To highlight this difference, here I have an empty table with 3 columns: When you ask 100 people how they would add DISTINCT to the original query (or how they would eliminate duplicates), I would guess you might get 2 or 3 who do it the way you did. Design and content © 2012-2020 SQL Sentry, LLC. SELECT distinct OrderID FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") The PostgreSQL GROUP BY condition is used with SELECT command, and it can also be used to reduce the redundancy in the result. We also see examples of how GROUP BY clause working with SUM() function, COUNT(), JOIN clause, multiple columns, and the without an aggregate function.. A video replay and other materials are available here: One of the items I always mention in that session is that I generally prefer GROUP BY over DISTINCT when eliminating duplicates. The only requirement is that we ORDER BY the field we group by (department in this case). (This isn't scientific data; just my observation/experience.). 2) Using PostgreSQL GROUP BY with SUM() function example. La principale… Lire plus . 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. Yet in the DISTINCT plan, most of the I/O cost is in the index spool (and here's that tooltip; the I/O cost here is ~41.4 "query bucks"). Given that all other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY? I think this is the new URL: The rule I have always required is that if the are two queries and performance is roughly identical then use the easier query to maintain. In real-life scenarios, there always has been a need for constraints on data so that we may have data that is mostly bug-free and consistent to ensure data integrity. In this case, the GROUP BY works like the DISTINCT clause that removes duplicate rows from the result set. It does not send any column to display. Note: The DISTINCT clause is only used with the SELECT command. DISTINCT vs. GROUP BY: Date: 2010-02-09 21:46:16: Message-ID: 1265751976.2513.34.camel@localhost: Views: Raw Message | Whole Thread | Download mbox | Resend email: Thread: Lists: pgsql-performance >From what I've read on the net, these should be very similar, and should generate equivalent plans, in such cases: SELECT DISTINCT x FROM mytable SELECT x FROM mytable GROUP … This is one reason it always bugs me when people say they need to "fix" the operator in the plan with the highest cost. Distinct vs Distinct on. 6. 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. The big difference, for me, is understanding the DISTINCT is logically performed well after GROUP BY. from Sales.OrderLines User contributions are licensed under, he says that these queries are semantically different, Grouped Concatenation : Ordering and Removing Duplicates, Four Practical Use Cases for Grouped Concatenation, SQL Server v.Next : STRING_AGG() performance, SQL Server v.Next : STRING_AGG Performance, Part 2, https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. > DISTINCT in a more efficient way: Probably (although the interactions with ORDER BY might be tricky). FROM (select distinct OrderID from Sales.OrderLines) AS o. While Adam Machanic is correct when he says that these queries are semantically different, the result is the same – we get the same number of rows, containing exactly the same results, and we did it with far fewer reads and CPU. (Remember, these queries return the exact same results.). But I want to confirm - Is the GROUP BY faster because it doesn't have to sort results, whereas DISTINCT must produce sorted results? groupby.org seems to have rebuilt their website without leaving 301 GONE redirects. We also show the re-costed values (which are based on the actual costs observed during query execution, a feature also only found in Plan Explorer). TOP. 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 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. condition: It is the criteria of a query. We'll talk about "query bucks" another time, but the point is that the index spool is more than 10X as expensive as the scan – yet the scan is still the same 3.4 in both plans. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. But at least 90 would just slap DISTINCT at the beginning of the keyword list. Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. Jul 22, 2018. Sep 19, 2005 at 2:51 pm: On Mon, 2005-19-09 at 16:27 +0200, Hans-Jürgen Schönig wrote: I was wondering whether it is possible to teach the planner to handle DISTINCT in a more efficient way: [...] Isn't it possible to perform the same operation using a HashAggregate? GROUP BY can (again, in some cases) filter out the duplicate rows before performing any of that work. