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 relevant records by sequentially scanning the input record set....
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...
<dsql_query>最上層/文件元素。 <sql>回應SQL_statement。 <params>目前不使用此標記。 <materialized_view_candidates> (預覽)包含建議具體化檢視的 CREATE 陳述式,可讓 SQL 陳述式有較好的效能。 <dsql_operations>摘要說明和包含查詢步驟,並包含查詢的成本資訊。 也包含所有<dsql_operation>區塊。 此標記包含...
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 ...
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 ...
1.setautotraceon2.explain planforsql语句;selectplan_table_outputfromtable(dbms_xplan.display());3.通过第3方工具,如plsql developer(f5查看执行计划)、toad等; 三、看懂执行计划 1.执行计划中字段解释 SQL> select * from scott.emp a,scott.emp b where a.empno=b.mgr; ...
1.set autotrace on 2.explain plan for sql语句; select plan_table_output from table(dbms_xplan.display()); 3.通过第3方工具,如plsql developer(f5查看执行计划)、toad等; 1. 2. 3. 4. 5. 6. 7. 三、看懂执行计划 1.执行计划中字段解释 ...
EXPLAIN [EXTENDED | DEPENDENCY | AUTHORIZATION] query-sql 1.3 案例实操 1 )查看下面这条语句的执行计划 hive (default)> explain select * from bigtable;hive (default)> explain select click_url, count(*) ct from bigtable groupby click_url; ...
SET odps.sql.allow.fullscan=true; create table shop as select shop_name, customer_id, total_price from sale_detail; Examples Execute the following statements based on the sample data: -- Execute the query statement. select a.customer_id as ashop, sum(a.total_price) as ap,count(b.tot...
theMySQLoptimizerconsidersmanytechniquestoefficientlyperformthelookupsinvolvedinanSQLquery.Aqueryonahugetablecanbeperformedwithoutreadingalltherows;ajoininvolvingseveraltablescanbeperformedwithoutcomparingeverycombinationofrows.Thesetofoperationsthattheoptimizerchoosestoperformthemostefficientqueryiscalledthe“queryexecution...