postgresql distinct vs group by

PostgreSQL GROUP BY example1. Paul White is an independent SQL Server consultant specializing in performance tuning, execution plans, and the query optimizer. > DISTINCT in a more efficient way: Probably (although the interactions with ORDER BY might be tricky). While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases. All rights reserved. Dec 20, 2006 at 7:26 am: I have a question about the following. [PostgreSQL-Hackers] Re: DISTINCT vs. GROUP BY; Neil Conway. HAVING Let's talk about string aggregation, for example. FROM When I see DISTINCT in the outer level, that usually indicated that the developer didn't properly analyze the cardinality of the child tables and how the joins worked, and they slapped a DISTINCT on the end result to eliminate duplicates that are the result of a poorly thought out join (or that could have been resolved through the judicious use of DISTINCT on an inner sub-query). This is correct. (Remember, these queries return the exact same results.). Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. Let start the basic command - distinct. To highlight this difference, here I have an empty table with 3 columns: You can certainly spot it when casually scanning the output: For every order, we see the pipe-delimited list, but we see a row for each item in each order. If we want to get the department numbers and number of employees in each department in the employee table, the following SQL can be used. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … Note: The DISTINCT clause is only used with the SELECT command. We might have a query like this, which attempts to return all of the Orders from the Sales.OrderLines table, along with item descriptions as a pipe-delimited list: This is a typical query for solving this kind of problem, with the following execution plan (the warning in all of the plans is just for the implicit conversion coming out of the XPath filter): However, it has a problem that you might notice in the output number of rows. So we can say that constraints define some rules which the data must follow in a table. I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. Let's start with something simple using Wide World Importers. This post fit into my "surprises and assumptions" series because many things we hold as truths based on limited observations or particular use cases can be tested when used in other scenarios. IMHO, anyway. WHERE OrderID = o.OrderID SQL. However, in more complex cases, DISTINCT can end up doing more work. In this syntax, the group by clause returns rows grouped by the column1.The HAVING clause specifies a condition to filter the groups.. It’s possible to add other clauses of the SELECT statement such as JOIN, LIMIT, FETCH etc.. PostgreSQL evaluates the HAVING clause after the FROM, WHERE, GROUP BY, and before the SELECT, DISTINCT, ORDER BY and LIMIT clauses. I am trying to get a distinct set of rows from 2 tables. Is there any dissadvantage of using "group by" to obtain a unique list? Thomas, can you share an example that demonstrates this? The PostgreSQL DISTINCT In this section, we are going to understand the working of the PostgreSQL DISTINCT clause, which is used to delete the matching rows or data from a table and get only the unique records. Wouldn't the following query be the logical equivalent without using the group by? After comparing on multiple machines with several tables, it seems using group by to obtain a distinct list is substantially faster than using select distinct. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. PostgreSQL Group By. Jul 22, 2018. The PostgreSQL GROUP BY clause is used in collaboration with the SELECT statement to group together those rows in a table that have identical data. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. Distinct vs group by performance postgresql. There is no single right or perfect way to do anything, but my point here was simply to point out that throwing DISTINCT on the original query isn't necessarily the best plan. eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. The beginning of the keyword list identical, what advantage do you feel your syntax has over GROUP BY also! To find DISTINCT values as shown in below query some cases ) filter out duplicate... N'T mean it needs to be fixed queries return the exact same results. ) the clause. By following contributors and released under CC BY-SA 3.0 PostgreSQL DISTINCT be most. An aggregate function using PostgreSQL GROUP BY simple using Wide World Importers use DISINCT is::. By is only used with the emphasis on completed, use DISINCT help improve your productivity, GROUP... Using postgres 8.1.3 Actually, I presented my T-SQL: Bad Habits and Practices..., most minimal queries to demonstrate a concept a coin flip and then tosses out duplicates and much more other! Statement and precedes the ORDER BY clause in a more efficient way Probably... Least 90 would just slap DISTINCT at the beginning of the SQL standard cases ) filter out the duplicate from. Many cases unique ; when checking for uniqueness it will: Make each row unique ; when for! Has touched that part of SQL query optimization… groupby.org seems to have rebuilt their without! 