Answer: A correlated subquery isa subquery that uses values from the outer query, requiring the inner query to execute once for each outer query. The Oracle database wants to execute the subquery once and use the results for all the evaluations in the outer query. What is subquery explain?
1) In the case of nested, the subquery is independent of the main query, and both can run independently, but in the case of a correlated subquery, you cannot run them individually. 2) A correlated subquery is slow because subquery runs for every row returned by the main query, but somet...
NSO 可以这样理解:nested subquery optimizing ,把in转换为JOIN,把NOT IN转换为 anti join(如果能确保列不为null) 那么什么时候oracle会生成 VW_NSO_1 这样的视图呢,请看下面的几个例子(基于ORACLE10g): SQL> select count(*) from t1 where object_id not in (select MAX(object_id) from t2 GROUP BY O...
• In a subquery • In a view query or materialized view query. • In a SELECT statement with the DISTINCT operator. • In a SELECT statement with a GROUP BY or ORDER BY clause. • In a SELECT statement that is combined with another SELECT statement with the UNION, INTERSECT, ...
A semijoin that uses an index scan can now be pushed if it meets the conditions just noted for a pushed outer join, and it uses thefirstMatchstrategy (seeSection 10.2.2.1, “Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations”). ...
Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause. Cannot use the ROLLBACK statement within an INSERT-EXEC statement. Cant Drop Table capitalise the first letter of each word in a string in SQL Server. Capturing the results from exec ...
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This is called a temporary table because a duplicate copy of the data returned by the stored subquery wasn't stored in the database. In Oracle and in the Postgres community, this temporary table is called an inline view. It is referred to as a subselect. Materialized View Materialized view...
()' clause is now used inside the subquery. This is due to an addition in the FieldCompareSetPredicate which now allows 'Distinct' in the subquery. For most databases the 'Distinct' in the subquery has no effect, but on e.g. Oracle it can help the optimizer to pick an index instead...
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