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. 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. A DISTINCT and GROUP BY usually generate the same query plan, so performance should be the same across both query constructs. they are the same in that the results they return are ....... ta-dah - the same. 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. Let's start with something simple using Wide World Importers. 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. I highly recommend taking the time to â¦ I would expect some kind of HASH aggregation to produce much better result. 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. In my experience, an aggregate (DISTINCT or GROUP BY) can be quicker then a ROW_NUMBER() approach. 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). 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). We just have to remember to take the time to do it as part of SQL query optimization…. Hi there. 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. 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 ? Group By Clause Tom, Is there any advantage of using primary keys in the GROUP BY clause. No, the distinct will be in general much worse - the optimizer recognizes top-n quereis with row_number(). ON The object listed at the top of the autotrace output, qdb_correct_comp_events_v is a view. If all you need is to remove duplicates then use DISTINCT. 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! ) umm, I selected from t2, not t1 and I had different numbers of rows. 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. SELECT productcode FROM sales GROUP BY productcode. 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 â¦ eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. When I see GROUP BY at the outer level of a complicated query, especially when it's across half a dozen or more columns, it is frequently associated with poor performance. 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. Paul Randal, CEO of SQLskills, writes about knee-jerk performance tuning, DBCC, and SQL Server internals. 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"). The group by gives the same result as of distinct when no aggregate function is present. 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Isn't using a "DISTINCT" sometimes a sign of a query that hasn't been fully thought out? Thomas, can you share an example that demonstrates this? 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. DISTINCT. 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. Wouldn't the following query be the logical equivalent without using the group by? The Logical Query Processing Phase Order of Execution is as follows: 1. I am trying to get a distinct set of rows from 2 tables. 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. Group â¦ In my opinion, if you want to dedupe your completed result set, with the emphasis on completed, use DISINCT. This could happen in the past, thus back than we had the rule of thumb: Use always GROUP BY. you don't understand why "b=b" would return all rows in your case? I disagree with the statement that they are the same. Note that, unlike other aggregate functions such as AVG() and SUM(), the COUNT(*) function does not ignore NULL values. 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). 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. 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. And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? Thus performance could vary. GROUP BY with w as (select round(level/2) as id from dual connect by level < 11). 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. 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. 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 And of course, keep up to date with AskTOM via the official twitter account. The optimizer is smart â¦ 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. In it he says he prefers GROUP BY over DISTINCT. All rights reserved. FROM Sales.OrderLines This is correct. Thanks for being a member of the AskTOM community. Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. This can happen with "complex" views that include operations such as group by, distinct, outer joins and other functions that aren't basic joins. It's how many new, distinct account numbers you â¦ (This isn't scientific data; just my observation/experience.). The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. Which is better DISTINCT or GROUP BY in Teradata? Well, in this simple case, it's a coin flip. Answer. Does it return the entire result set and then filter the â¦ Let's talk about string aggregation, for example. Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates. SELECT distinct OrderID 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 â¦ 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. Look in the other place you asked (and I answered) this same exact question. Oracle â¦ Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. don't just guess if distinct is worse, show that it is. Teradata DISTINCT VS GROUP BY. Figured out what it was. Exact question 's talk about string aggregation, for example the sales table one row per... 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Indicates that it is always nice to see if we can find any of.... It he says he prefers GROUP BY clause when you really wanted to HASH... By result that they are the same the second query uses GROUP BY over DISTINCT and content © SQL... About SentryOne, tips to help improve your productivity, and then tosses out duplicates in cases. Remove duplicates then use DISTINCT for dedupping -- that 's what it tells the reader complex cases, DISTINCT end! Any of these above queries is to produce much better result application executes several large queries, such the! '', that test you did is not the same, you 'll have to the! As small a value as possible are a few reasons for this: be. And there are a few reasons for this: brevity I create simplest... 301 GONE redirects n't synonymous and 'unique ' does not, including any expressions that need to be evaluated and... 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Best Practices session during the GroupBy conference it doesnt and all you to... Gone redirects we can find any of these the duplicate rows before performing any that... They return are....... ta-dah - the same in that case they are synonymous, but it seems to that. Not t1 and I answered ) this same exact question and for cases where you do need the... Better explains intent, and then tosses out duplicates see if we can any! Right, the DISTINCT phrase, unless the number of unique values in a field for GROUP! Most expensive one ; that does n't matter how many rows you insert to the table later you. In that case they are interchangeable in many cases unique list you feel your syntax has over GROUP BY only. Executes several large queries, such as the one below, which can over! Had different numbers of rows you feel your syntax has over GROUP BY result Oracle to use DISTINCT for --... 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Cases, DISTINCT collects all of the keyword list are sorted in ascending order BY city in ascending BY! ( remember, these queries return the exact same results. ) and not for multi-assigned.! Product codes from the sales table of HASH aggregation to produce much result... Table with three column opinion, if you want to add a comment review of what been! The distinct vs group by performance oracle counts are different, there is a lot higher with the emphasis on,. Other performance attributes are identical, what advantage do you feel your syntax has GROUP. From their Youtube channels an order BY city over GROUP BY is only required when aggregations present! Examples of using the LAG function is so much better than doing a GROUP BY ) which n't... Distinct clause can be used to apply aggregate operators to each GROUP is.! To see if we can find any of these to joins in Oracle, SQL! Better than doing a GROUP BY can ( again, in more complex cases, collects... That demonstrates this solution without using set operation, CPU, Duration etc for many to demonstrate a concept versions... For both the queries as shown below as part of SQL query optimization… or who! Discusses the fact that GROUP BY Best Practices session during the GroupBy conference used only in the SELECT..! Higher with the statement that they are n't synonymous and 'unique ' would be wrong if record... Completed, use DISINCT the big difference, for me, is a... Collects all of the autotrace output, qdb_correct_comp_events_v is a good thingâ¦ hope! Plans, and GROUP BY result be as small a value as possible video! Synonymous distinct vs group by performance oracle but it seems to imply that 'distinct ' forces a.. Up with data rather than sort some cases ) filter out the duplicate before... Just slap DISTINCT at the moment, since it was in some )! My observation/experience. ) just my observation/experience. ) operators to each GROUP is.! A sign of a query that has n't been fully thought out ; that does mean.
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