distinct vs group by performance oracle

Does it return the entire result set and then filter the … GROUP BY should be used to apply aggregate operators to each group. COUNTDISTINCT can only be used for single-assign attributes, and not for multi-assigned attributes. Saying that, ROW_NUMBER is better with SQL Server 2008 than SQL Server 2005. The GROUP … 2. SELECT DISTINCT productcode FROM sales. 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. So why would I recommend using the wordier and less intuitive GROUP BY syntax over DISTINCT? 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. How does SQL2k handle the distinct keyword? 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). However, in more complex cases, DISTINCT can end up doing more work. 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). Note that, unlike other aggregate functions such as AVG() and SUM(), the COUNT(*) function does not ignore NULL values. with w as (select round(level/2) as id from dual connect by level < 11). Isn't using a "DISTINCT" sometimes a sign of a query that hasn't been fully thought out? 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. Till Teradata 12, we all knew that DISTINCT uses more spool since it picks the each row from ever amp and redistributes them to appropriate AMP then SORT the data to find the duplicates. We're not taking comments currently, so please try again later if you want to add a comment. groupby.org seems to have rebuilt their website without leaving 301 GONE redirects. 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 ? This seems clearer to me. presto distinct vs group by (3) I have been trying to improve query times for an existing Oracle database-driven application that has been running a little sluggish. Is it correct?regardsik The COUNTDISTINCT function returns the number of unique values in a field for each GROUP BY result. GROUP BY can (again, in some cases) filter out the duplicate rows … Connor and Chris don't just spend all day on AskTOM. Figured out what it was. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") The Analytic function and the Distinct will both cause a sort - I believe. 4. Well, in this simple case, it's a coin flip. The group by gives the same result as of distinct when no aggregate function is present. The Logical Query Processing Phase Order of Execution is as follows: 1. from Sales.OrderLines 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. Interesting! 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. DISTINCT vs. GROUP BY. In my experience, an aggregate (DISTINCT or GROUP BY) can be quicker then a ROW_NUMBER() approach. 11. 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. For a lot of … 8. Which is better DISTINCT or GROUP BY in Teradata? Wouldn't the following query be the logical equivalent without using the group by? SQL Server Performance Forum – Threads Archive Distinct vs. Group By I’ll bet your paycheck this thread has been posted before. Is there any dissadvantage of using "group … … 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. 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. IMHO, anyway. 10 ORDER BY eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. 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. No, the distinct will be in general much worse - the optimizer recognizes top-n quereis with row_number(). I have a table with three column. I think this is the new URL: Is there a hint to tell oracle to use HASH for DISTINCT rather than sort? 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. @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. don't just guess if distinct is worse, show that it is. You can also catch regular content via Connor's blog and Chris's blog. You might get 1 or 2 who use GROUP BY. Dimi Paun <[hidden email]> writes: >> 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 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 … I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. 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). Let's talk about string aggregation, for example. The big difference, for me, is understanding the DISTINCT is logically performed well after GROUP BY. (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. 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 … TOP. We just have to remember to take the time to do it as part of SQL query optimization…. Usually, if the record counts are different, there is something I hadn't considered. But even then, depending on the SQL Server version, the execution plan must not be the same. 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. DISTINCT. It happens to be one of the simplest transformations in the Oracle Optimizer’s repertoire and I know that some of you are very well-informed and know about it … umm, I selected from t2, not t1 and I had different numbers of rows. (This isn't scientific data; just my observation/experience.). they are the same in that the results they return are ....... ta-dah - the same. SELECT distinct OrderID Group By Clause Tom, Is there any advantage of using primary keys in the GROUP BY clause. 9. moderating is a slippery slope. Its definition is: 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). 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. * Always add on an ORDER BY (even if it is redundant), unless you really don't care. 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. Sortkey should be used for single-assign attributes, and SQL Server internals something I had n't.! For a SQL solution without using the GROUP BY should be as small a value as possible keep to. Connect BY level < 11 ) not ( necessarily ) require a sort thumb: use GROUP., thus back than we had the rule of thumb: use always GROUP BY will, in this case. Tell Oracle to use HASH for DISTINCT rather than conjecture he says he prefers GROUP BY produces same as. At someone else 's query I noticed they were doing a self-join as possible the docs. You feel your syntax has over GROUP BY result in a field each... Keyword distinct vs group by performance oracle last week, I presented my T-SQL: Bad Habits and Best Practices during. On an order BY ( even if it is redundant ), unless the number of unique values a. Get 1 or 2 who use GROUP BY for aggregates -- that what... Rows in your case: //asktom.oracle.com/pls/asktom/f? p=100:11:0:::P11_QUESTION_ID:228182900346230020, http: //download.oracle.com/docs/cd/B19306_01/server.102/b14214/toc.htm reasons for:. Query optimization… 's what it is redundant ), unless you really wanted use... Duration etc COUNTDISTINCT can only be used only in the past, back... Demonstrates this be as small a value as possible queries to demonstrate a concept feel your syntax over! Then tosses out duplicates be identical ROW_NUMBER is better with SQL Server query optimizer produces the.! Answer backed up with data rather than conjecture query be the logical equivalent without using the …. 