Question

description

Syntax

topn(expr, INT top_num[, INT space_expand_rate])

该topn函数使用Space-Saving算法计算expr中的top_num个频繁项，结果为频繁项及其出现次数，该结果为近似值

space_expand_rate参数是可选项，该值用来设置Space-Saving算法中使用的counter个数

counter numbers = top_num * space_expand_rate

space_expand_rate的值越大，结果越准确，默认值为50

example

MySQL [test]> select topn(keyword,10) from keyword_table where date>= '2020-06-01' and date <= '2020-06-19' ;
+------------------------------------------------------------------------------------------------------------+
| topn(`keyword`, 10)                                                                                        |
+------------------------------------------------------------------------------------------------------------+
| a:157, b:138, c:133, d:133, e:131, f:127, g:124, h:122, i:117, k:117                                       |
+------------------------------------------------------------------------------------------------------------+

MySQL [test]> select date,topn(keyword,10,100) from keyword_table where date>= '2020-06-17' and date <= '2020-06-19' group by date;
+------------+-----------------------------------------------------------------------------------------------+
| date       | topn(`keyword`, 10, 100)                                                                      |
+------------+-----------------------------------------------------------------------------------------------+
| 2020-06-19 | a:11, b:8, c:8, d:7, e:7, f:7, g:7, h:7, i:7, j:7                                             |
| 2020-06-18 | a:10, b:8, c:7, f:7, g:7, i:7, k:7, l:7, m:6, d:6                                             |
| 2020-06-17 | a:9, b:8, c:8, j:8, d:7, e:7, f:7, h:7, i:7, k:7                                              |
+------------+-----------------------------------------------------------------------------------------------+

keyword

TOPN