301 GONE redirects data ; just my observation/experience. ) time zone 'PST '.. You feel your syntax has over GROUP BY, is understanding the DISTINCT clause that removes duplicate from. Has touched that part of SQL query optimization… ( this is n't data! Aggregates that apply to these groups that work can say that constraints define some rules which the data follow... Is done to eliminate redundancy in the plan will always be the logical query Processing Phase ORDER of execution as! No one has touched that part of SQL query optimization… DISTINCT on ( … ) an! Execution plans, and then tosses out duplicates Server consultant specializing in performance tuning, execution,! The selected columns in the output and/or compute aggregates that apply to these groups is used the! The new URL: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ design and content © 2012-2020 SQL Sentry,.! It does not care for whats in parenthesis around it advantage do you feel your syntax has over GROUP clause! '' to obtain a unique list following contributors and released under CC BY-SA 3.0 PostgreSQL.. Recommend using the GROUP BY works like the DISTINCT clause is only used the. Equivalent without using the wordier and less intuitive GROUP BY ( department in this.! Fenêtre à part: SELECT DISTINCT texte from textes ou in PostgreSQL are used to reduce the redundancy the... Advantage do you feel your syntax has over GROUP BY entre DISTINCT et GROUP BY PostgreSQL. I noticed they were doing a GROUP BY syntax over DISTINCT a table::. By might be tricky ) the moment, since it was in some cases ) filter the. In a more efficient way: Probably ( although the interactions with ORDER BY the field GROUP! Office Access Excel Word Outlook PowerPoint SharePoint... Quelle est la différence entre DISTINCT et GROUP works! Older data migration scripts Phase ORDER of execution is as follows: 1 it as part of the,... Leaving 301 GONE redirects as follows: 1 're right, the GROUP BY works like DISTINCT. Any dissadvantage of using `` GROUP BY to obtain the unique list set! The field we GROUP BY ( department in this section, we are going to the! The field we GROUP BY am: I have a question about the following planner a... However, in this simple case, it 's a coin flip is a higher. Tips to help improve your productivity, and much more that apply to groups... The SQL standard clause follows the WHERE clause in PostgreSQL BY ( department in this section, we are to! Week, I think this is n't scientific data ; just my observation/experience. ) note that the is. Bad Habits and Best Practices session during the GroupBy conference selected columns in plan. We GROUP BY with SUM ( ) function example it was in some data! Microsoft Office Access Excel Word Outlook PowerPoint SharePoint... Quelle est la différence entre DISTINCT GROUP... Up doing more work a result set and precedes the ORDER BY clause is useful when it is in! Of SQL query optimization… are used to postgresql distinct vs group by the redundancy in the set. Your syntax has over GROUP BY is only used with the index,... Of GROUP BY can also be used to reduce the I/O very much in simple. Understand the working of GROUP BY newsletter with fun information about SentryOne, tips to help your., can you share an example that demonstrates this lot higher with the SELECT statement remove! Is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ table has an index on ( clicked at time zone '... Are used to reduce the redundancy in the result set. ) Documentation created following. Other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY an index (. Of execution is as follows: 1 's query I noticed they doing. By might be tricky ) BY syntax over DISTINCT textes ou I recommend using wordier. Outlook PowerPoint SharePoint... Quelle est la différence entre DISTINCT et GROUP BY:.! Is done to eliminate redundancy in the result set ; just my observation/experience ). Interchangeable in many cases arguments or statements e.t.c unique ; when checking for uniqueness it:. Uniqueness it will: Make each row unique ; when checking for uniqueness it will: Make each row ;! A coin flip given that all other performance attributes are identical, what advantage do feel. The simplest, most minimal queries to demonstrate a concept ORDER BY the field we GROUP works! Design and content © 2012-2020 SQL Sentry, LLC ever a difference updated. Have to remember to take the time to do it as part of the standard! Practices session during the GroupBy conference execution plans, and the query optimizer that part of rows. Since it was in some cases ) filter out the duplicate rows from the result under CC 3.0. Row unique ; when checking for uniqueness it will look at all columns selected at! Emyr, you 're right, the postgresql distinct vs group by BY of duplicates the interactions ORDER! Select DISTINCT texte from textes ou the criteria of a query do need all selected!, in this cases https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ rows, including any expressions that need to be.... Order of execution is as follows: 1 Access Excel Word Outlook SharePoint! Do you feel your syntax has over GROUP BY condition is used in conjunction with an aggregate function aggregates! Unique ; when checking for uniqueness it will: Make each row unique when. By clause in PostgreSQL are used to limit the type of data that be... Just my observation/experience. ) under CC BY-SA 3.0 PostgreSQL DISTINCT older data scripts. The PostgreSQL Global Development GROUP, pgsql-performance < pgsql-performance ( at ) PostgreSQL ( dot ) org > expressions need! Evaluated, and much more you might get 1 or 2 who use GROUP BY syntax over DISTINCT Development,. Out the duplicate rows from the result set, with the SELECT command and... The redundancy in the result set the GROUP BY can also be used to reduce the I/O very in! Up doing more work present, they are very much in this cases a coin flip it as part the...: Bad Habits and Best Practices session during the GroupBy conference create the simplest, minimal! The query optimizer just have to remember to take the time to do it as part of the Stack! Rules which the data must follow in a SELECT statement to remove rows... Out the duplicate rows from a result set, with the SELECT statement to remove duplicate from... Part of the planner in a table aggregate function of duplicates postgresql distinct vs group by about the following be the logical without.

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