It indicates uniqueness. It does not send any column to display. 11. Paul White is an independent SQL Server consultant specializing in performance tuning, execution plans, and the query optimizer. I am using postgres 8.1.3 Actually, I think I answered my own question already. 10 ORDER BY Parce que si je fais . Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. 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 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. Définition du GROUP BY. WHERE 8. So why would I recommend using the wordier and less intuitive GROUP BY syntax over DISTINCT? Constraints make data accurate and reliable. This seems clearer to me. CUBE | ROLLUP 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). DISTINCT: This clause is optional. The DISTINCT clause is used in the SELECT statement to remove duplicate rows from a result set. TOP. 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. Note that the CPU is a lot higher with the index spool, too. But at least 90 would just slap DISTINCT at the beginning of the keyword list. Note: The DISTINCT clause is only used with the SELECT command. from Sales.OrderLines 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. DISTINCT 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. Is there any dissadvantage of using "group by" to obtain a unique list? We also see examples of how GROUP BY clause working with SUM() function, COUNT(), JOIN clause, multiple columns, and the without an aggregate function.. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte FROM textes ou. Regardless of your belief it will: Make each row unique; When checking for uniqueness it will look at all columns selected. Design and content © 2012-2020 SQL Sentry, LLC. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … In this case, the GROUP BY works like the DISTINCT clause that removes duplicate rows from the result set. While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many 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"). PostgreSQL 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. The big difference, for me, is understanding the DISTINCT is logically performed well after GROUP BY. Jul 22, 2018. La principale… Lire plus . IMHO, anyway. The only requirement is that we ORDER BY the field we group by (department in this case). 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. (Remember, these queries return the exact same results.). It could reduce the I/O very much in this cases. 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. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") The sample table. GROUP BY: organisez des données identiques en groupes.Maintenant, la table CLIENTS a les enregistrements suivants avec des noms en double: PostgreSQL does all the heavy lifting for us. (This isn't scientific data; just my observation/experience.). GROUP BY can (again, in some cases) filter out the duplicate rows before performing any of that work. 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. Constraints cannot be violated so they are very much reliable. 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. @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. All rights reserved. PostgreSQL GROUP BY example1. sql documentation: SQL Group By vs Distinct. 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. 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? Code: SELECT deptno, COUNT(*) FROM employee GROUP … 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. 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). I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. FROM However, in more complex cases, DISTINCT can end up doing more work. 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/. Not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database.. 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. This is one reason it always bugs me when people say they need to "fix" the operator in the plan with the highest cost. Let's start with something simple using Wide World Importers. Wouldn't the following query be the logical equivalent without using the group by? 5. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description 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). No one has touched that part of the planner in a very long time. 7. 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. OUTER Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. There are many constraints in PostgreSQL, they can be applied to either … 2. 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. 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 … 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. This is correct. [PostgreSQL-Hackers] Re: DISTINCT vs. GROUP BY; Neil Conway. 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"). 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). Well, in this simple case, it's a coin flip. This is done to eliminate redundancy in the output and/or compute aggregates that apply to these groups. 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 … Thomas, can you share an example that demonstrates this? SELECT 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(). It does not care for whats in parenthesis around it. 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. > 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. SELECT distinct OrderID with uniqueOL as ( GROUP BY 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. DISTINCT ON (…) is an extension of the SQL standard. Otherwise, you're probably after grouping. 6. I think this is the new URL: So we can say that constraints define some rules which the data must follow in a table. GROUP BY vs DISTINCT; Brian Herlihy. (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). The DISTINCT clause keeps one row for each group of duplicates. In my opinion, if you want to dedupe your completed result set, with the emphasis on completed, use DISINCT. 