http://www.jasongj.com/2015/03/15/count_distinct/ Web8.2.1.15 GROUP BY Optimization. The most general way to satisfy a GROUP BY clause is to scan the whole table and create a new temporary table where all rows from each group are consecutive, and then use this temporary table to discover groups and apply aggregate functions (if any). In some cases, MySQL is able to do much better than that and ...
MySQL :: MySQL 5.7 Reference Manual :: 8.2.1.15 GROUP BY …
WebMar 31, 2024 · 从1.3版本开始,针对多个Count Distinct场景进行了优化,当Count Distinct数量大于1个时,会自动改写为UNIQ实现(结果和语义和当前实现是一样的,对上层透明),以减少基于当前模式下不必要的subquery重复计算,提升查询性能。 Web背景在分布式数据库中,计算count(distinctxxx),需要对distinct的字段,1、去重,2、重分布去重后的数据,(这一步,如果distin...,CodeAntenna技术文章技术问题代码片段及聚合 ... 《Greenplum 最佳实践 - 估值插件hll的使用(以及hll分式聚合函数优化)》 《PostgreSQL hll ... diamond wave coupon
使用子查询可提升 COUNT DISTINCT 速度 50 倍 - OSCHINA - 中文 …
WebThus, for more details on the optimization possibilities for DISTINCT queries, see Section 8.2.1.15, “GROUP BY Optimization” . When combining LIMIT row_count with DISTINCT, MySQL stops as soon as it finds row_count unique rows. If you do not use columns from all tables named in a query, MySQL stops scanning any unused tables as soon as it ... WebJun 19, 2024 · 当Distinct Key数目过大时,State的读写开销太大,因此Count Distinct优化存在性能瓶颈。 但在很多场景,完全精确的统计并不那么必要。 如果您希望牺牲部分精确度来换取性能上的提升,您可以使用新增的内置函数 APPROX_COUNT_DISTINCT 提升作业 … cistern\u0027s 41