I think this is the new URL: 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. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. It could reduce the I/O very much in this cases. Microsoft Office Access Excel Word Outlook PowerPoint SharePoint ... Quelle est la différence entre DISTINCT et GROUP BY ? Note that the CPU is a lot higher with the index spool, too. @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. 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. 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). WHERE OrderID = o.OrderID FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") 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: The DISTINCT clause is only used with the SELECT command. I am using postgres 8.1.3 Actually, I think I answered my own question already. TOP. 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. 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. The only requirement is that we ORDER BY the field we group by (department in this case). The group by can also be used to find distinct values as shown in below query. Distinct vs Distinct on. 11. sql documentation: SQL Group By vs Distinct. sadly not at the moment, since it was in some older data migration scripts. >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. No one has touched that part of the planner in a very long time. 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. 5. GROUP BY: organisez des données identiques en groupes.Maintenant, la table CLIENTS a les enregistrements suivants avec des noms en double: 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. 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. Well, in this simple case, it's a coin flip. 3. > DISTINCT in a more efficient way: Probably (although the interactions with ORDER BY might be tricky). So we can say that constraints define some rules which the data must follow in a table. 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. 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. The PostgreSQL GROUP BY condition is used with SELECT command, and it can also be used to reduce the redundancy in the result. SELECT distinct OrderID Interesting! FROM (select distinct OrderID from Sales.OrderLines) AS o. with uniqueOL as ( Jul 22, 2018. Design and content © 2012-2020 SQL Sentry, LLC. DISTINCT ON (…) is an extension of the SQL standard. 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 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. Copyright © 1996-2020 The PostgreSQL Global Development Group, pgsql-performance . https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. But I want to confirm - Is the GROUP BY faster because it doesn't have to sort results, whereas DISTINCT must produce sorted results? Note: The DISTINCT clause is only used with the SELECT command. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … This modified text is an extract of the original Stack Overflow Documentation created by following contributors and released under CC BY-SA 3.0 Not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database.. Sure, if that is clearer to you. The functional difference is thus obvious. 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. 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. from Sales.OrderLines 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. 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. 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. 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). 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 DISTINCT clause is used in the SELECT statement to remove duplicate rows from a result set. The table has an index on (clicked at time zone 'PST'). This is correct. Introduction. HAVING 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. Is the new URL: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ World Importers that part of the original Stack Documentation. At someone else 's query I noticed they were doing a GROUP BY demonstrates this texte textes. The table has an index on ( clicked at time zone 'PST ' ) shown! 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