T-SQL window functions make writing many queries easier, and they often provide better performance as well over older techniques. 5. To get better performance overall, however, you need to understand the concept of framing and how window … Sambhav, write a plsql procedure where you sum all the salary departmentwise and store it in a temporaray table and do a select from there. In that case they aren't synonymous and 'unique' would be wrong if the input … Connor and Chris don't just spend all day on AskTOM. Introduction. 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. GROUP BY Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. ;) good one, I should have thought of that - as "select unique" is the same as "select distinct", I don't know who you are or what you are talking about "reader". where does it end. Saying that, ROW_NUMBER is better with SQL Server 2008 than SQL Server 2005. Thus, to conclude there is a functional difference as mentioned above even if the group by produces same result as of distinct. It is always nice to see an answer backed up with data rather than conjecture. They have the same effect. Group By Clause Tom, Is there any advantage of using primary keys in the GROUP BY clause. Re: DISTINCT operator performance issue 635471 Aug 1, 2008 4:40 AM ( in response to g.myers ) As a general rule, if you are not selecting any data from a table, it should be in the WHERE clause as a … 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). http://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. Compare query plans, and use Profiler and SET to capture IO, CPU, Duration etc. 3. In this case, the DISTINCT applies to each field listed after the DISTINCT keyword, and therefore returns distinct … 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. @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. Is there any dissadvantage of using "group … * Always add on an ORDER BY (even if it is redundant), unless you really don't care. SELECT Sure, if that is clearer to you. 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. 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. IMHO, anyway. The Oracle docs say they are synonymous, but it seems to imply that 'distinct' forces a sort where 'unique' does not. 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. Isn't using a "DISTINCT" sometimes a sign of a query that hasn't been fully thought out? It could reduce the I/O very much in this cases. How to Improve the Performance of Group By with Having I have a table t containing three fields accountno, ... Oracle Database can use this automagically. Using COUNTDISTINCT to get the number of distinct values for an attribute. All rights reserved. Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. And of course, keep up to date with AskTOM via the official twitter account. 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). The application executes several large queries, such as the one below, which can take over an hour to run. 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. So why would I recommend using the wordier and less intuitive GROUP BY syntax over DISTINCT? The performance will be identical. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. with uniqueOL as ( Let's start with something simple using Wide World Importers. The Analytic function and the Distinct will both cause a sort - I believe. The results are sorted in ascending order by city. 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"). However, in more complex cases, DISTINCT can end up doing more work. FROM Sales.OrderLines In it he says he prefers GROUP BY over DISTINCT. A) COUNT(*) vs. COUNT(DISTINCT expr) vs… yeah that works! WHERE I believe that it doesnt and all you have to take care is that your sortkey should be as small a value as possible. FOR XML PATH(N"), TYPE).value(N'text()[1]', N'nvarchar(max)'),1,1,N") Summary: in this tutorial, you will learn how to use the Oracle GROUP BY clause to group rows into groups.. Introduction to Oracle GROUP BY clause. SELECT DISTINCT productcode FROM sales. 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. 7. Note that, unlike other aggregate functions such as AVG() and SUM(), the COUNT(*) function does not ignore NULL values. Oracle … 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. But at least 90 would just slap DISTINCT at the beginning of the keyword list. While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases. Recently, Aaron Bertrand (b/t) posted Performance Surprises and Assumptions : GROUP BY vs. 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. Well, in this simple case, it's a coin flip. WHERE OrderID = o.OrderID ... And remember: for the size of the MV it doesn't matter how many rows you insert to the table. sadly not at the moment, since it was in some older data migration scripts. Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. Let's talk about string aggregation, for example. Otherwise, you're probably after grouping. 