表示先走二级索引,再走一级索引找到数据 finds relevant records based on an Index. Index Scans perform 2 read operations: one to read the index and another to read the actual value from the table. 2、顺序扫描 - Seq Scan Node finds
IN ('North America', 'Pacific') 14 AND Gender = 'F' 15 GROUP BY G.StateProvinceName, T.SalesTerritoryGroup 16 ORDER BY AVG(YearlyIncome) DESC</sql> 17 <dsql_operations total_cost="0.926237696" total_number_operations="9"> 18 <dsql_operation operation_type="RND_ID"> 19 <identifi...
1 )开启动态分区功能(默认 true ,开启) set hive.exec.dynamic.partition=true; ( 2 )设置为非严格模式(动态分区的模式,默认 strict ,表示必须指定至少一个分区为 静态分区, nonstrict 模式表示允许所有的分区字段都可以使用动态分区。) set hive.exec.dynamic.partition.mode=nonstrict; ( 3 )在所有执行 MR ...
ClickHouse是俄罗斯的Yandex于2016年开源的列式存储数据库(DBMS),使用C++语言编写,主要用于在线分析处理查询(OLAP),能够使用SQL查询实时生成分析数据报告。 OLAP(On-Line Analytical Processing)翻译为联机分析处理,专注于分析处理,从对数据库操作来看,OLAP是对数据的查询; OLTP(on-line transaction processing)翻译为联机...
the MySQL optimizer considers many techniques to efficiently performthe lookups involved in an SQL query. A query on a huge table can be performedwithout reading all the rows; a join involving several tables can be performedwithout comparing every combination of rows. The set of operations that ...
Oracle Database maintains tables used for internal processing. When it needs to change these tables, Oracle Database generates an internal SQL statement, which in turn generates a recursive call. In short, recursive calls are basically SQL performed on behalf of your SQL. So, if you had to ...
There are a number of different operations that promote sorts: (1)order by clauses (2)group by (3)sort merge join –-这三个会产生排序运算 2.filter --过滤,如not in、min函数等容易产生 Has a number of different meanings, used to indicate partition elimination, may also indicate an actual...
set hive.exec.dynamic.partition=true;( 2 )设置为非严格模式(动态分区的模式,默认 strict ,表示必须指定至少一个分区为静态分区, nonstrict 模式表示允许所有的分区字段都可以使用动态分区。) set hive.exec.dynamic.partition.mode=nonstrict;( 3 )在所有执行 MR 的节点上,最大一共可以创建多少个动态分区。默...
theMySQLoptimizerconsidersmanytechniquestoefficientlyperformthelookupsinvolvedinanSQLquery.Aqueryonahugetablecanbeperformedwithoutreadingalltherows;ajoininvolvingseveraltablescanbeperformedwithoutcomparingeverycombinationofrows.Thesetofoperationsthattheoptimizerchoosestoperformthemostefficientqueryiscalledthe“queryexecution...
看懂执行计划也就成了SQL优化的先决条件。 这里的SQL优化指的是SQL性能问题的定位,定位后就可以解决问题。 一.查看执行计划的三种方法 1.1设置autotrace SQL>set autotrace on SQL>select * from dave; ID NAME --- --- 8 安庆 1 dave 2 bl 1 bl ...