Union vs union all performance

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.

See: SQL SERVER – Difference Between Union vs.

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 ? When I was comparing performance of UNION vs MySQL 5. 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. But now let’s come back to performance considerations: UNION ALL always performs better than UNION.

The reason is that UNION removes duplicates from the result set – the AMPs have to sort the resulting rows of all spools to be able to remove duplicates. More than a year ago I had written article SQL SERVER – Union vs. I have got many request to update this article. One could combine more than tables. UNION and UNION ALL are used to combine data or record sets from two different tables.

Just like JOINS, UNION combines data into a single record-set but vertically by adding rows from another table. It shows duplicate records also. 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. Union all operator does not eliminate duplicate values. As you will see the final ets will differ, but there is some interesting info on how SQL Server actually completes the process.

Where things get a little bit interesting is how UNION handles generating that distinct list. You will notice that the UNION output is in order while the UNION ALL is not. The way PostgreSQL handles this query also explains, why many users report performance problems when using UNION.

PostgreSQL has to sort all the data to remove duplicate – this is clearly more expensive than just adding data to some result. Published by sqlandplsql To learn more about the Oracle, SQL, PL SQL, Performance Tuning, Database Modeling, Ubuntu, MySQL etc. What most people really want.

Suggestions, comments, feedbacks and referrals are highly appreciated. 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. I am having a performance issue with a query that contains UNION ALL. Unions happen in SAP BW as well as enterprise HANA graphical views and in SQL Script UNION statements.

Suppose we want to perform the following activities on our sample tables. While SQL Server will generate the same query plan for either syntax, there is another technique you can try that can sometimes can improve performance under certain conditions: UNION ALL. This post is a continuation of my series to document ways of refactoring queries for improved performance.

UNION insures you get DISTINCT records from both the tables. UNION ALL pulls out all records from both the tables with duplicates. Generally “ UNION ALL ” is considered best in performance when compared with UNION.

But you should only use “ UNION ALL ” when you are sure that you have distinct or no common records in both the records-sets.