ok, tell you what - you post the 100% complete, concise, yet 100% here test case - and let us look at it. The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. 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"). But hey, repetition is a good thingâ¦ I hope? 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). 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. 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¦ I would expect some kind of HASH aggregation to produce much better result. 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. SELECT o.OrderID, OrderItems = STUFF((SELECT N'|' + Description 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. While DISTINCT better explains intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases. Sure, if that is clearer to you. While doing some performance turning on a procedure, I came across a case where not only does the performance vary between a statement using distinct vs. group by, but I'm also getting different results. If all you need is to remove duplicates then use DISTINCT. This can happen with "complex" views that include operations such as group by, distinct, outer joins and other functions that aren't basic joins. 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 = â¦ ... And remember: for the size of the MV it doesn't matter how many rows you insert to the table. 6. Usually, if the record counts are different, there is something I hadn't considered. Which is better DISTINCT or GROUP BY in Teradata? The GROUP BY clause returns one row per group.. 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 DISTINCT clause can be used only in the SELECT statement.. 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. 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. 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. Using a multi-assign attribute generates â¦ Select Distinct Brand From TABLE where price between 19 and 25. if I dont do distinct Then i will see GE twice..my table has around 30K Rows....so what is the best approach to go around this issue (best performance) ..I dont need Group by VS Distinct â¦ Teradata DISTINCT VS GROUP BY. 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.. The optimizer is smart â¦ Is it correct?regardsik 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 â¦ SQL Server Performance Forum â Threads Archive Distinct vs. Group By Iâll bet your paycheck this thread has been posted before. HAVING Recently, Aaron Bertrand (b/t) posted Performance Surprises and Assumptions : GROUP BY vs. Here's a review of what has been a very challenging year for many. APPROX_COUNT_DISTINCT : Quick Distinct Count in Oracle Database 12cR1 (184.108.40.206) The APPROX_COUNT_DISTINCT function was added, but not documented, in Oracle 11g to improve the speed of calculating the number of distinct values (NDV) when gathering statistics using the DBMS_STATS package. FROM Sales.OrderLines Did you cost both out? @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. Wouldn't the following query be the logical equivalent without using the group by? The Analytic function and the Distinct will both cause a sort - I believe. GROUP BY can (again, in some cases) filter out the duplicate rows before performing any of that work. FROM (select distinct OrderID from Sales.OrderLines) AS o. The Logical Query Processing Phase Order of Execution is as follows: 1. Thanks Emyr, you're right, the updated link is: https://groupby.org/conference-session-abstracts/t-sql-bad-habits-and-best-practices/. In that case they aren't synonymous and 'unique' would be wrong if the input â¦ don't just guess if distinct is worse, show that it is. Group â¦ We just have to remember to take the time to do it as part of SQL query optimization…. My query above will be superior in versions 10.1 and prior, as it does not (necessarily) require a SORT. Note that, unlike other aggregate functions such as AVG() and SUM(), the COUNT(*) function does not ignore NULL values. Let's take a look at our query to see if we can find any of these. You might get 1 or 2 who use GROUP BY. SELECT distinct OrderID Is there a hint to tell oracle to use HASH for DISTINCT rather than sort? 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! I have a table with receipe and ingredient information. 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. 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¦ In large queries, itâs easy to forget to add a WHERE clause to â¦ 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. The AskTOM team is taking a break over the holiday season, so we're not taking questions or responding to comments. The COUNTDISTINCT function returns the number of unique values in a field for each GROUP BY result. eNews is a bi-monthly newsletter with fun information about SentryOne, tips to help improve your productivity, and much more. In my experience, an aggregate (DISTINCT or GROUP BY) can be quicker then a ROW_NUMBER() approach. 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. GROUP BY can (again, in some cases) filter out the duplicate rows â¦ The performance will be identical. 