PostgreSQL 9.5新特性 width_bucket求位置插值，可展示柱状图-阿里云开发者社区

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bucket+1。bucket除了用数字来表示，还可以直接用数组来表示。

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(operandnumeric, b1 numeric, b2numeric, 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(operandanyelement, thresholdsanyarray)	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 arraymust 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

--------------

(1 row)

postgres=# select width_bucket(5.0, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(-0.0000001, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(5.1, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

在边界内：

postgres=# select width_bucket(0, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(1, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(1.9, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(1.9999999, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(2, 0.0, 5.0, 5);

width_bucket

--------------

(1 row)

postgres=# select width_bucket(4.9999, 0.0, 5.0, 5);

width_bucket

--------------

(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

--------------

(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

--------------

(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