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
--------------
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]
