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...
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 filter step where one row source is filtering, another, functions such as min may introduce filter steps into query plans. 3.view --视图,大都...
Both queries return the same result (55, the sum of numbers from 1 to 10), but the first query reads all 122071 granules (each granule contains 8192 rows so that comes out to the entire dataset of 1 billion rows) and the second query reads just the single granule that contains the ten...
MySQL Query Profiler Watch the video to see the MySQL Query Profiler in action. Learn how it makes database development a whole lot easier and more efficient. In addition, it is a great optimization tool for tuning MySQL queries and investigating performance issues. ...
In the example below the table SALES does not have statistics, so the optimizer has used dynamic sampling during the query optimization, which is displayed in the plan using the +note' in the query: SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('plan_table',null,'basic +note'))...
With the help ofEXPLAIN, you can see where you should add indexes to tables so that the statement executes faster by using indexes to find rows. You can also useEXPLAINto check whether the optimizer joins the tables in an optimal order. To give a hint to the optimizer to use a join or...
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 1. SQL> select * from scott.emp a,scott.emp b where a.empno=b.mgr; ...
Based on the execution duration of each operator, you can perform SQL statement optimization. Note In Hologres V1.3.4x and later, results returned by the EXPLAIN and EXPLAIN ANALYZE statements are optimized. If you want to query execution plans with higher readability, we recommend that you ...
To view the EXPLAIN PLAN for this query, use the command: EXPLAIN PLAN FOR select /*+ result_cache */ deptno, avg(sal) from emp group by deptno; select PLAN_TABLE_OUTPUT from TABLE (DBMS_XPLAN.DISPLAY()); The EXPLAIN PLAN output for this query should look similar to the following...
So the PLAN_TABLE that is resolved first by EXPLAIN PLAN FOR is not the same PLAN_TABLE that is resolved first by standard SQL including DBMS_XPLAN, which is defined with “invoker rights”. SQL> select * from table(dbms_xplan.display('mnash.plan_table')); ...