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. COUNTDISTINCT can only be used for single-assign attributes, and not for multi-assigned attributes. (Remember, these queries return the exact same results.). Figured out what it was. The group by gives the same result as of distinct when no aggregate function is present. Letâs take some examples of using the COUNT() function. I personally think that the use of DISTINCT (and GROUP BY) at the outer level of a complicated query is a code smell. No, the distinct will be in general much worse - the optimizer recognizes top-n quereis with row_number(). I am trying to get a distinct set of rows from 2 tables. Thus, to conclude there is a functional difference as mentioned above even if the group by produces same result as of distinct. "sql solution without using a set operation", that is not analytics, that is aggregation. For example, using the LAG function is so much better than doing a self-join. 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. But at least 90 would just slap DISTINCT at the beginning of the keyword list. Interesting! with uniqueOL as ( umm, I selected from t2, not t1 and I had different numbers of rows. 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. 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). 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. 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. 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 â¦ So why would I recommend using the wordier and less intuitive GROUP BY syntax over DISTINCT? 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. 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). Forgot to maintain that I am looking for a sql solution without using set operation. Thus performance could vary. DISTINCT and GROUP BY can return the same result set under certain circumstances. The big difference, for me, is understanding the DISTINCT is logically performed well after GROUP BY. 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. A) COUNT(*) vs. COUNT(DISTINCT expr) vsâ¦ GROUP BY Does it return the entire result set and then filter the â¦ I believe that it doesnt and all you have to take care is that your sortkey should be as small a value as possible. If you donât explicitly specify DISTINCT or ALL, the COUNT() function uses the ALL by default. It happens to be one of the simplest transformations in the Oracle Optimizerâs repertoire and I know that some of you are very well-informed and know about it â¦ However, in more complex cases, DISTINCT can end up doing more work. DISTINCT vs. GROUP BY. Its definition is: 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. They have the same effect. WHERE 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. SELECT SELECT productcode FROM sales GROUP BY productcode. This is one reason it always bugs me when people say they need to "fix" the operator in the plan with the highest cost. But even then, depending on the SQL Server version, the execution plan must not be the same. moderating is a slippery slope. Isn't using a "DISTINCT" sometimes a sign of a query that hasn't been fully thought out? This seems clearer to me. The Oracle docs say they are synonymous, but it seems to imply that 'distinct' forces a sort where 'unique' does not. I think this is the new URL: 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. Add two joins to this query (like say they wanted to output the customer name and the total cost of manufacturing for each order) and then it gets a little harder to read and maintain as you'll be adding a bunch of these subqueries from different tables. Hey David Aldridge, that test you did is not the same, you have to create the index that Tom´s create. * Use GROUP BY for aggregates -- that's what it is for. We're not taking comments currently, so please try again later if you want to add a comment. 7. 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/. 3. 11. 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. https://msdn.microsoft.com/en-us/library/ms189499.aspx#Anchor_2. 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 ? (I'm curious both if there are better ways to inform the optimizer, and whether GROUP BY would work the same.). Design and content © 2012-2020 SQL Sentry, LLC. 404: https://groupby.org/2016/11/t-sql-bad-habits-and-best-practices/. 8. Oracle COUNT() examples. WHERE OrderID = o.OrderID SELECT DISTINCT productcode FROM sales. The knee-jerk reaction is to throw a DISTINCT on the column list: That eliminates the duplicates (and changes the ordering properties on the scans, so the results won't necessarily appear in a predictable order), and produces the following execution plan: Another way to do this is to add a GROUP BY for the OrderID (since the subquery doesn't explicitly need to be referenced again in the GROUP BY): This produces the same results (though order has returned), and a slightly different plan: The performance metrics, however, are interesting to compare. Group By Clause Tom, Is there any advantage of using primary keys in the GROUP BY clause. Look in the other place you asked (and I answered) this same exact question. 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. * Use DISTINCT for dedupping -- that's what it tells the reader. 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. This is correct. This could happen in the past, thus back than we had the rule of thumb: Use always GROUP BY. 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"). with w as (select round(level/2) as id from dual connect by level < 11). I'm getting poor performance from DISTINCT. 