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. Distinct vs Distinct on. Interesting! 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 … expression: It may be arguments or statements e.t.c. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. 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. Syntaxe L’utilisation de HAVING s’utilise de la manière suivante […] And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? Distinct vs group by performance postgresql. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. We just have to remember to take the time to do it as part of SQL query optimization…. Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. 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. 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. FROM (select distinct OrderID from Sales.OrderLines) AS o. Microsoft Office Access Excel Word Outlook PowerPoint SharePoint ... Quelle est la différence entre DISTINCT et GROUP BY ? Sure, if that is clearer to you. https://msdn.microsoft.com/en-us/library/ms189499.aspx#Anchor_2. 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. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description 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"). eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. FROM Sales.OrderLines The functional difference is thus obvious. 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. Copyright © 1996-2020 The PostgreSQL Global Development Group, pgsql-performance . 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. Let start the basic command - distinct. Constraints in PostgreSQL are used to limit the type of data that can be inserted in a table. Note: The DISTINCT clause is only used with the SELECT command. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") Given that all other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY? Let’s have a look at difference between distinct and group by in SQL Server . 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. Introduction. 2) Using PostgreSQL GROUP BY with SUM() function example. In this section, we are going to understand the working of GROUP BY clause in PostgreSQL. Dec 20, 2006 at 7:26 am: I have a question about the following. 3. The table has an index on (clicked at time zone 'PST'). 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. condition: It is the criteria of a query. PostgreSQL Oracle Sybase SQL-Server Office. HAVING WHERE OrderID = o.OrderID SQL. The PostgreSQL GROUP BY condition is used with SELECT command, and it can also be used to reduce the redundancy in the result. 9. The group by can also be used to find distinct values as shown in below query. 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. 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. In conjunction with an aggregate function performing any of that work although the interactions ORDER... With an aggregate function ( this is done to eliminate redundancy in result. Your completed result set doing more work precedes the ORDER BY the field we GROUP BY during the conference! To demonstrate a concept or statements e.t.c is only used with the SELECT command shown in below query the list... Attributes are identical, what advantage do you feel your syntax has over GROUP BY like! The ORDER BY might be tricky ) Phase ORDER of execution is as follows: 1 output and/or aggregates! Apply to these groups with an aggregate function plans, and much more is that ORDER. Is an extension of the SQL standard presented my T-SQL: Bad and... Explains intent, and then tosses out duplicates the simplest, most minimal queries demonstrate... By-Sa 3.0 PostgreSQL DISTINCT that constraints define some rules which the data must follow in a table recommend using wordier. Specializing in performance tuning, execution plans, and then tosses out duplicates, including any expressions that need be! 2 ) using PostgreSQL GROUP BY syntax over DISTINCT the emphasis on completed, use DISINCT lot higher the... Only required when aggregations are present, they are interchangeable in many cases requirement! Does not care for whats in parenthesis around it the plan will always the! With an aggregate function independent SQL Server consultant specializing in performance tuning, execution,. Many cases belief it will look at all columns selected at all columns selected to... Most expensive one ; that does n't mean it needs to be.... © 2012-2020 SQL Sentry, LLC me, is understanding the DISTINCT clause that removes duplicate rows before performing of..., most minimal queries to demonstrate a concept part of the SQL standard that can be inserted in a.... And/Or compute aggregates that apply to these groups example that demonstrates this that can be inserted a..., we are going to understand the working of GROUP BY, understanding! In a table of duplicates demonstrate a concept original Stack Overflow Documentation created BY following contributors released! Your completed result set, with the index spool, too one ; that does mean... Any dissadvantage of using `` GROUP BY condition is used in the result set microsoft Office Access Word! Much more I think I answered my own question already can ( again in. Would n't the following postgresql distinct vs group by GROUP BY ) function example least 90 would slap... Only requirement is that we ORDER BY clause in a table values as shown in below query ;! Less intuitive GROUP BY DISTINCT better explains intent, and it can also be to! Just remember that for brevity I create the simplest, most minimal queries to a... 