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte FROM textes ou. The DISTINCT clause is used in the SELECT statement to remove duplicate rows from a result set. When I remember correct there was a second 'trick' on it by using a UNION with a SELECT NULL, NULL, NULL … I'll bookmark this article and come back, when I find a current statement, that benefits this behavior. WHERE OrderID = o.OrderID So why would I recommend using the wordier and less intuitive GROUP BY syntax over DISTINCT? However, in more complex cases, DISTINCT can end up doing more work. IF YOU HAVE A BAD QUERY… publish that query in a document on what not to do and why so other developers can learn from past mistakes. (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). The SQLPerformance.com bi-weekly newsletter keeps you up to speed on the most recent blog posts and forum discussions in the SQL Server community. Here is the DISTINCT plan: You can see that, in the GROUP BY plan, almost all of the I/O cost is in the scans (here's the tooltip for the CI scan, showing an I/O cost of ~3.4 "query bucks"). DISTINCT ON (…) is an extension of the SQL standard. Définition du GROUP BY. When performance is critical then DOCUMENT why and store the slower but query to read away so it could be reviewed as I've seen slower performing queries perform later in subsequent versions of SQL Server. ) So while DISTINCT and GROUP BY are identical in a lot of scenarios, here is one case where the GROUP BY approach definitely leads to better performance (at the cost of less clear declarative intent in the query itself). Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. Add two joins to this query (like say they wanted to output the customer name and the total cost of manufacturing for each order) and then it gets a little harder to read and maintain as you'll be adding a bunch of these subqueries from different tables. Syntaxe L’utilisation de HAVING s’utilise de la manière suivante […] Looking at the list you can see that GROUP BY and HAVING will happen well before DISTINCT (which is itself an adjective of the SELECT CLAUSE). Parce que si je fais . This is done to eliminate redundancy in the output and/or compute aggregates that apply to these groups. The ma j or difference between the DISTINCT and GROUP BY is, GROUP BY operator is meant for the aggregating or grouping rows whereas DISTINCT is just used to get distinct values. [PostgreSQL-Hackers] Re: DISTINCT vs. GROUP BY; Neil Conway. So while DISTINCT and GROUP BY are identical in a lot of scenarios, here is one case where the GROUP BY approach definitely leads to better performance (at the cost of less clear declarative intent in the query itself). sadly not at the moment, since it was in some older data migration scripts. This modified text is an extract of the original Stack Overflow Documentation created by following contributors and released under CC BY-SA 3.0 5. No one has touched that part of the planner in a very long time. DISTINCT Constraints cannot be violated so they are very much reliable. When I see GROUP BY at the outer level of a complicated query, especially when it's across half a dozen or more columns, it is frequently associated with poor performance. FROM uniqueOL AS o; You've made a query perform relatively okay using the keyword DISTINCT – I think you've made the point, but you've missed the spirit. Note: The DISTINCT clause is only used with the SELECT command. So we can say that constraints define some rules which the data must follow in a table. You might get 1 or 2 who use GROUP BY. Constraints in PostgreSQL are used to limit the type of data that can be inserted in a table. We just have to remember to take the time to do it as part of SQL query optimization…. https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. SQL. 8. 7. Let's start with something simple using Wide World Importers. I am using postgres 8.1.3 Actually, I think I answered my own question already. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. One of the query comparisons that I showed in that post was between a GROUP BY and DISTINCT for a sub-query, showing that the DISTINCT is a lot slower, because it has to fetch the Product Name for every row in the Sales table, rather than just for each different ProductID. It does not care for whats in parenthesis around it. 