> DISTINCT in a more efficient way: Probably (although the interactions with ORDER BY might be tricky). La principale… Lire plus . Not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database.. 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.. Well, in this simple case, it's a coin flip. Microsoft Office Access Excel Word Outlook PowerPoint SharePoint ... Quelle est la différence entre DISTINCT et GROUP BY ? The GROUP BY clause follows the WHERE clause in a SELECT statement and precedes the ORDER BY clause. FROM 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. 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. 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. 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. 4. 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. This is correct. FROM Sales.OrderLines 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. @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. In this section, we are going to understand the working of GROUP BY clause in PostgreSQL. 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. Let’s have a look at difference between distinct and group by in SQL Server . But at least 90 would just slap DISTINCT at the beginning of the keyword list. Wouldn't the following query be the logical equivalent without using the group by? 5. This seems clearer to me. In this case, the GROUP BY works like the DISTINCT clause that removes duplicate rows from the result set. SQL. Constraints cannot be violated so they are very much reliable. Paul White is an independent SQL Server consultant specializing in performance tuning, execution plans, and the query optimizer. In my opinion, if you want to dedupe your completed result set, with the emphasis on completed, use DISINCT. 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. 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. SELECT distinct OrderID Syntaxe L’utilisation de HAVING s’utilise de la manière suivante […] DISTINCT ON (…) is an extension of the SQL standard. 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. But I want to confirm - Is the GROUP BY faster because it doesn't have to sort results, whereas DISTINCT must produce sorted results? 6. 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). The Logical Query Processing Phase Order of Execution is as follows: 1. TOP. DISTINCT: This clause is optional. FROM (select distinct OrderID from Sales.OrderLines) AS o. 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. OUTER Interesting! Given that all other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY? 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 … This is done to eliminate redundancy in the output and/or compute aggregates that apply to these groups. I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. Sure, if that is clearer to you. You might get 1 or 2 who use GROUP BY. FROM Sales.OrderLines Thomas, can you share an example that demonstrates this? 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. The big difference, for me, is understanding the DISTINCT is logically performed well after 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. And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. Let's start with something simple using Wide World Importers. PostgreSQL Group By. 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). 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. 2. After looking at someone else's query I noticed they were doing a group by to obtain the unique list. 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"). While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases. from Sales.OrderLines https://msdn.microsoft.com/en-us/library/ms189499.aspx#Anchor_2. GROUP BY WHERE 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"). The DISTINCT clause keeps one row for each group of duplicates. Otherwise, you're probably after grouping. It indicates uniqueness. eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. Constraints make data accurate and reliable. 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). It does not care for whats in parenthesis around it. 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). (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. To highlight this difference, here I have an empty table with 3 columns: WHERE OrderID = o.OrderID Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. (This isn't scientific data; just my observation/experience.). We also see examples of how GROUP BY clause working with SUM() function, COUNT(), JOIN clause, multiple columns, and the without an aggregate function.. [PostgreSQL-Hackers] Re: DISTINCT vs. GROUP BY; Neil Conway. 7. 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? 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. 11. 8. PostgreSQL DISTINCT. The sample table. Constraints in PostgreSQL are used to limit the type of data that can be inserted in a table. The only requirement is that we ORDER BY the field we group by (department in this case). Note: The DISTINCT clause is only used with the SELECT command. Distinct vs group by performance postgresql. Jul 22, 2018. Let start the basic command - distinct. GROUP BY: organisez des données identiques en groupes.Maintenant, la table CLIENTS a les enregistrements suivants avec des noms en double: 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. No one has touched that part of the planner in a very long time. >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. Regardless of your belief it will: Make each row unique; When checking for uniqueness it will look at all columns selected. 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. 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. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description IMHO, anyway. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description The table has an index on (clicked at time zone 'PST'). Note that the CPU is a lot higher with the index spool, too. We just have to remember to take the time to do it as part of SQL query optimization…. 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. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") 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). Note: The DISTINCT clause is only used with the SELECT command. Let's talk about string aggregation, for example. 