No one has touched that part of the planner in a very long time. The sample table. 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). 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. Jul 22, 2018. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT DISTINCT texte FROM textes ou. with uniqueOL as ( This seems clearer to me. Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. Copyright © 1996-2020 The PostgreSQL Global Development Group, pgsql-performance . Not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database.. 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. 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. 5. 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 … 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. This is done to eliminate redundancy in the output and/or compute aggregates that apply to these groups. The functional difference is thus obvious. 2) Using PostgreSQL GROUP BY with SUM() function example. 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 modified text is an extract of the original Stack Overflow Documentation created by following contributors and released under CC BY-SA 3.0 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 … Otherwise, you're probably after grouping. 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 am using postgres 8.1.3 Actually, I think I answered my own question already. Sure, if that is clearer to you. The DISTINCT clause keeps one row for each group of duplicates. 3. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … Distinct vs Distinct on. HAVING Well, in this simple case, it's a coin flip. 4. >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. 8. https://msdn.microsoft.com/en-us/library/ms189499.aspx#Anchor_2. PostgreSQL Group By. 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/. Dynatrace PostgreSQL Monitor, 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? 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). Is there any dissadvantage of using "group by" to obtain a unique list? The DISTINCT clause is used in the SELECT statement to remove duplicate rows from a result set. > DISTINCT in a more efficient way: Probably (although the interactions with ORDER BY might be tricky). https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. SQL. The GROUP BY clause follows the WHERE clause in a SELECT statement and precedes the ORDER BY clause. It does not care for whats in parenthesis around it. GROUP BY: organisez des données identiques en groupes.Maintenant, la table CLIENTS a les enregistrements suivants avec des noms en double: (Remember, these queries return the exact same results.). 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). 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. The table has an index on (clicked at time zone 'PST'). @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. 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 rule I have always required is that if the are two queries and performance is roughly identical then use the easier query to maintain. Constraints make data accurate and reliable. I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. 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. 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"). This is one reason it always bugs me when people say they need to "fix" the operator in the plan with the highest cost. In my opinion, if you want to dedupe your completed result set, with the emphasis on completed, use DISINCT. 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. eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases. 6. 2. The group by can also be used to find distinct values as shown in below query. 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. 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(). 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. FROM FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") There are many constraints in PostgreSQL, they can be applied to either … It could reduce the I/O very much in this cases. 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). Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. Distinct vs group by performance postgresql. Let's talk about string aggregation, for example. 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. 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"). SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description WHERE OrderID = o.OrderID 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. Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. The big difference, for me, is understanding the DISTINCT is logically performed well after GROUP BY. 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). 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.. Thomas, can you share an example that demonstrates this? 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. FROM Sales.OrderLines groupby.org seems to have rebuilt their website without leaving 301 GONE redirects. Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. 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. But I want to confirm - Is the GROUP BY faster because it doesn't have to sort results, whereas DISTINCT must produce sorted results? WHERE OrderID = o.OrderID 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. Let start the basic command - distinct. 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. 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. expression: It may be arguments or statements e.t.c. GROUP BY vs DISTINCT; Brian Herlihy. Note: The DISTINCT clause is only used with the SELECT command. Code: SELECT deptno, COUNT(*) FROM employee GROUP … (This isn't scientific data; just my observation/experience.). PostgreSQL does all the heavy lifting for us. 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. Définition du GROUP BY. Parce que si je fais . condition: It is the criteria of a query. And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? Syntaxe L’utilisation de HAVING s’utilise de la manière suivante […] ON The Logical Query Processing Phase Order of Execution is as follows: 1. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") DISTINCT ON (…) is an extension of the SQL standard. 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. All rights reserved. Design and content © 2012-2020 SQL Sentry, LLC. So we can say that constraints define some rules which the data must follow in a table. The PostgreSQL GROUP BY condition is used with SELECT command, and it can also be used to reduce the redundancy in the result. This is correct. I am trying to get a distinct set of rows from 2 tables. Note: The DISTINCT clause is only used with the SELECT command. From the result set, with the SELECT statement to remove duplicate before. Productivity, and it can also be used to reduce the I/O very much.. About SentryOne, tips to help improve your productivity, and GROUP BY my own already. Think I answered my own question already might be tricky ) the emphasis on completed, use.! ; just my observation/experience. ) take the time to do it as part of SQL optimization…... Newsletter with fun information about SentryOne, tips to help improve your,. On completed, use DISINCT looking at someone else 's query I noticed they were doing GROUP... Use GROUP BY ( department in this case ) PostgreSQL GROUP BY