Oracle union vs union all performance

Hi tom, I have a question regarding the internals (and costs) of a UNION ALL statement. Up to now we are running some of our selects on a huge table (table1) which consists of more than billion rows. Published by sqlandplsql To learn more about the Oracle , SQL, PL SQL, Performance Tuning, Database Modeling, Ubuntu, MySQL etc.

Suggestions, comments, feedbacks and referrals are highly appreciated. This article is completely re-written with better example SQL SERVER – Difference Between Union vs. Union All – Optimal Performance Comparison. I suggest all of my readers to go here for update article.

UNION ALL command is equal to UNION comman except that UNION ALL selects all the values. The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all the rows from all the tables fitting your query specifics and combines them into a table. More than a year ago I had written article SQL SERVER – Union vs. I have got many request to update this article. UNION and UNION ALL are used to combine data or record sets from two different tables.

One could combine more than tables. Just like JOINS, UNION combines data into a single record-set but vertically by adding rows from another table. UNION removes duplicate records (where all columns in the are the same), UNION ALL does not. There is a performance hit when using UNION instead of UNION ALL , since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports). UNION versus SELECT DISTINCT and UNION ALL.

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. 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.

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. Notice the heavily skewed … Continue reading OR vs UNION ALL – Is One Better For Performance ? A UNION places a result set on top another, meaning that it appends result sets vertically.

However, a join such as INNER JOIN or LEFT JOIN combines result sets horizontally. SQL union vs union all : SQL Set Operators combines the result of queries or components on to the single result. The queries containing the different set operators like union , union all , intersection minus are simply called as Compound Query.

SQL set operators used to get meaningful data from or more different tables. In real world scenarios set operators are very useful in reporting. Sinisa pointed out I should be using UNION ALL instead of simple UNION in my benchmarks, and he was right. Numbers would be different but it should not change general point of having optimization of moving LIMIT inside of. We tried pushing the SQL into a WITH clause, and some other subquery rewrites, but no perfomance boost.

UNION statements can sometimes introduce performance penalties into your query. This post has a look at how to tune your query up!