programmers.
I have a question.
I know that Oracle's WITH is used when using subquery a lot, what is the difference from View?
With: The WITH clause is a kind of inline view or a temporary table. The advantage of using is that if there is multiple reference to a subquery it can be replaced temporary table, rather being written multiple times.
View: It is a virtual table based on the result of an SQL statement.
A View is a type of persistent sql object like a table or index used to produce a query result while a with statement is sql syntax similar in effect to a sub query.
With statements allow for the ability to break down a query into smaller efficient steps and ultimately are used to produce a query result.
Since the goal of with statements is to produce a query result, with statements can exist inside views.
Related
I have a statement (in simple we could see it like this: )
Select * from VIEW where View.target='alpha'
The view is using multiple tables - table X, Y, Z for example, and the target will be in X. The view returns a dataset of 2 billion rows and with this query the database will load all of them and search where target equals 'alpha'. Now is there any possibility to make this query faster?
Maybe there is a possibility to make the view from which I load a little bit smaller (?) I think when I could make the 'target='alpha' statement within the view and the view would then be smaller this would really help... but I think when I do this statement within the view the problem would be the same because then the view would do the same (Am I right?)
At the end it would be better when I could have the view as it is and do the work within the new statement but if someone would have an idea that would work when I change the view this could be done also.
Thank you!
A view is a stored SQL statement, and to make a view faster you will have to make the stored SQL statement faster.
The SQL statement depends on tables and to speed up queries against tables you use index. The benefits with index are plenty, read this great answer to a SO question here. It explains what index is, how it works and why its needed.
So to prevent a full table scan you add indexes to make the query faster. In your case you need to identify the columns in the SQL statement that make a good fit for an index. Usually columns that are commonly used in WHERE, ORDER BY, JOIN and GROUP BY clauses make a good fit to be included in an index.
In your case, start looking at the table where the target column exists. Start by adding an index there and then continue with its relations to other tables in the query. This will eventually result in a faster response time when using your view.
To create index on your Oracle table. Read the Oracle docs here
Using more columns within a CTE query affects the performance? I am currently trying to execute a query with the WITH sentence, and it seams that if I use more colum,s, it takes more time to load the data. Am I correct?
The number of columns defined in a CTE should have no effect on the actual performance of the query (it might affect the compile-time, which is generally miniscule).
Why? Because SQL Server "embeds" the code for the CTE in the query itself and then optimizes all the code together. Unused columns should be eliminated.
This might be an over generalization. There might be some cases where SQL Server doesn't eliminate the work for columns -- such as extra aggregation functions in an aggregation query or certain subqueries. But, in general, what is important is how the CTE is used, not how many columns are defined in it.
You can think of CTE as a View but it doesnt materialize to Disk.So A view expands it definition at run time ,same goes for CTE.
WITH and INTO seem very similar. Both create temporary tables. What differentiates them?
WITH doesn't create a temporary table. It allows you assign names to subqueries and then reference them in your actual query as if they were tables or views. It allows you to do stepwise-refinement on a query in a way that would otherwise require either a series of defined table variables/temp tables, or a horrific tangle of nested subqueries.
One way to think of it is as a way of pre-defining your subqueries and then referencing them by name rather than putting the query expression in your final query.
SELECT INTO, on the other hand, just creates a table based on the results of the SELECT.
Suppose I have a SQL Query that isn't really "nice".
The SQL Query has a lot inner join with long conditions, and finally a simple "where" statement.
Now suppose I make a View that corresponds to that SQL Query, excepting the final "where" statement. So that instead of make that long query, I use the view and simply add the "where" statement.
My question is the following:
Which one would have best performance?
I'm not sure if the "View" option corresonds to two sql statement, instead of the one sql statement of the first option. Or before executing the query, the "View" query is "pasted" in the more simple query and is "the same".
No difference. The view uses the same query as the query alone.
To be different it would need to store it in a view table view don't, they're just queries made to look easy from the outside.
Assuming the DBMS you're using has a cost-based query optimizer, you'll probably not get any better performance when a portion of the query is converted to a view. You can confirm this with your database's query explain tool by comparing the estimated cost of both versions of the query.
I recommend using the query explain utility to determine exactly where the query becomes expensive, since the location of a query's pain points are often unexpected. It may seem that all those joins are slowing things down, but they may be exploiting indexes that minimize the I/O and CPU cycles required to connect the rows. Many times the query bogs down because an unindexed column is referenced in a WHERE or ON clause.
Can anybody in here tell me the difference of VIEW and WITH, as I have searched many placed and I can't find anything about it.
My thoughts is that VIEW and WITH are the same, except for VIEWs is saved as a Schema-object, but I can be wrong
SQL views and with clauses are very similar. Here are some differences.
Views create an actual object in the database, with associated metadata and security capabilities. With statements are only part of a single query.
In many databases, views have options, for instance, to index them or to "instantiate" them.
With statements offer the opportunity to have recursive CTEs, in some databases. This is not possible for views.
For simple subqueries incorporated into queries, they are quite similar. The choice really depends on whether you want to create a reusable code (views) or are focused on a single query (with).
Fundamentally, the definition of a view is saved in the database and can be reused by any query, whereas a WITH clause (or Common Table Expression, or CTE) is tied to one specific query and can only be reused by copying.
Otherwise, they will be substantially the same.
If you use a recursive WITH clause, then you can't achieve the same result in a VIEW unless the view definition itself uses a WITH clause (which is legitimate).
In a few words, WITH is a clause that is used in DML, VIEW is a database object. View definition may contain query that uses WITH. You can consider WITH as a variation of derived table (in Microsoft terminology) or inline view(in Oracle) that is defined before main DML and has an ability to refer to itself (recursive queries)
WITH is also used in SQLServer in a different context(query hints).