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. The GROUP BY clause follows the WHERE clause in a SELECT statement and precedes the ORDER BY clause. In my opinion, if you want to dedupe your completed result set, with the emphasis on completed, use DISINCT. It's generally an aggregation that could have been done in a sub-query and then joined to the associated data, resulting in much less work for SQL Server. PostgreSQL GROUP BY example1. I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. Wouldn't the following query be the logical equivalent without using the group by? 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. Code: SELECT deptno, COUNT(*) FROM employee GROUP … expression: It may be arguments or statements e.t.c. All rights reserved. We can also compare the execution plans when we change the costs from CPU + I/O combined to I/O only, a feature exclusive to Plan Explorer. Well, in this simple case, it's a coin flip. DISTINCT: This clause is optional. PostgreSQL Oracle Sybase SQL-Server Office. 3. I'd be interested to know if you think there are any scenarios where DISTINCT is better than GROUP BY, at least in terms of performance, which is far less subjective than style or whether a statement needs to be self-documenting. >From what I've read on the net, these should be very similar,and should generate equivalent plans, in such cases: SELECT DISTINCT x FROM mytableSELECT x FROM mytable GROUP BY x. The table has an index on (clicked at time zone 'PST'). OUTER FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description GROUP BY vs DISTINCT; Brian Herlihy. Summary: in this tutorial, you will learn how to use the PostgreSQL SELECT DISTINCT clause to remove duplicate rows from a result set returned by a query.. Introduction to PostgreSQL SELECT DISTINCT clause. 9. And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? 4. 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. Otherwise, you're probably after grouping. This is correct. 10 ORDER BY 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. Sure, if that is clearer to you. 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. While in SQL Server v.Next you will be able to use STRING_AGG (see posts here and here), the rest of us have to carry on with FOR XML PATH (and before you tell me about how amazing recursive CTEs are for this, please read this post, too). 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. DISTINCT is used to filter unique records out of the records that satisfy the query criteria.The "GROUP BY" clause is used when you need to group the data and it s hould be used to apply aggregate operators to each group.Sometimes, people get confused when to use DISTINCT and when and why to use GROUP BY in SQL queries. Microsoft Office Access Excel Word Outlook PowerPoint SharePoint ... Quelle est la différence entre DISTINCT et GROUP BY ? https://msdn.microsoft.com/en-us/library/ms189499.aspx#Anchor_2. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description Copyright © 1996-2020 The PostgreSQL Global Development Group, pgsql-performance . SELECT WHERE OrderID = o.OrderID There are many constraints in PostgreSQL, they can be applied to either … It could reduce the I/O very much in this cases. These two queries produce the same result: And in fact derive their results using the exact same execution plan: Same operators, same number of reads, negligible differences in CPU and total duration (they take turns "winning"). Note that the CPU is a lot higher with the index spool, too. FROM Sales.OrderLines Sometimes I use DISTINCT in a subquery to force it to be "materialized", when I know that this would reduce the number of results very much but the compiler does not "believe" this and groups to late. Dec 20, 2006 at 7:26 am: I have a question about the following. HAVING Constraints make data accurate and reliable. WHERE I am trying to get a distinct set of rows from 2 tables. However, in my case (postgresql-server-8.1.18-2.el5_4.1),they generated different results with quite differentexecution times (73ms vs 40ms for DISTINCT and GROUP BYrespectively): tts_server_db=# EXPLAIN ANALYZE select userdata from tagrecord where clientRmaInId = 'CPC-RMA-00110' group by userdata; QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------------- HashAggregate (cost=775.68..775.69 rows=1 width=146) (actual time=40.058..40.058 rows=0 loops=1) -> Bitmap Heap Scan on tagrecord (cost=4.00..774.96 rows=286 width=146) (actual time=40.055..40.055 rows=0 loops=1) Recheck Cond: ((clientrmainid)::text = 'CPC-RMA-00110'::text) -> Bitmap Index Scan on idx_tagdata_clientrmainid (cost=0.00..4.00 rows=286 width=0) (actual time=40.050..40.050 rows=0 loops=1) Index Cond: ((clientrmainid)::text = 'CPC-RMA-00110'::text) Total runtime: 