postgresql distinct vs group by

This seems clearer to me. This modified text is an extract of the original Stack Overflow Documentation created by following contributors and released under CC BY-SA 3.0 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). https://msdn.microsoft.com/en-us/library/ms189499.aspx#Anchor_2. Let’s have a look at difference between distinct and group by in SQL Server . 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. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description Code : Sélectionner tout-Visualiser dans une fenêtre à part: SELECT texte FROM textes GROUP BY … SELECT distinct OrderID 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). PostgreSQL DISTINCT. 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. Introduction. Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. 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. IMHO, anyway. 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. Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. 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. SELECT Interesting! 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. I think this is the new URL: 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. 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.. However, in more complex cases, DISTINCT can end up doing more work. So we can say that constraints define some rules which the data must follow in a table. Jul 22, 2018. GROUP BY sql documentation: SQL Group By vs Distinct. I am using postgres 8.1.3 Actually, I think I answered my own question already. Paul White is an independent SQL Server consultant specializing in performance tuning, execution plans, and the query optimizer. GROUP BY vs DISTINCT; Brian Herlihy. Parce que si je fais . The GROUP BY clause follows the WHERE clause in a SELECT statement and precedes the ORDER BY clause. So why would I recommend using the wordier and less intuitive GROUP BY syntax over DISTINCT? 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 group by can also be used to find distinct values as shown in below query. 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). Syntaxe L’utilisation de HAVING s’utilise de la manière suivante […] 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. The only requirement is that we ORDER BY the field we group by (department in this case). FROM (select distinct OrderID from Sales.OrderLines) AS o. Thomas, can you share an example that demonstrates this? 3. >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. 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. Note that the CPU is a lot higher with the index spool, too. Regardless of your belief it will: Make each row unique; When checking for uniqueness it will look at all columns selected. 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. Well, in this simple case, it's a coin flip. 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. HAVING To highlight this difference, here I have an empty table with 3 columns: ) 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. Not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database.. (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. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. 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 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. GROUP BY: organisez des données identiques en groupes.Maintenant, la table CLIENTS a les enregistrements suivants avec des noms en double: Définition du GROUP BY.

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