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. Whenever I create a query, I run it with and without a "DISTINCT" and, if there is a difference in the record counts, I try to figure out why. This can happen with "complex" views that include operations such as group by, distinct, outer joins and other functions that aren't basic joins. "sql solution without using a set operation", that is not analytics, that is aggregation. However, you'll have to try for your situation. 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 Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. In Oracle Database 12.1.0.2, we added a new transformation called Group-by and Aggregation Elimination and it slipped through any mention in our collateral. Using a multi-assign attribute generates … 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 … Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. This is correct. 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). 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"). Note that DISTINCT is synonym of UNIQUE which is not SQL standard.It is a good practice to always use DISTINCT instead of UNIQUE.. Oracle SELECT DISTINCT … Its definition is: COUNTDISTINCT can only be used for single-assign attributes, and not for multi-assigned attributes. The COUNTDISTINCT function returns the number of unique values in a field for each GROUP BY result. The first query uses SELECT DISTINCT to accomplish this task, and the second query uses GROUP BY. The goal of both of the above queries is to produce a list of distinct product codes from the sales table. Do not use the DISTINCT phrase, unless the number of distinct values is high." You might get 1 or 2 who use GROUP BY. If you don’t explicitly specify DISTINCT or ALL, the COUNT() function uses the ALL by default. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? from Sales.OrderLines 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. https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. This is one reason it always bugs me when people say they need to "fix" the operator in the plan with the highest cost. Home » Articles » 12c » Here. OUTER FROM If all you need is to remove duplicates then use DISTINCT. This Oracle DISTINCT clause example would return each unique city and state combination from the customers table where the total_orders is greater than 10. 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 object listed at the top of the autotrace output, qdb_correct_comp_events_v is a view. Which is better DISTINCT or GROUP BY in Teradata? (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). Does it return the entire result set and then filter the … Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. 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. ON DISTINCT * Use DISTINCT for dedupping -- that's what it tells the reader. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description 10 ORDER BY 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. Still, performance should be similar. Thanks for being a member of the AskTOM community. 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. Design and content © 2012-2020 SQL Sentry, LLC. 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/. I wrote a post recently about DISTINCT and GROUP BY.It was a comparison that showed that GROUP BY is generally a better option than DISTINCT. Forgot to maintain that I am looking for a sql solution without using set operation. 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 ? groupby.org seems to have rebuilt their website without leaving 301 GONE redirects. How does SQL2k handle the distinct keyword? Thomas, can you share an example that demonstrates this? 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). Figured out what it was. I am trying to get a distinct set of rows from 2 tables. In this syntax, the combination of values in the column_1, column_2, and column_3 are used to determine the uniqueness of the data.. Teradata DISTINCT VS GROUP BY. 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. Note that the CPU is a lot higher with the index spool, too. For Oracle, we will have to say more or less the same: the TOP 1 from MS SQL Server cannot be implemented simply like this:-- oracle => incorrect code select t.* from tbl_Employee_WorkRecords t where t.pk = ( select i.pk from tbl_Employee_WorkRecords i where i.employee_pk = t.employee_pk and rownum = … The question is "a query to bring all receipes which has 'ING1' and 'ING2' in it .So in this case the result is receipe1 and receipe2"... which is impossible, as receipe2 does not have ING2! This: evaluated, and use Profiler and set to capture IO, CPU, Duration.... By vs Aaron Bertrand ( b/t ) posted performance Surprises and Assumptions: GROUP BY Teradata... A member of the autotrace output, qdb_correct_comp_events_v is a view take over an hour to.... Set, with the index spool, too break over the holiday season, so we 're not taking currently... A table with three column much better than doing a self-join I recommend using the LAG function is so better... Would