650536d53e
For #4674 This is a udaf for approximate topn using Space-Saving algorithm. At present, we can only calculate the frequent items and their frequencies in a certain column, based on which we can implement similar topN functions supported by Kylin in the future. I have also added a test to calculate the accuracy of this algorithm. The following is a rough running result. The total amount of data is 1 million lines and follows the Zipfian distribution, where Element Cardinality represents the data cardinality, 20X, 50X.. The value representing space_expand_rate is 20,50, which is used to set the counter number in the space-saving algorithm ``` zf exponent = 0.5 Element cardinality 20X 50X 100X 1000 100% 100% 100% 10000 100% 100% 100% 100000 100% 100% 100% 500000 94% 98% 99% zf exponent = 0.6，1 Element cardinality 20X 50X 100X 1000 100% 100% 100% 10000 100% 100% 100% 100000 100% 100% 100% 500000 100% 100% 100% ```
60 lines
2.8 KiB
Markdown
60 lines
2.8 KiB
Markdown
---
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{
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"title": "TOPN",
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"language": "zh-CN"
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}
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---
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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# TOPN
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## description
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### Syntax
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`topn(expr, INT top_num[, INT space_expand_rate])`
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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
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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
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```
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counter numbers = top_num * space_expand_rate
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```
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space_expand_rate的值越大，结果越准确，默认值为50
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## example
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```
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MySQL [test]> select topn(keyword,10) from keyword_table where date>= '2020-06-01' and date <= '2020-06-19' ;
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+------------------------------------------------------------------------------------------------------------+
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| topn(`keyword`, 10) |
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+------------------------------------------------------------------------------------------------------------+
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| a:157, b:138, c:133, d:133, e:131, f:127, g:124, h:122, i:117, k:117 |
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+------------------------------------------------------------------------------------------------------------+
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MySQL [test]> select date,topn(keyword,10,100) from keyword_table where date>= '2020-06-17' and date <= '2020-06-19' group by date;
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+------------+-----------------------------------------------------------------------------------------------+
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| date | topn(`keyword`, 10, 100) |
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+------------+-----------------------------------------------------------------------------------------------+
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| 2020-06-19 | a:11, b:8, c:8, d:7, e:7, f:7, g:7, h:7, i:7, j:7 |
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| 2020-06-18 | a:10, b:8, c:7, f:7, g:7, i:7, k:7, l:7, m:6, d:6 |
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| 2020-06-17 | a:9, b:8, c:8, j:8, d:7, e:7, f:7, h:7, i:7, k:7 |
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+------------+-----------------------------------------------------------------------------------------------+
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```
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## keyword
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TOPN |