承接上一篇博客《GaussDB(DWS)性能调优系列基础篇一:万物之始analyze统计信息》,在GaussDB(DWS)的优化器根据analyze统计信息生成计划后,这里继续介绍如何详细解读计划以及计划的执行过程,从中发现可能存在的性能瓶颈点及其产生的原因。 前言 执行计划(又称解释计划)是数据库执行SQL语句的具体步骤,例如通过索引还
这个filesort是说, MySQL要多做一次额外的排序, 确切的说是快速排序(Quicksort). 先初步了解下Quicksort排序的概念(From Wikipedia). Quicksort is a divide and conquer algorithm. Quicksort first divides a large array into two smaller sub-arrays: the low elements and the high elements. Quicksort can ...
The algorithm isn't simply returning the values that account for the biggest amount of the change. For example, if most (98%) sales came from the USA, then it would commonly be the case that most of the increase was also in the USA. Yet unless the USA or other countries/regions had...
defchangeremove_index:users,name: :index_users_on_discarded_at,algorithm: :concurrently,column: :discarded_atadd_index:users,:discarded_at,where:"discarded_at IS NOT NULL",algorithm: :concurrently,name: :index_users_on_not_null_discarded_atendend Always usealgorithm: :concurrentlyif you're addi...
Sort Selected rows were sorted using a sort algorithm. Union Merge The results of multiple subselects were merged or combined into a single result. Subquery Merge The results of multiple subselects were merged or combined into a single result. Bitmap Merge Multiple bitmaps were merged or combined...
The longest match number string dial-peer algorithm finds the dial-peer with the most numbers in a sequence that exactly match a sequence of numbers in a number string. This concept is clarified in the subsequent scenario. Scenario: Eligible dial-peers have been configured with th...
download large files, for example, you'll sometimes be given what's called an MD5 hash code to check, which is a long number (often in hexadecimal or base 16 format, made up of the numbers 0–9 and the letters A–F) computed from the original file by a complex mathematical algorithm...
If, for any reason, the algorithm of a large online platform favors one candidate, there is no way to counteract its impact, and if multiple online platforms all favor the same candidate, the impact of these dif- ferent sources of influence might be additive. Implications and Concerns ...
1. What is the difference between the worse case and the best case running time of a merge sort? 2. True or false? Let Ai and Ai+1 be the ith largest and the (i+1) largest elements in A. Any algorithm E...
with 48 commentsFrom Stack Overflow: Can anyone give me a good example of when CROSS APPLY makes a difference in those cases where INNER JOIN will work as well? This is of course SQL Server. A quick reminder on the terms. INNER JOIN is the most used construct in SQL: it joins two ...