235Z＆＃39;到＆＃39; 2000-01-01T00：00：00.5024355Z＆＃39;与半偶数舍入一致。

我猜测异常是由于浮点错误从输入中的浮点数转换为timestamp使用的整数微秒。

test=# select '2000-01-01T00:00:00.5024235Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502424
(1 row)

test=# select '2000-01-01T00:00:00.5024245Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502425
(1 row)

test=# select '2000-01-01T00:00:00.5024255Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502425
(1 row)

test=# select '2000-01-01T00:00:00.5024265Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502426
(1 row)

test=# select '2000-01-01T00:00:00.5024275Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502428
(1 row)

test=# select '2000-01-01T00:00:00.5024285Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502428
(1 row)

test=# select '2000-01-01T00:00:00.5024295Z'::timestamp;
         timestamp         
---------------------------
 2000-01-01 00:00:00.50243
(1 row)

test=# select '2000-01-01T00:00:00.5024305Z'::timestamp;
         timestamp         
---------------------------
 2000-01-01 00:00:00.50243
(1 row)

test=# select '2000-01-01T00:00:00.5024315Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502432
(1 row)

test=# select '2000-01-01T00:00:00.5024325Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502432
(1 row)

test=# select '2000-01-01T00:00:00.5024335Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502434
(1 row)

test=# select '2000-01-01T00:00:00.5024345Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502434
(1 row)

test=# select '2000-01-01T00:00:00.5024355Z'::timestamp;
         timestamp          
----------------------------
 2000-01-01 00:00:00.502436
(1 row)

这也与interval N microsecond一起发挥作用。较小的小数位表示浮动点错误较少。

test=# select interval '0.5 microsecond';
 interval 
----------
 00:00:00
(1 row)

test=# select interval '1.5 microsecond';
    interval     
-----------------
 00:00:00.000002
(1 row)

test=# select interval '2.5 microsecond';
    interval     
-----------------
 00:00:00.000002
(1 row)

test=# select interval '3.5 microsecond';
    interval     
-----------------
 00:00:00.000004
(1 row)

test=# select interval '4.5 microsecond';
    interval     
-----------------
 00:00:00.000004
(1 row)

test=# select interval '5.5 microsecond';
    interval     
-----------------
 00:00:00.000006
(1 row)

test=# select interval '6.5 microsecond';
    interval     
-----------------
 00:00:00.000006
(1 row)

一个小型C程序确认存在浮点精度问题，单精度浮点数为7位小数，会影响舍入。

#include <math.h>
#include <stdio.h>

int main() {
    float nums[] = {
        0.5024235f,
        0.5024245f,
        0.5024255f,
        0.5024265f,
        0.5024275f,
        0.5024285f,
        0.5024295f,
        0.5024305f,
        NAN
    };

    for( int i = 0; !isnan(nums[i]); i++ ) {
        printf("%0.8f\n", nums[i]);
    }
}

这会产生：

0.50242352
0.50242448
0.50242549
0.50242651
0.50242752
0.50242847
0.50242949
0.50243050

虽然有双打，但没有问题。

0.50242350
0.50242450
0.50242550
0.50242650
0.50242750
0.50242850
0.50242950
0.50243050