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