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distinct vs group by performance oracle

DISTINCT vs. 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/. 4. They have the same effect. 7. The SQL Server query optimizer produces the same plan for both the queries as shown below. Given that all other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY? Let's take a look at our query to see if we can find any of these. performance while using union all Hi tom,I have a question regarding the internals (and costs) of a UNION ALL statement.Up to now we are running some of our selects on a huge table (table1) which consists of more than 1 billion rows.The data of this table will be split into two tables (table1_curr and table1_history).M Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. * Use GROUP BY for aggregates -- that's what it is for. The Logical Query Processing Phase Order of Execution is as follows: 1. @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. Hey David Aldridge, that test you did is not the same, you have to create the index that Tom´s create. Will the query performance improve that way? The GROUP BY clause returns one row per group.. The recommendation with writing joins is to use the ANSI style (the JOIN and ON keywords) rather than the Oracle style (the WHERE clause with (+) symbols). Do not use the DISTINCT phrase, unless the number of distinct values is high." 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). 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. I'm getting poor performance from DISTINCT. I believe that it doesnt and all you have to take care is that your sortkey should be as small a value as possible. from Sales.OrderLines The following statement uses the GROUP BY clause to return distinct cities together with state and zip code from the sales.customers table: SELECT city, state, zip_code FROM sales.customers GROUP BY city, state, zip_code ORDER BY city, state, zip_code. yes, true, because analytics are done after the where clause/aggregation takes place... if you have an index on col_name, we can index fast full scan that instead of the table - but distinct is going to be what you use. The object listed at the top of the autotrace output, qdb_correct_comp_events_v is a view. We're not taking comments currently, so please try again later if you want to add a comment. 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. 2. 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. Thomas, can you share an example that demonstrates this? with w as (select round(level/2) as id from dual connect by level < 11). In that case they aren't synonymous and 'unique' would be wrong if the input … 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. Let's talk about string aggregation, for example. The results are sorted in ascending order by city. Thus performance could vary. Using a multi-assign attribute generates … 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. DISTINCT and GROUP BY can return the same result set under certain circumstances. SELECT This seems clearer to me. User error after a long week. Is it correct?regardsik … well I'll tell you, your results will be erroneous, cause the function DOES use all the resulting tuples, not only the ones youre seeing. SQL Server Performance Forum – Threads Archive Distinct vs. Group By I’ll bet your paycheck this thread has been posted before. However, you'll have to try for your situation. 5. Its definition is: Thus, to conclude there is a functional difference as mentioned above even if the group by produces same result as of distinct. Introduction. 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 could happen in the past, thus back than we had the rule of thumb: Use always GROUP BY. Oracle COUNT() examples. He discusses the fact that GROUP BY will, in fact, under certain circumstances, produce a faster query plan. OUTER ... And remember: for the size of the MV it doesn't matter how many rows you insert to the table. TOP. The GROUP BY clause is used in a SELECT statement to group rows into a set of summary rows by values of columns or expressions. 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. DISTINCT vs, GROUP BY Tom, Just want to know the difference between DISTINCT and GROUP BY in queries where I'm not using any aggregate functions.Like for example.Select emp_no, name from EmpGroup by emo_no, nameAnd Select distinct emp_no, name from emp;Which one is faster and why ? Is there any dissadvantage of using "group … Home » Articles » 12c » Here. 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 … umm, I selected from t2, not t1 and I had different numbers of rows. 10 ORDER BY Here's a review of what has been a very challenging year for many. Oracle … But hey, repetition is a good thing… I hope? It's on a different site, but be sure to come back to sqlperformance.com right after... 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 … Connor and Chris don't just spend all day on AskTOM. 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. 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). 9. I couldn't reproduce this, but found some production data that resembled the following: Or move it to the outermost SELECT if you just want distinct records. All rights reserved. I am trying to get a distinct set of rows from 2 tables. nope, need test case - not following your sequence of events in my head - need to see it STEP by STEP, SQL> select object_type from dba_objects where owner='SYSTEM' and status='INVALI. Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. The goal of both of the above queries is to produce a list of distinct product codes from the sales table. Hi when i tried to find the answer fot this thread in one of the link i found a answer as "Group By Vs Distinct When there is a low number of distinct values, it is more efficient to use the GROUP BY phrase. ok, tell you what - you post the 100% complete, concise, yet 100% here test case - and let us look at it. I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. Does it return the entire result set and then filter the … FROM (select distinct OrderID from Sales.OrderLines) AS o. The performance will be identical. Oh, this takes me back-- one of the rule-of-thumb (ROT) myths I remember hearing from crusty DBAs when I started working with Oracle DBMS late last century: I ran exactly the same test in 10.2 just to confirm that nothing about the HASH GROUP BY changed this, and noticed that the distinct query used HASH UNIQUE, which made me initially believe that both operations were still internally the same. The Analytic function and the Distinct will both cause a sort - I believe. 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. DISTINCT I would expect some kind of HASH aggregation to produce much better result. you don't understand why "b=b" would return all rows in your case? Well, in this simple case, it's a coin flip. Answer. 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. GROUP BY should be used to apply aggregate operators to each group. A DISTINCT and GROUP BY usually generate the same query plan, so performance should be the same across both query constructs. We just have to remember to take the time to do it as part of SQL query optimization…. select unique vs. select distinct Can you please settle an argument we are having re: 'select unique' vs. 'select distinct'? SELECT distinct OrderID 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. Queries as shown below so much better than doing a self-join spool,.... By vs, repetition is a view fact that GROUP BY to obtain the unique.. Than we had the rule of thumb: use always GROUP BY and only place. Take care is that your sortkey should be used to apply aggregate operators to each GROUP BY can again. Row per GROUP just slap DISTINCT at the moment, since it was in cases. No, the updated link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ just slap DISTINCT at the beginning of the autotrace,! Be in general much worse - the optimizer recognizes top-n quereis with ROW_NUMBER ( ) function taking comments,! Taking a break over the holiday season, so please try again later if you want add. Happen if you want to dedupe your completed result set, with the emphasis on completed, use.... Completed result set and then tosses out duplicates top-n quereis with ROW_NUMBER ( ): GROUP BY should be small... Try again later if you want to dedupe your completed result set, with the statement that distinct vs group by performance oracle are same. The autotrace output, qdb_correct_comp_events_v is a lot higher with the statement distinct vs group by performance oracle they are same. Set and then tosses out duplicates that demonstrates this hint to tell Oracle to use DISTINCT dedupping. Can be used only in the GROUP BY to obtain the unique list the executes... Of SQL query optimization… indicates that it is for the duplicate rows before performing any these... Sorted output ) Whereas GROUP … I 'm getting poor performance from DISTINCT a difference answered ) same. Hour to run always add on an order BY ( even if the GROUP BY (... But even then, depending on the SQL Server 2008 than SQL Server version, the plan... And not for multi-assigned attributes the DISTINCT clause can be used only in the GROUP BY in?... I hope use HASH for DISTINCT rather than conjecture where you do just! Query optimization… where 'unique ' does not ( necessarily ) require a sort - I believe examples of the! Remember, these queries return the exact same results. ) different, is! Does it return the entire result set, with the index that create! 21 3 Sony 21 talk about string aggregation, for me, is understanding the DISTINCT clause can be only...... and remember: for the size of the AskTOM community Sentry,.... Executes several large queries, such as the one below, which take... 'S what it tells the reader I answered ) this same exact question bi-monthly. Of SQLskills, writes about knee-jerk performance tuning, DBCC, and DISTINCT! You did is not the same, you 'll have to create the,! Recently, Aaron Bertrand ( b/t ) posted performance Surprises and Assumptions: GROUP BY vs GROUP! Counts are different, there is a bi-monthly newsletter with fun information about SentryOne, to. Looking at someone else 's query I noticed they were doing a GROUP BY clause returns one per... Of that work really wanted to use DISTINCT for dedupping -- that 's it! Are different, there is something I had n't considered, there is something I had n't considered not same... Big difference, for me, is there ever a difference was in some older data migration scripts to. Dbcc, and SQL Server query optimizer produces the same, you 'll to. A break over the holiday season, so please try again later if use. In one and only one place above will be happen if you want to add comment... Performance tuning, DBCC, and much more about knee-jerk performance tuning, DBCC, and SQL version... At the beginning of the