40.121 ms, tts_server_db=# EXPLAIN ANALYZE select distinct userdata from tagrecord where clientRmaInId = 'CPC-RMA-00109'; QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------------------- Unique (cost=786.63..788.06 rows=1 width=146) (actual time=73.018..73.018 rows=0 loops=1) -> Sort (cost=786.63..787.34 rows=286 width=146) (actual time=73.016..73.016 rows=0 loops=1) Sort Key: userdata -> Bitmap Heap Scan on tagrecord (cost=4.00..774.96 rows=286 width=146) (actual time=72.940..72.940 rows=0 loops=1) Recheck Cond: ((clientrmainid)::text = 'CPC-RMA-00109'::text) -> Bitmap Index Scan on idx_tagdata_clientrmainid (cost=0.00..4.00 rows=286 width=0) (actual time=72.936..72.936 rows=0 loops=1) Index Cond: ((clientrmainid)::text = 'CPC-RMA-00109'::text) Total runtime: 73.144 ms. -- Dimi Paun Lattica, Inc. > SELECT x FROM mytable GROUP BY x > However, in my case (postgresql-server-8.1.18-2.el5_4.1), > they generated different results with quite different > execution times (73ms vs 40ms for DISTINCT and GROUP BY > respectively): The results certainly ought to be the same (although perhaps not with the same ordering) --- if they aren't, please provide a reproducible test case. FROM Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … CUBE | ROLLUP ON with uniqueOL as ( Introduction. The sample table. The DISTINCT clause keeps one row for each group of duplicates. Distinct is used to find unique/distinct records where as a group by is used to group a selected set of rows into summary rows by one or more columns or an expression. Dynatrace PostgreSQL Monitor, Regardless of your belief it will: Make each row unique; When checking for uniqueness it will look at all columns selected. Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. Thomas, can you share an example that demonstrates this? sql documentation: SQL Group By vs Distinct. Différence entre HAVING et WHERE Les clauses WHERE et HAVING sont principalement utilisées dans des requêtes SQL, elles permettent de limiter une résultat en utilisant un prédicat spécifique. The Logical Query Processing Phase Order of Execution is as follows: 1. While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases. Let's talk about string aggregation, for example. Is there any dissadvantage of using "group by" to obtain a unique list? 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.. Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. Distinct vs group by performance postgresql. Not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database.. eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. They just aren't logically equivalent, and therefore shouldn't be used interchangeably; you can further filter groupings with the HAVING clause, and can apply windowed functions that will be processed prior to the deduping of a DISTINCT clause. PostgreSQL Group By. Interesting! Let start the basic command - distinct. In this section, we are going to understand the working of GROUP BY clause in PostgreSQL. La condition HAVING en SQL est presque similaire à WHERE à la seule différence que HAVING permet de filtrer en utilisant des fonctions telles que SUM(), COUNT(), AVG(), MIN() ou MAX(). The functional difference is thus obvious. It indicates uniqueness. IMHO, anyway. I'd be interested to know if you think there are any scenarios where DISTINCT is better than GROUP BY, at least in terms of performance, which is far less subjective than style or whether a … Let’s have a look at difference between distinct and group by in SQL Server . SELECT b,c,d FROM a GROUP BY b,c,d; vs SELECT DISTINCT b,c,d FROM a; We see a few scenarios where Postgres optimizes by removing unnecessary columns from the GROUP BY list (if a subset is already known to be Unique) and where Postgres could do even better. 2. 11. After looking at someone else's query I noticed they were doing a group by to obtain the unique list. This seems clearer to me. The GROUP BY clause is useful when it is used in conjunction with an aggregate function. PostgreSQL does all the heavy lifting for us. FROM Sales.OrderLines Postgresql Performance Subject: Re: GROUP BY vs DISTINCT: Date: 2006-12-20 11:00:07: Message-ID: 20061220105739.GB31739@uio.no: Views: Raw Message | Whole Thread | Download mbox | Resend email: Thread: Lists: pgsql-performance: On Tue, Dec 19, 2006 at 11:19:39PM -0800, Brian Herlihy wrote: > Actually, I think I answered my own question … Paul White is an independent SQL Server consultant specializing in performance tuning, execution plans, and the query optimizer. GROUP BY Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. The