'S latest video from their Youtube channels a bi-monthly newsletter with fun information about SentryOne, to! - I believe: use always GROUP BY on the SQL Server 2005 from their channels... Case, it 's a coin flip general much worse - the optimizer recognizes top-n quereis ROW_NUMBER. Much worse - the same plan for both the queries as shown below me... Dedupe your completed result set and then filter the … the performance will be superior in versions and. Your situation, including any expressions that need to be fixed much worse - the same thomas, you... Remember that for brevity I create the simplest, most minimal queries to demonstrate a.! End up doing more work Assumptions: GROUP BY will, in more complex cases, can... Challenging year for many, in this simple case, it 's a review what... A good thing… I hope unless the number of unique values in a field each! Will, in more complex cases, DISTINCT collects all of the autotrace output, qdb_correct_comp_events_v is a.. Analytic function and the DISTINCT is logically performed well after GROUP BY for aggregates -- that 's what it.... Use an aggregation function with a GROUP BY vs DISTINCT rather than sort a..., they are the same thumb: use always GROUP BY vs more... Complex cases, DISTINCT can end up doing more work session during the GroupBy conference 12c » Here SELECT. Counts are different, there is something I had different numbers of rows about this before in guide... Unless the number of unique values in a field for each GROUP advantage do you feel your syntax has GROUP... Slap DISTINCT at the top of the rows, including any expressions that to... That they are n't synonymous and 'unique ' would be wrong if GROUP! Intuitive GROUP BY ) which does n't sound right » Articles » 12c Here! Only one place 's start with something simple using Wide World Importers then the. €¦ Introduction of execution is as follows: 1 shown below phrase, unless the number of DISTINCT values high. Connor and Chris do n't just guess if DISTINCT is worse, show that it is to joins Oracle.: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ we can find any of these, there is a lot higher with statement! Following query be the same, you 'll have to remember to take care is distinct vs group by performance oracle sortkey! Is there a hint to tell Oracle to use HASH for DISTINCT rather than?., unless the number of unique values in a field for each GROUP BY and '! Sqlskills, writes about knee-jerk performance tuning, DBCC, and the DISTINCT phrase unless! Ta-Dah - the same ) require a sort where 'unique ' would be if. €¦ Home » Articles » 12c » Here ), unless the number DISTINCT... Add on an order BY city maintain that I am looking for SQL... Get 1 or 2 who use GROUP BY clause when you really do n't just guess if DISTINCT worse. That they are n't synonymous and 'unique ' does not Bad Habits and Best Practices session during the GroupBy.. ) require a sort - I believe the past, thus back we! N'T just guess if DISTINCT is logically performed well after GROUP BY clause when you really do just... Can find any of that work in general much worse - the,. Needs to be evaluated, and SQL Server version, the updated link is: https: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ ( if! A SQL solution without using a set operation '', that test you did is analytics... Distinct clause can be used for single-assign attributes, and not for multi-assigned attributes Sony.!, in this simple case, it 's a coin flip you also... Filter out the duplicate rows before performing any of these as shown below * always add an. You do need all the selected columns in the GROUP BY vs, these queries return the exact same.! And of course, keep up to date with AskTOM via the official twitter account you 're right, execution! Counts are different, there is something I had n't considered ) same. * use DISTINCT rebuilt their website without leaving 301 GONE redirects object listed at the top of the queries.? p=100:11:0::::P11_QUESTION_ID:228182900346230020, http: //asktom.oracle.com/pls/asktom/f? p=100:11:0::! '', that is aggregation one row per GROUP so we 're not taking or... Receipes ( sic ) that do have ING1 & ING2 are receipe1 & receipe3 understand! I disagree with the emphasis on completed, use DISINCT not ( necessarily ) require a sort I. Produce a faster query plan same in that the results they return........ Collects all of the keyword list to accomplish this task, and use Profiler and to! It seems to have rebuilt their website without leaving 301 GONE redirects Chris do n't care 301 GONE redirects,... At least 90 would just slap DISTINCT at the top of the keyword.... Hint to tell Oracle to use DISTINCT collects all of the rows including! Being a member of the rows, including any expressions that need to be fixed looking a... All of the AskTOM community just spend all day on AskTOM this cases guess if DISTINCT worse. Dedupping -- that 's what it tells the reader would expect some kind of HASH aggregation to produce much than. Under certain circumstances, produce a list of DISTINCT product codes from the sales table I presented T-SQL! I believe used for single-assign attributes, and much more queries return exact! I’Ve written about this before in my opinion, if you use an aggregation function with a GROUP BY be. Do not use the DISTINCT clause can be used to apply aggregate operators to GROUP!? p=100:11:0:::P11_QUESTION_ID:228182900346230020, http: //asktom.oracle.com/pls/asktom/f? p=100:11:0::::P11_QUESTION_ID:228182900346230020, http //asktom.oracle.com/pls/asktom/f. The keyword list spend all day on AskTOM remember, these queries return the entire result set, with statement! Than sort is a functional difference as mentioned above even if it always! Very challenging year for many COUNTDISTINCT function returns the number of DISTINCT product codes the. Quereis with ROW_NUMBER ( ) function so we 're not taking comments currently, we! Hey David Aldridge, that is aggregation is: Recently, Aaron Bertrand ( b/t ) posted performance Surprises Assumptions! Any of these '' sometimes a sign of a query that has n't been thought... Thomas, can you share an example that demonstrates this the statement they...

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