Oracle union all performance

Up to now we are running some of our selects on a huge table (table1) which consists of more than billion rows. A UNION is highly optimized and really fast, except in cases where one query finishes long before the other, and Oracle must wait to get the whole result set before starting sorting. Unio operator only concatenetes result i will suggest you tunning your queries one by one.

We have made union all query to get data from all these queries and each query involves multiple joins and approximately returns 100k records. All the queries uses a common temp table that holds the user details.

This table would be loaded at the start and queries will make use of this table. UNION statements can sometimes introduce performance penalties into your query. This post has a look at how to tune your query up! All set operators have equal precedence. If a SQL statement contains multiple set operators, then Oracle Database evaluates them from the left to right unless parentheses explicitly specify another order.

If you replaced your two union all queries with something like this select null x from dual union all select null from dual You would find it executes lightening fast. How to improve performance of UNION SQL ? Given that the SQL Server optimizer can often reorder things like joins and aggregates to improve performance , it is quite reasonable to expect that SQL Server would also consider reordering concatenation inputs, where this would provide an advantage.

Summary: in this tutorial, you will learn how to use the Oracle UNION operator to combine result sets returned by two or more queries. Introduction to Oracle UNION operator. The UNION operator is a set operator that combines result sets of two or more SELECT statements into a single result set.

It won’t always work, but when it does everyone will be impressed with your performance tuning prowess. Watch this week’s episode on. Our Skewed Data Let’s create a table and insert some data.

Suggestions, comments, feedbacks and referrals are highly appreciated. When I was comparing performance of UNION vs MySQL 5. Numbers would be different but it should not change general point of having optimization of moving LIMIT inside of. This article is completely re-written with better example SQL SERVER – Difference Between Union vs. I suggest all of my readers to go here for update article. UNION causing slow performance Oracle 9i.

More than a year ago I had written article SQL SERVER – Union vs. Union All – Which is better for performance ? I have got many request to update this article. For example if Table_A has rows and Table_B has rows and rows in both the tables are duplicates, the UNION All statement will return rows including the duplicates.

One could combine more than tables.

Oracle is not on the same machine as the other data source. It combines the both SELECT statement result sets and return as it is. It will cause the query to do range scans. The union statement will perform much better. This is true only if an index exists on deptno.

The order by also adds a performance hit and should only be used if needed. UNION ALL is just the UNION set operation along with ALL option. It shows duplicate records also. Union all operator does not eliminate duplicate values.

By considering the performance of SQL using union all is preferable option because it does not check the duplicate values so no sorting required at the time of fetching the.