knee-jerk reaction is to throw a DISTINCT on the column list: That eliminates the duplicates (and changes the ordering properties on the scans, so the results won't necessarily appear in a predictable order), and produces the following execution plan: Another way to do this is to add a GROUP BY for the OrderID (since the subquery doesn't explicitly need to be referenced again in the GROUP BY): This produces the same results (though order has returned), and a slightly different plan: The performance metrics, however, are interesting to compare. GROUP BY: organisez des données identiques en groupes.Maintenant, la table CLIENTS a les enregistrements suivants avec des noms en double: @AaronBertrand those queries are not really logically equivalent — DISTINCT is on both columns, whereas your GROUP BY is only on one, — Adam Machanic (@AdamMachanic) January 20, 2017. By can ( again, in more complex cases, DISTINCT can end up doing more work org. The redundancy in the output and/or compute aggregates that apply to these groups Quelle est différence. A coin flip have to remember to take the time to do it as part of the keyword.... Where clause in a table a result set why would I recommend using the wordier less. Remember, these queries return the exact same results. ) and precedes ORDER! Out the duplicate rows from the result set, with the SELECT command, and then tosses out duplicates in. By, is understanding the DISTINCT is logically performed well after GROUP BY is only used the! Performed well after GROUP BY very long time I presented my T-SQL: Bad Habits and Best Practices session the. The duplicate rows before performing any of that work in PostgreSQL are used reduce! Microsoft Office Access Excel Word Outlook PowerPoint SharePoint... Quelle est la entre..., execution plans, and GROUP BY works like the DISTINCT clause is only required when aggregations postgresql distinct vs group by! 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Development GROUP, pgsql-performance < pgsql-performance ( at ) postgresql distinct vs group by ( dot ) >. A very long time data must follow in a table and the query optimizer any dissadvantage using... String aggregation, for me, is understanding the DISTINCT clause keeps one for! It could reduce the I/O very much reliable SQL Server consultant specializing in tuning. Get 1 or 2 who use GROUP BY works like the DISTINCT clause removes. Index spool, too is there ever a difference 's start with something simple using Wide Importers. To remove duplicate rows before performing any of that work a GROUP clause... Enews is a lot higher with the index spool, too section, we going... That does n't mean it needs to be evaluated, and GROUP BY works like DISTINCT. All the selected columns in the output and/or compute aggregates that apply to these groups help improve your productivity and. With SELECT command use GROUP BY condition is used in the SELECT.... About string aggregation, for example in more complex cases, DISTINCT can up! Simple using Wide World Importers, it 's a coin flip be tricky ) after looking at someone else query. Complex cases, DISTINCT can end up doing more work scientific data ; just my observation/experience. ) index (! Very long time constraints can not be violated so they are interchangeable in many.! Emyr, you 're right, the GROUP BY, is there a. ( ) function example est la différence entre DISTINCT et GROUP BY condition is in... T-Sql: Bad Habits and Best Practices session during the GroupBy conference to obtain the unique list unique ; checking. Limit the type of data that can be inserted in a more efficient way Probably. 'S a coin flip selected columns in the output and/or compute aggregates that apply to these groups very long.... My T-SQL: Bad Habits and Best Practices session postgresql distinct vs group by the GroupBy conference clicked at time zone 'PST )... Or statements e.t.c doing more work when checking for uniqueness it will: each... To find DISTINCT values as shown in below query a lot higher with the emphasis on completed use... Link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ index on ( … ) is an extract of SQL... It can also be used to reduce the redundancy in the SELECT command, then! Global Development GROUP, pgsql-performance < pgsql-performance ( at ) PostgreSQL ( dot ) org.... Org > row for each GROUP of duplicates BY the field we GROUP?. The unique list filter out the duplicate rows before performing any of that work not care whats! For brevity I create the simplest, most minimal queries to demonstrate a concept website without leaving 301 GONE.!