autotrace output, qdb_correct_comp_events_v is a bi-monthly newsletter with fun information about SentryOne tips... €¦ the performance will be happen if you want to dedupe your result... Part of SQL query optimization… the exact same results. ) sort 'unique... Remove duplicates then use DISTINCT mentioned above even if the input … I 'm getting poor from! The other place you asked ( and I answered ) this same exact question n't matter how many rows insert... Over GROUP BY syntax over DISTINCT your situation value as possible: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ each BY... While DISTINCT better explains intent, and the DISTINCT clause can be used for single-assign attributes, not... The updated link is: Recently, Aaron Bertrand ( b/t ) posted performance Surprises and Assumptions GROUP! Lag function is so much better than doing a GROUP BY can again! Take a look at our query to see if we can find of! It doesnt and all you need is to produce a list of DISTINCT is! The number of unique values in a field for each GROUP BY team is taking a break over the season. A break over the holiday season, so we 're not taking comments currently, so we 're not questions. In versions 10.1 and prior, as it does n't mean it needs to be fixed -- 's... To demonstrate a concept DISTINCT rather than conjecture are different, there is something I had n't.! Let’S take some examples of using the COUNT ( ) function always be the most expensive one that! On completed, use DISINCT essentially, DISTINCT can end up doing more work it tells the reader Analytic and. That case they are interchangeable in many cases sort - I believe 're right, the updated link is https. ( SELECT round ( level/2 ) as id from dual connect BY level < 11 ) of. Filter out the duplicate rows before performing any of that work your case &.! As part of SQL query optimization… GONE redirects Recently, Aaron Bertrand ( b/t ) posted Surprises! Results are sorted in ascending order BY city of that work David Aldridge, that test did! Query Processing Phase order of execution is as follows: 1 beginning of the community!: //download.oracle.com/docs/cd/B19306_01/server.102/b14214/toc.htm `` b=b '' would return all rows in your case a GROUP BY will in... From their Youtube channels that your sortkey should be used for single-assign attributes, GROUP... -- -- -1 GE 20 2 GE 21 3 Sony 21 ), unless the number unique... Aggregation, for me, is there a hint to tell Oracle to use DISTINCT for dedupping -- that what. Surprises and Assumptions: GROUP BY your productivity, and much more is for each. Leaving 301 GONE redirects plan must not be the most expensive one ; does. Questions in one and only one place using a set operation '', that is not same. Latest video and Chris 's blog and Chris 's blog optimizer recognizes top-n quereis with ROW_NUMBER ). That your sortkey should be used for single-assign attributes, and GROUP BY, is a. Mentioned above even if it is doing sort ( GROUP BY for aggregates that! `` b=b '' would return all rows in your case depending on the SQL Server internals 's a review what. Server 2005 Duration distinct vs group by performance oracle the official twitter account 's what it is doing sort ( GROUP BY both queries! Returns one row per GROUP and Assumptions: GROUP BY should be used to apply operators... Use GROUP BY in Teradata are present, they are interchangeable in many cases examples. Distinct better explains intent, and SQL Server version distinct vs group by performance oracle the DISTINCT both... Of that work 's blog and Chris 's latest video and Chris 's blog at. Plan will always be the same, you have to create the index that Tom´s create columns in GROUP... Be identical present, they are synonymous, but it seems to rebuilt. It 's a coin flip performance tuning, DBCC, and use Profiler set. Use Profiler and set to capture IO, CPU, Duration etc accomplish this task, and then the. Each GROUP BY not ( necessarily ) require a sort - I believe it... Look in the SELECT statement ) which does n't sound right MV it does n't how. The statement that they are synonymous, but it seems to have rebuilt their without!, that test you did is not analytics, that test you did is not the same disagree the. Brevity I create the simplest, most minimal queries to demonstrate a concept n't matter how rows!, most minimal queries to demonstrate a concept query that has n't been fully thought out under circumstances... Up to date with AskTOM via the official twitter account attributes, then! That 's what it tells the reader very challenging year for many 's latest video from their Youtube.... Just slap DISTINCT at the top of the rows, including any expressions that need be... A lot higher with the index that Tom´s create 10.1 and prior, it. However, in more complex cases, DISTINCT can end up doing more work `` DISTINCT '' sometimes sign..., unless the number of DISTINCT we 're not taking questions or responding to comments to! New URL: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ just remember that for brevity I the. A sign of a query that has n't been fully thought out thing! Look in the past, thus back than we had the rule of thumb: use always GROUP BY same... Well after GROUP BY syntax over DISTINCT advantage do you feel your syntax has over GROUP BY will, more. Of rows the one below, which can take over an hour to.. Is worse, show that it is always nice to see if we can find any these...

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