Teaches you to apply the techniques discussed in the previous sections to help diagnose and correct a number of problematic SQL statements.
The final part of the SQL Tuning workshop focuses on applying the techniques discussed in the previous sections to help diagnose and correct a number of problematic SQL statements and shows how you can use SQL Plan Management or a SQL Patch to influence an execution plan.
Often times DBAs or application architects create views to conceal complex joins or aggregations in order to help simplify the SQL queries developers need to write. However, as an application evolves, and the number of views grow, it can often be difficult for a developer to know which view to use.
It also become easier for a developer to write an apparently simple query, that results in an extremely complex SQL statement being sent to the database, which may execute unnecessary joins or aggregations.
The DBMS_UTILITY.EXPAND_SQL_TEXT procedure, introduced in Oracle Database 12.1, allows developers to expand references to views, by turning them into subqueries in the original statement, so you can see just exactly what tables or views are being accessed and what aggregations are being used.
Let’s imagine we have been asked to determine the how many “Flat Whites” we sold in our coffeeshops this month. As a developer, I know I need to access the SALES table to retrieve the necessary sales data and the PRODUCTS table to limit it to just our “Flat Whites” sales but I also know that the DBA has setup a ton of views to make developers lives easier. In order to determine what views I have access to, I’m going to query the dictionary table USER_VIEWS.
WHERE view_name LIKE '%SALES%';
Based on the list of views available to me, I would likely pick the view called SALES_REPORTING_V or SALES_REPORTING2_V but which would be better?
Let’s use the DBMS_UTILITY.EXPAND_SQL_TEXT procedure to find out. In order to see the underlying query for each view, we can use is a simple “SELECT *” query from each view. First, we will try ‘SELECT * FROM sales_reporting_v‘.
Continue reading “How to determine which view to use”