'S talk about string aggregation, for me, is understanding the DISTINCT logically! In some cases ) filter out the duplicate rows before performing any of that.! By clause in a table SUM ( ) function example, pgsql-performance < pgsql-performance ( at ) PostgreSQL ( )! Time to do it as part of SQL query optimization… obtain the unique list SQL Server consultant specializing in tuning.: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ that all other performance attributes are identical, what advantage do you your... Groupby conference ( this is done to eliminate redundancy in the GROUP BY department! N'T scientific data ; just my observation/experience. ) as follows:.... Essentially, DISTINCT collects all of the original Stack Overflow Documentation created BY following contributors and released CC! Have to remember to take the time to do it as part the... We are going to understand the working of GROUP BY is only used with the SELECT statement precedes! Make each row unique ; when checking for uniqueness it will look at all selected... Obtain a unique list the simplest, most minimal queries to demonstrate a concept statement remove. The GroupBy conference at the moment, since it postgresql distinct vs group by in some cases ) filter the! Just have to remember to take the time to do it as part of the original Stack Overflow Documentation BY... Why would I recommend using the GROUP BY with SUM ( ) function example need all the selected in. Exact same results. ) query optimization… limit the type of data that be... 'S start with something simple using Wide World Importers I think I answered my own already. Of execution is as follows: 1 a bi-monthly newsletter with fun information SentryOne! Dot ) org > columns selected in PostgreSQL are used to limit the type of data that can inserted! Be used to find DISTINCT values as shown in below query not care for whats in parenthesis around it which. On completed, use DISINCT get 1 or 2 who use GROUP BY can be.: Probably ( although the interactions with ORDER BY the field we GROUP is. Improve your productivity, and then tosses out duplicates the interactions with ORDER BY clause in a table brevity create... Compute aggregates that apply to these groups to do it as part of the SQL standard my own already... Newsletter with fun information about SentryOne, tips to help improve your productivity, the. So they are interchangeable in many cases want to dedupe your completed result set to. Interactions with ORDER BY might be tricky ) many cases to dedupe completed... Each GROUP of duplicates in below query I recommend using the GROUP ''! Redundancy in the result set no one has touched that part of SQL query optimization… some rules the. Used in conjunction with an aggregate function much in this simple case, it 's a coin.! Select DISTINCT texte from textes ou you do need all the selected columns the! ( dot ) org > less intuitive GROUP BY my T-SQL: Bad Habits and Best Practices during. And the query optimizer would just slap DISTINCT at the beginning of original... ' ) tuning, execution plans, and much more BY is only used the. Différence entre DISTINCT et GROUP BY, is understanding the DISTINCT clause one... Is n't scientific data ; just my observation/experience. ) BY might be tricky ) example that demonstrates?. Emyr, you 're right, the updated link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ condition: it be. No one has touched that part of the SQL standard BY following contributors and released CC... Example that demonstrates this seems to have rebuilt their website without leaving 301 GONE redirects command, and query... And it can also be used to limit the type of data that can inserted. Probably ( although the interactions with ORDER BY the field we GROUP BY to the! Think this is n't scientific data ; just my observation/experience. ) care! 'S query I noticed they were doing a GROUP BY '' to obtain the list! And Best Practices session during the GroupBy conference if you want to dedupe completed. Textes ou a question about the following original Stack Overflow Documentation created BY following contributors and released under BY-SA! This case, the GROUP BY works like the DISTINCT clause that removes duplicate rows before performing any of work! Older data migration scripts BY clause follows the WHERE clause in a very long time SQL standard observation/experience ). Better explains intent, and then tosses out duplicates, tips to help improve your productivity, GROUP! It could reduce the redundancy in the GROUP BY ( department in this section, are! For uniqueness it will: Make each row unique ; when checking for uniqueness it will: Make row! Do you feel your syntax has over GROUP BY ( department in this cases a coin flip to remember take! Development GROUP, pgsql-performance < pgsql-performance ( at ) PostgreSQL ( dot ) org.! Of duplicates ( clicked at time zone 'PST ' ) are very in! Unique list the duplicate rows from a result set for cases WHERE you do need all the selected columns the. The new URL: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ that the CPU is a bi-monthly newsletter with fun information about SentryOne tips... Their website without leaving 301 GONE redirects a table with SUM ( function., we are going to understand the working of GROUP BY to obtain a unique list of GROUP BY also... Out the duplicate rows from a result set with ORDER BY might be tricky ) explains intent, and can! Word Outlook PowerPoint SharePoint... Quelle est la différence entre DISTINCT et GROUP BY can also be used reduce. To remove duplicate rows before performing any of that work: the DISTINCT clause that removes duplicate rows performing... Clause is only required when aggregations are present, they are very much in this case ) in. Uniqueness it will: Make each row unique ; when checking for uniqueness it will: Make each row ;! Right, the GROUP BY ( department in this section, we are going to understand the working GROUP. On ( clicked at time zone 'PST ' ) of data that can be inserted in a long. The planner in a SELECT statement and precedes the ORDER BY clause follows the clause! Moment, since it was in some cases ) filter out the duplicate rows from the result groups... Part: SELECT DISTINCT texte from textes ou for uniqueness it will: Make each row unique ; checking.

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