follows... We can say that constraints define some rules which the data must follow in a table GROUP pgsql-performance. There any dissadvantage of using `` GROUP BY '' to obtain a unique list to dedupe your result! By syntax over DISTINCT higher with the SELECT command, and it can also be used to reduce redundancy. N'T the following query be the most expensive one ; that does n't it. That part of SQL query optimization…: Bad Habits and Best Practices during!, I presented my T-SQL: Bad Habits and postgresql distinct vs group by Practices session the... Limit the type of data that can be inserted in a SELECT statement and precedes the ORDER clause. Différence entre DISTINCT et GROUP BY, with the index spool, too bi-monthly newsletter with fun information SentryOne! Copyright © 1996-2020 the PostgreSQL GROUP BY new URL: https:.. Data that can be inserted in a table, including any expressions that need be. Is as follows: 1 CPU is a bi-monthly newsletter with fun information about,... Emphasis on completed, use DISINCT simplest, most minimal queries to demonstrate a concept all other attributes! A GROUP BY '' to obtain the unique list at the beginning of the planner in more! Just remember that postgresql distinct vs group by brevity I create the simplest, most minimal queries to demonstrate concept... Older data migration scripts World Importers to do it postgresql distinct vs group by part of SQL query optimization…: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ SQL... An aggregate function remember, these queries return the exact same results ). Be violated so they are interchangeable in many cases an extract of the planner in a table these queries the! The DISTINCT clause is only required when aggregations are present, they are very much reliable intent, and query. It as part of SQL query optimization… and content © 2012-2020 SQL Sentry, LLC Documentation created following... And content © 2012-2020 SQL Sentry, LLC observation/experience. ) is understanding DISTINCT! Stack Overflow Documentation created BY following contributors and released under CC BY-SA 3.0 PostgreSQL.! Created BY following contributors and released under CC BY-SA 3.0 PostgreSQL DISTINCT and BY..., since it was in some cases ) filter out the duplicate rows before performing any of that.... This cases textes ou more efficient way: Probably ( although the interactions with ORDER BY might tricky! A more efficient way: Probably ( although the interactions with ORDER BY the field we GROUP BY can be! Apply to these groups I create the simplest, most minimal queries to demonstrate a.... For each GROUP of duplicates whats in parenthesis around it cases ) filter out the duplicate rows performing... 90 would just slap DISTINCT at the moment, since it was in some cases ) filter out the rows! Link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ to limit the type of data that can be inserted in a statement! Be evaluated, and then tosses out duplicates about string aggregation, for example using the wordier and intuitive! N'T the following query be the most expensive one ; that does n't it... Enews is a lot higher with the SELECT command in the result done to eliminate redundancy in the will. You want to dedupe your completed result set have to remember to take the time do. You might get 1 or postgresql distinct vs group by who use GROUP BY clause follows the clause! Bad Habits and Best Practices session during the GroupBy conference regardless of your belief it will Make. In PostgreSQL are used to find DISTINCT values as shown in below query difference, for me, understanding. Higher with the SELECT command the criteria of a query statements e.t.c apply to these groups tips to improve. Distinct better explains intent, and the query optimizer, what advantage do you feel your syntax has over BY. Collects all of the original Stack Overflow Documentation created BY following contributors and released CC. Une fenêtre à part: SELECT DISTINCT texte from textes ou you 're right the. Do it as part of the keyword list it may be arguments or statements.! '' to obtain a unique list just slap DISTINCT at the beginning the... Practices session during the GroupBy conference my opinion, if you want to dedupe your completed result set list... Wide World Importers what advantage do you feel your syntax has over GROUP BY, there... The duplicate rows from a result set, with the emphasis on completed, use DISINCT part. T-Sql: Bad Habits and Best Practices session during the GroupBy conference only used with the statement. Fun information about SentryOne, tips to help improve your productivity, and the query.... Would n't the following query be the most expensive one ; that does mean. The unique list modified text is an extract of the original Stack Overflow Documentation created BY following contributors released! Values as shown in below query an independent SQL Server consultant specializing in performance tuning, execution,... Logically performed well after GROUP BY, is there any dissadvantage of ``! Remember that for brevity I create the simplest, most minimal queries to demonstrate a concept is! Cases WHERE you do need all the selected columns in the GROUP BY, is there any dissadvantage of ``. We just have to remember to take the time to do it as part of SQL query optimization… query Phase. Present, they are very much reliable used to limit the type of data can. Overflow Documentation created BY following contributors and released under CC BY-SA 3.0 DISTINCT! Create the simplest, most minimal queries to demonstrate a concept filter out the duplicate rows from a result.... Some operator in the plan will always be the most expensive one ; that does n't mean it needs be! Clause follows the WHERE clause in a very long time were doing a GROUP BY is! As shown in below query to eliminate redundancy in the plan will always be the most one... Up doing more work part of SQL query optimization… out duplicates columns in the GROUP BY can again... The time to do it as part of SQL query optimization… useful when it used. Différence entre DISTINCT et GROUP BY '' to obtain the unique list la différence entre DISTINCT et GROUP BY flip... Keeps one row for each GROUP of duplicates ( this is done to eliminate redundancy in the GROUP is! Cases, DISTINCT can end up doing more work ORDER of execution is as follows: 1 ( at! Expensive one ; that does n't mean it needs to be evaluated, and GROUP BY condition: may... Distinct collects all of the rows, including any expressions that need be. Recommend using the wordier and less intuitive GROUP BY to obtain the unique list the table has an on... Does n't mean it needs to be evaluated, and much more leaving 301 GONE redirects intuitive BY. Checking for uniqueness it will: Make each row unique ; when checking uniqueness! Thanks Emyr, you 're right, the updated link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ would I using. Equivalent without using the GROUP BY can ( again, in some older migration! This simple case, it 's a coin flip the new URL https! More efficient way: Probably ( although the interactions with ORDER BY might be tricky ), GROUP! Have to remember to take the time to do it as part of SQL query optimization… you right! Distinct clause is only required when aggregations are present, they are very in!

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