HISTOGRAM

HISTOGRAM

description

Syntax

histogram(expr[, INT num_buckets])

The histogram function is used to describe the distribution of the data. It uses an "equal height" bucking strategy, and divides the data into buckets according to the value of the data. It describes each bucket with some simple data, such as the number of values that fall in the bucket. It is mainly used by the optimizer to estimate the range query.

The result of the function returns an empty or Json string.

Parameter description：

• num_buckets：Optional. Limit the number of histogram buckets. The default value is 128.

Alias function: hist(expr[, INT num_buckets])

example

MySQL [test]> SELECT histogram(c_float) FROM histogram_test;
+-------------------------------------------------------------------------------------------------------------------------------------+
| histogram(`c_float`)                                                                                                                |
+-------------------------------------------------------------------------------------------------------------------------------------+
| {"num_buckets":3,"buckets":[{"lower":"0.1","upper":"0.1","count":1,"pre_sum":0,"ndv":1},...]} |
+-------------------------------------------------------------------------------------------------------------------------------------+

MySQL [test]> SELECT histogram(c_string, 2) FROM histogram_test;
+-------------------------------------------------------------------------------------------------------------------------------------+
| histogram(`c_string`)                                                                                                               |
+-------------------------------------------------------------------------------------------------------------------------------------+
| {"num_buckets":2,"buckets":[{"lower":"str1","upper":"str7","count":4,"pre_sum":0,"ndv":3},...]} |
+-------------------------------------------------------------------------------------------------------------------------------------+

Query result description：

{
    "num_buckets": 3, 
    "buckets": [
        {
            "lower": "0.1", 
            "upper": "0.2", 
            "count": 2, 
            "pre_sum": 0, 
            "ndv": 2
        }, 
        {
            "lower": "0.8", 
            "upper": "0.9", 
            "count": 2, 
            "pre_sum": 2, 
            "ndv": 2
        }, 
        {
            "lower": "1.0", 
            "upper": "1.0", 
            "count": 2, 
            "pre_sum": 4, 
            "ndv": 1
        }
    ]
}

Field description：

• num_buckets：The number of buckets
• buckets：All buckets
  • lower：Upper bound of the bucket
  • upper：Lower bound of the bucket
  • count：The number of elements contained in the bucket
  • pre_sum：The total number of elements in the front bucket
  • ndv：The number of different values in the bucket

Total number of histogram elements = number of elements in the last bucket(count) + total number of elements in the previous bucket(pre_sum).

keywords

HISTOGRAM, HIST