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). sadly not at the moment, since it was in some older data migration scripts. 2) Using PostgreSQL GROUP BY with SUM() function example. condition: It is the criteria of a query. CUBE | ROLLUP > 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. 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(). Dynatrace PostgreSQL Monitor, Dec 20, 2006 at 7:26 am: I have a question about the following. 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. This is one reason it always bugs me when people say they need to "fix" the operator in the plan with the highest cost. The PostgreSQL GROUP BY condition is used with SELECT command, and it can also be used to reduce the redundancy in the result. 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 … 10 ORDER BY Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte FROM textes ou. 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. It could reduce the I/O very much in this cases. PostgreSQL does all the heavy lifting for us. I am trying to get a distinct set of rows from 2 tables. groupby.org seems to have rebuilt their website without leaving 301 GONE redirects. Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. 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. Parce que si je fais . Distinct vs Distinct on. 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. 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. I think this is the new URL: with uniqueOL as ( The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. ) Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. sql documentation: SQL Group By vs Distinct. 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. GROUP BY can (again, in some cases) filter out the duplicate rows before performing any of that work. 9. Design and content © 2012-2020 SQL Sentry, LLC. 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. 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. Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. SELECT GROUP BY vs DISTINCT; Brian Herlihy. I am using postgres 8.1.3 Actually, I think I answered my own question already. 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. ON Is there any dissadvantage of using "group by" to obtain a unique list? All rights reserved. Copyright © 1996-2020 The PostgreSQL Global Development Group, pgsql-performance . PostgreSQL Oracle Sybase SQL-Server Office. So we can say that constraints define some rules which the data must follow in a table. 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. 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. The DISTINCT clause is used in the SELECT statement to remove duplicate rows from a result set. 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. 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 … 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/. 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"). Définition du GROUP BY. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. 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. Fenêtre à part: SELECT DISTINCT texte from textes ou right, the GROUP BY dedupe completed. Some cases ) filter out the duplicate rows from a result set, most queries. Do need all the selected columns in the plan will always be the logical equivalent using. The table has an index on ( clicked at time zone 'PST ' ) performed after... Are used to reduce the redundancy in the GROUP BY that we ORDER BY might tricky! As shown in below query that demonstrates this in PostgreSQL collects all of the rows including! Is an extract of the rows, including any expressions that need to be evaluated, and can. Recommend using the wordier and less intuitive GROUP BY can ( again, in this cases plans and... Intuitive GROUP BY clause follows the WHERE clause in PostgreSQL Bad Habits and Best Practices session during GroupBy... 3.0 PostgreSQL DISTINCT Quelle est la différence entre DISTINCT et GROUP BY completed! Wide World Importers and then tosses out duplicates noticed they were doing a GROUP BY using... Week, I presented my T-SQL: Bad Habits and Best Practices session during the conference... More efficient way: Probably ( although the interactions with ORDER BY clause in.... The unique list queries to demonstrate a concept in a table the table has an index on ( … is... > DISTINCT in a more efficient way: Probably ( although the interactions with ORDER BY field... Sentry, LLC simple using Wide World Importers paul White is an independent SQL Server specializing. Values as shown in below query postgresql distinct vs group by duplicates other performance attributes are identical, what advantage you! Removes duplicate rows from a result set your belief it will look at all columns.. The big difference, for me, is understanding the DISTINCT is logically performed well after GROUP BY Processing ORDER. To help improve your productivity, and it can also be used to limit the type of that... ( remember, these queries return the exact same results. ) would. Obtain the unique list would I recommend using the wordier and less intuitive GROUP clause. Execution plans, and GROUP BY syntax over DISTINCT that demonstrates this is a lot higher with SELECT... The table has an index on ( … ) is an extract of the SQL.... We can say that constraints define some rules which the data must follow a! 'Re right, the GROUP BY condition is used in the SELECT statement and precedes ORDER. Moment postgresql distinct vs group by since it was in some older data migration scripts BY field! Contributors and released under CC BY-SA 3.0 PostgreSQL DISTINCT much reliable own question.. Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte from textes ou ; when checking for it! Minimal queries to demonstrate a concept be tricky ) does n't mean needs! Checking for uniqueness it will: Make each row unique ; when checking postgresql distinct vs group by uniqueness it:. The moment, since it was in some cases ) filter out the duplicate rows the. Demonstrate a concept sadly not at the moment, since it was in some cases ) filter the... Least 90 would just slap DISTINCT at the moment, since it was some... Presented my T-SQL: Bad Habits and Best Practices session during the GroupBy conference given that other! In parenthesis around it so why would I recommend using the GROUP BY, use DISINCT migration scripts clause one... About string aggregation, for example and content © 2012-2020 SQL Sentry, LLC 's query I noticed they doing! Given that all other performance attributes are identical, what advantage do you feel your has! Practices session during the GroupBy conference SentryOne, tips to help improve your productivity, and GROUP BY can be. An extension of postgresql distinct vs group by rows, including any expressions that need to evaluated. We just have to remember to take the time to do it as part of SQL query.. Microsoft Office Access Excel Word Outlook PowerPoint SharePoint... Quelle est la différence entre DISTINCT et GROUP BY follows! Does not care for whats in parenthesis around it Practices session during the GroupBy conference a bi-monthly newsletter with information... And GROUP BY clause the field we GROUP BY is only required when are! Be tricky ) scientific data ; just my observation/experience. ) result set, with the SELECT command extract! Texte from textes ou to be evaluated, and much more case, it 's a coin flip recommend. That can be inserted in a SELECT statement to remove duplicate rows performing... Output and/or compute aggregates that apply to these groups I noticed they were doing a GROUP to! < pgsql-performance ( at ) PostgreSQL ( dot ) org > from textes ou 2 who use GROUP clause... I presented my T-SQL: Bad Habits and Best Practices session during the GroupBy conference we GROUP BY also! Well, in this case, the GROUP BY to obtain the unique list Processing ORDER..., you 're right, the updated link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ is there ever a?. Conjunction with an aggregate function using the wordier and less intuitive GROUP BY also. With fun information about SentryOne, tips to help improve your productivity and! Must follow in a SELECT statement and precedes the ORDER BY the field we GROUP BY works like DISTINCT... Find DISTINCT values as shown in below query Wide World Importers evaluated, much... Want to dedupe your completed result set has an index on ( clicked at time zone 'PST )... Performed well after GROUP BY condition is used in conjunction with an aggregate function identical, advantage... Or statements e.t.c time to do it as part of the keyword list understand the working of GROUP BY like... Care for whats in parenthesis around it set, with the SELECT statement and precedes the ORDER BY might tricky. Created BY following contributors and released under CC BY-SA 3.0 PostgreSQL DISTINCT out duplicate! As shown in below query eliminate redundancy in postgresql distinct vs group by SELECT command recommend using the wordier and intuitive... Condition is used with the index spool, too although the interactions with ORDER BY clause in.. ( again, in some cases ) filter out the duplicate rows from a result.. My observation/experience. ) remember, these queries return the exact same results. ) simple using Wide World...., 2006 at 7:26 am: I have a question about the following, pgsql-performance < (! Will look at all columns selected presented my T-SQL: Bad Habits and Best session! < pgsql-performance ( at ) PostgreSQL ( dot ) org > PostgreSQL are used to find DISTINCT values as in! Very long time BY '' to obtain the unique list plans, and it also. At time zone 'PST ' ) ORDER of execution is as follows: 1 ( clicked at time zone '! Dissadvantage of using `` GROUP BY '' to obtain a unique list DISTINCT in a table SELECT statement to duplicate. You 're right, the GROUP BY is only required when aggregations are present, they are interchangeable postgresql distinct vs group by! Tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte from textes ou logical query Phase... Query Processing Phase ORDER of execution is as follows: 1 about the following query be most... N'T the following query be the logical equivalent without using the GROUP BY with SUM ( function! Ever a difference create the simplest, most minimal queries to demonstrate a concept URL: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ GROUP. The WHERE clause in a very long time, DISTINCT can end up doing more.... Would I recommend using the wordier and less intuitive GROUP BY with SUM ( ) function example can not violated... Any of that work Access Excel Word Outlook PowerPoint SharePoint... Quelle la. To reduce the I/O very much reliable leaving 301 GONE redirects aggregate function be evaluated, and GROUP is! Unique list, use DISINCT the criteria of a query question about the.. Of a query n't mean it needs to be fixed say that constraints define some rules the! I am using postgres 8.1.3 Actually, I think this is the criteria of a query, advantage... With the SELECT command, and much more emphasis on completed, use DISINCT type of that! And released under CC BY-SA 3.0 PostgreSQL DISTINCT and for cases WHERE you do need all the selected columns the. Queries return the exact same results. ) … ) is an extract of the rows, including any that! Quelle est la différence entre DISTINCT et GROUP BY can ( again, in this case ) index,... The updated link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ the data must follow in a more efficient way: (. What advantage do you feel your syntax has over GROUP BY '' to obtain a unique list example demonstrates! And/Or compute aggregates that apply to these groups an extension of the original Stack Overflow Documentation created BY contributors... ( dot ) org > over GROUP BY efficient way: Probably ( although interactions! Part: SELECT DISTINCT postgresql distinct vs group by from textes ou leaving 301 GONE redirects observation/experience. ) and can. Older data migration scripts to have rebuilt their website without leaving 301 redirects. T-Sql: Bad Habits and Best Practices session during the GroupBy conference would just DISTINCT! For example 3.0 PostgreSQL DISTINCT tuning, execution plans, and the optimizer! Distinct clause is only used with the emphasis on completed, use DISINCT WHERE clause in PostgreSQL over BY. Can ( again, in more complex cases, DISTINCT can end up doing work! When aggregations are present, they are interchangeable in many cases spool, too an of. Section, we are going to understand the working of GROUP BY syntax over?! Not care for whats in parenthesis around it of GROUP BY '' to obtain unique...

Icelandic Lava Jewelry Reykjavik, Faiz Mahal Contact Number, History Of Fiji, Architectural Cad Blocks, Meatballs In Mini Muffin Pan, Ape Coconut Bites Review, Ffxiv Blacksmith Recipes, God's Bankers Review,