When a view cannot be merged into the main query you will often see a projection view operation. This indicates that the 'view' will be selected from directly as opposed to being broken down into joins on the base tables. A number of constructs make a view non mergeable. Inline views ar...
With the help ofEXPLAIN, you can see where you should add indexes to tables so that thestatement executes faster by using indexes to find rows. You can also useEXPLAIN to check whether the optimizer joins the tables in an optimal order. Togive a hint to the optimizer to use a join orde...
value IN (SELECT primary_key FROM single_table WHERE some_expr) 此处按照官网的格式未测试出例子来,若有例子的请留言,我测试更新 9、index_subquery 该联接类型类似于unique_subquery。可以替换IN子查询,但只适合下列形式的子查询中的非唯一索引: value IN (SELECT key_column FROM single_table WHERE some_ex...
JOINS:Amazon Redshift uses different join operators based on the physical design of the tables being joined, the location of the data required for the join, and specific attributes of the query itself. Subquery Scan -- Subquery scan and append are used to run UNION queries. ...
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 1. SQL> select * from scott.emp a,scott.emp b where a.empno=b.mgr...
The following example returns a plan for a statement that joins multiple tables: EXPLAIN SELECT f.film_id, title, name category_name FROM film f INNER JOIN film_category fc ON fc.film_id = f.film_id INNER JOIN category c ON c.category_id = fc.category_id ORDER BY title; The output...
We recommend that you prevent non-equivalent joins in SQL statements. The following code provides an example of a nested loop join. BEGIN; CREATE TABLE public.nestedloop_test_1 ( a integer not null, b integer not null ); CALL set_table_property('public.nestedloop_test_1', 'distribution_...
基于oracle的应用系统很多性能问题,是由应用系统sql性能低劣引起的,所以,sql的性能优化很重要,分析与优化sql的性能我们一般通过查看该sql的执行计划,本文就如何看懂执行计划,以及如何通过分析执行计划对sql进行优化做相应说明。 一、什么是执行计划(explain plan) ...
Different from EXPLAIN, EXPLAIN ANALYZE executes the corresponding SQL statement, records its runtime information, and returns the information together with the execution plan. Therefore, you can regard EXPLAIN ANALYZE as an extension of the EXPLAIN statement. Compared to EXPLAIN (...
displaying both the planned and actual execution details. It helps in understanding query performance, identifying bottlenecks, and optimizing complex queries. By analyzing the execution plan, database administrators and developers can make informed adjustments to indexes, joins, and other query elements ...