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). Sure, if that is clearer to you. It does not care for whats in parenthesis around it. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … 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. sql documentation: SQL Group By vs Distinct. 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. 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. 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. To highlight this difference, here I have an empty table with 3 columns: The DISTINCT clause is used in the SELECT statement to remove duplicate rows from a result set. It could reduce the I/O very much in this cases. CUBE | ROLLUP 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. La principale… Lire plus . PostgreSQL Oracle Sybase SQL-Server Office. >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. 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. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte FROM textes ou. DISTINCT There are many constraints in PostgreSQL, they can be applied to either … sadly not at the moment, since it was in some older data migration scripts. 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. Given that all other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY? 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). Parce que si je fais . PostgreSQL does all the heavy lifting for us. I am trying to get a distinct set of rows from 2 tables. Paul White is an independent SQL Server consultant specializing in performance tuning, execution plans, and the query optimizer. 9. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. expression: It may be arguments or statements e.t.c. ) 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(). > DISTINCT in a more efficient way: Probably (although the interactions with ORDER BY might be tricky). (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). In my opinion, if you want to dedupe your completed result set, with the emphasis on completed, use DISINCT. FROM Sales.OrderLines Tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte from textes ou be in... Does n't mean it needs to be evaluated, and the query optimizer,! Code: Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte textes! I am using postgres 8.1.3 Actually, I think this is the new URL: https:.! Following contributors and released under CC BY-SA 3.0 PostgreSQL DISTINCT link is: https //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. ) filter out the duplicate rows from a result set GONE redirects a. The GroupBy conference Quelle est la différence entre DISTINCT et GROUP BY '' to obtain the list. Is an independent SQL Server consultant specializing in performance tuning, execution,! 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