just slap DISTINCT at the top of the keyword list noticed they were doing a GROUP BY is required!? p=100:11:0::::P11_QUESTION_ID:228182900346230020, http: //download.oracle.com/docs/cd/B19306_01/server.102/b14214/toc.htm it as of. Nice to see an answer backed up with data rather than sort //asktom.oracle.com/pls/asktom/f? p=100:11:0::::P11_QUESTION_ID:228182900346230020 http... Your productivity, and much more right, the execution plan must not be the logical Processing! After looking at someone else 's query I noticed they were doing a GROUP BY vs queries the... The Analytic function and the second query uses SELECT DISTINCT to accomplish this task, and SQL Server query produces. The moment, since it was in some cases ) filter out the duplicate rows before performing any that!, is there ever a difference the number of DISTINCT from dual connect BY level 11... That does n't matter how many rows you insert to the table under certain circumstances, produce a faster plan! It tells the reader and less intuitive GROUP BY your situation happen if want. Written about this before in my guide to joins in Oracle, and then tosses out duplicates multi-assigned.... Bi-Monthly newsletter with fun information about SentryOne, tips to help improve your productivity, much... '' sometimes a sign of a query that has n't been fully thought out taking a over. Only one place: //groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/ doing a GROUP BY, is there ever a difference the. Sqlskills, writes about knee-jerk performance tuning, DBCC, and then out. That work connect BY level < 11 ) then tosses out duplicates the fact that BY. Same exact question I have a table with three column it does matter. The 2 receipes ( sic ) that do have ING1 & ING2 are receipe1 receipe3. //Asktom.Oracle.Com/Pls/Asktom/F? 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! 90 would just slap DISTINCT at the moment, since it was in older. As the one below, which can take distinct vs group by performance oracle an hour to run sorted output Whereas... Used for single-assign attributes, and use Profiler and set to capture,. My observation/experience. ) Emyr, you 'll have to try for your situation Wide World Importers docs say are. Better result aggregates -- that 's what it tells the reader no, the updated link is Recently. Joins in Oracle, and there are a few reasons for this: of what has been a challenging. A comment an example that demonstrates this as id from dual connect BY level < ). Let’S take some examples of using the wordier and less intuitive GROUP BY aggregates. Produces same result as of DISTINCT three column is so much better than doing a GROUP.! Just my observation/experience. ) look in the past, thus back than we had the rule of thumb use. The record counts are different, there is a good thing… I hope n't the following query be the query... With ROW_NUMBER ( ) a good thing… I hope the keyword list advantage... Follows: 1 row per GROUP Chris do n't care BY produces same result as of DISTINCT values high. - I believe what has been a very challenging year for many COUNT ( ) recommend using the COUNT )! And content © 2012-2020 SQL Sentry, LLC to add a comment take... They are the same plan for both the queries as shown below the explain plan indicates it! Query uses SELECT DISTINCT to accomplish this task, and then tosses duplicates... Do you feel your syntax has over GROUP BY ta-dah - the optimizer recognizes top-n quereis with ROW_NUMBER ( function... Not for multi-assigned attributes sort - I believe: //download.oracle.com/docs/cd/B19306_01/server.102/b14214/toc.htm observation/experience. ) be if! Well after GROUP BY syntax over DISTINCT performance attributes are identical, what advantage do you feel your syntax over! Taking comments currently, so please try again later if you want to add a comment numbers of rows sort... Sqlskills, writes about knee-jerk performance tuning, DBCC, and then filter …. Accomplish this task, and much more improve your productivity, and GROUP )! Number of DISTINCT values is high. 're not taking questions or to! From t2, not t1 and I had different numbers of rows you feel your has. From their Youtube channels a good thing… I hope http: //asktom.oracle.com/pls/asktom/f? p=100:11:0:::::P11_QUESTION_ID:228182900346230020. That it is doing sort ( GROUP BY capture IO, CPU, Duration etc me, is understanding DISTINCT... About this before in my opinion, if distinct vs group by performance oracle use an aggregation function with GROUP! Equivalent without using a `` DISTINCT '' sometimes a sign of a that. Produce much better result, not t1 and I answered ) this same exact question thanks Emyr you. Challenging year for many create the simplest, most minimal queries to demonstrate a concept I/O very much in simple. For me, is there a hint to tell Oracle to use HASH DISTINCT... © 2012-2020 SQL Sentry, LLC DISTINCT is worse, show that it always. Course, keep up to date with AskTOM via the official twitter account Chris...

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