PostgreSQL 9.5 new feature - width_bucket return the bucket number

背景

PostgreSQL 9.5 新增的一个函数width_bucket ， 用来计算一个值在一个bucket范围内的位置信息，如果这个值超出了给定的范围，返回 0 或者 总buckets+1 。

bucket除了用数字来表示，还可以直接用数组来表示。

Function	Return Type	Description	Example	Result
width_bucket(operand dp, b1 dp, b2 dp, count int)	int	return the bucket number to which operand would be assigned in a histogram having count equal-width buckets spanning the range b1 to b2; returns 0 or count+1 for an input outside the range	width_bucket(5.35, 0.024, 10.06, 5)	3
width_bucket(operand numeric, b1 numeric, b2 numeric, count int)	int	return the bucket number to which operand would be assigned in a histogram having count equal-width buckets spanning the range b1 to b2; returns 0 or count+1 for an input outside the range	width_bucket(5.35, 0.024, 10.06, 5)	3
width_bucket(operand anyelement, thresholds anyarray)	int	return the bucket number to which operand would be assigned given an array listing the lower bounds of the buckets; returns 0 for an input less than the first lower bound; the thresholds array must be sorted, smallest first, or unexpected results will be obtained	width_bucket(now(), array[‘yesterday’, ‘today’, ‘tomorrow’]::timestamptz[])	2

例子

0.0, 5.0是边界，其中包含0.0，但是不包含5.0, 最后一个参数表示一共分为5个bucket。

超出边界：

postgres=# select width_bucket(-1, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            0  
(1 row)  
postgres=# select width_bucket(5.0, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            6  
(1 row)  
postgres=# select width_bucket(-0.0000001, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            0  
(1 row)  
postgres=# select width_bucket(5.1, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            6  
(1 row)  

在边界内：

postgres=# select width_bucket(0, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            1  
(1 row)  
postgres=# select width_bucket(1, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            2  
(1 row)  
postgres=# select width_bucket(1.9, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            2  
(1 row)  
postgres=# select width_bucket(1.9999999, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            2  
(1 row)  
postgres=# select width_bucket(2, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            3  
(1 row)  
postgres=# select width_bucket(4.9999, 0.0, 5.0, 5);  
 width_bucket   
--------------  
            5  
(1 row)  

直接使用数组代表边界：

注意参数类型必须一致。

postgres=# select width_bucket(now(), array['yesterday', 'today', 'tomorrow']::timestamptz[]);  
 width_bucket   
--------------  
            2  
(1 row)  
postgres=# select width_bucket(now(), array['yesterday', 'today', 'tomorrow']::timestamp[]);  
ERROR:  function width_bucket(timestamp with time zone, timestamp without time zone[]) does not exist  
LINE 1: select width_bucket(now(), array['yesterday', 'today', 'tomo...  
               ^  
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.  
  
  
postgres=# select width_bucket(1,'{0,1,100,200,300}'::int[]);  
 width_bucket   
--------------  
            2  
(1 row)  

边界表示如下，所以1落在第二个bucket。

[0,1)  
[1,100)  
[100,200)  
[200,300)  

参考

1. http://www.postgresql.org/docs/devel/static/functions-math.html#FUNCTIONS-MATH-FUNC-TABLE

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