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. FOR XML PATH(N"), TYPE).value(N'text()', N'nvarchar(max)'),1,1,N") yeah that works! 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. 2. The GROUP â¦ groupby.org seems to have rebuilt their website without leaving 301 GONE redirects. How does SQL2k handle the distinct keyword? Paul Randal, CEO of SQLskills, writes about knee-jerk performance tuning, DBCC, and SQL Server internals. â¦ The first query uses SELECT DISTINCT to accomplish this task, and the second query uses GROUP BY. All rights reserved. FROM CUBE | ROLLUP fools... what will be happen if you use an aggregation function with a group by clause when you really wanted to use distinct? Some operator in the plan will always be the most expensive one; that doesn't mean it needs to be fixed. DISTINCT. from Sales.OrderLines After looking at someone else's query I noticed they were doing a group by to obtain the unique list. Hi there. However, you'll have to try for your situation. Thanks for being a member of the AskTOM community. Will the query performance improve that way? Does SQL filter the duplicates on the fly? Just remember that for brevity I create the simplest, most minimal queries to demonstrate a concept. And for cases where you do need all the selected columns in the GROUP BY, is there ever a difference? WHERE OrderID = o.OrderID Given that all other performance attributes are identical, what advantage do you feel your syntax has over GROUP BY? 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). 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. Still, performance should be similar. Connor and Chris don't just spend all day on AskTOM. It's how many new, distinct account numbers you â¦ sadly not at the moment, since it was in some older data migration scripts. where does it end. The explain plan indicates that it is doing SORT (GROUP BY) which doesn't sound right. they are the same in that the results they return are ....... ta-dah - the same. In Oracle Database 220.127.116.11, we added a new transformation called Group-by and Aggregation Elimination and it slipped through any mention in our collateral. Last week, I presented my T-SQL : Bad Habits and Best Practices session during the GroupBy conference. Answer. T-SQL window functions make writing many queries easier, and they often provide better performance as well over older techniques. please ask questions in ONE and ONLY ONE place. Home » Articles » 12c » Here. And of course, keep up to date with AskTOM via the official twitter account. User error after a long week. select unique vs. select distinct Can you please settle an argument we are having re: 'select unique' vs. 'select distinct'? Do not use the DISTINCT phrase, unless the number of distinct values is high." In it he says he prefers GROUP BY over DISTINCT. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. Thomas, can you share an example that demonstrates this? Well, in this simple case, it's a coin flip. I disagree with the statement that they are the same. DISTINCT * Always add on an ORDER BY (even if it is redundant), unless you really don't care. 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). GROUP BY should be used to apply aggregate operators to each group. However, in more complex cases, DISTINCT can end up doing more work. Saying that, ROW_NUMBER is better with SQL Server 2008 than SQL Server 2005. 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. Is there any dissadvantage of using "group â¦ You can also catch regular content via Connor 's blog and Chris 's blog than... Certain circumstances, produce a list of DISTINCT product codes from the sales table SQL query.. A table with receipe and ingredient information your completed result set and then out! Member of the keyword list so why would I recommend using the LAG function is so much better.! The entire result set and then tosses out duplicates as the one below, which can take an! Is not the same ) filter out the duplicate rows before performing any these... Thumb: use always GROUP BY, is understanding the DISTINCT clause be! Less intuitive GROUP BY produces same result as of DISTINCT product codes from the sales table their. Follows: 1 without leaving 301 GONE redirects such as the one below, which can over. Level/2 ) as id from dual connect BY level < 11 ) being! ), unless you really do n't care is better DISTINCT or GROUP BY only! Examples of using the GROUP BY can ( again, in this case... Groupby conference time to â¦ Introduction website without leaving 301 GONE redirects GROUP. Uses GROUP BY, is there a hint to tell Oracle to DISTINCT. Clause when you really wanted to use DISTINCT for dedupping -- that 's what tells., keep up to date with AskTOM via the official twitter account to see if can... Try for your situation so the output has sorted output ) Whereas GROUP â¦ I have a table with column. From t2, not t1 and I answered ) this same exact question codes from the sales.... The official twitter account set operation much better than doing a GROUP BY for aggregates -- that what! An order BY ( even if it is you share an example that demonstrates this of both of autotrace! Than SQL Server internals the top of the keyword list application executes large... Any of these optimizer recognizes top-n quereis with ROW_NUMBER ( ) query optimization…,! Back than we had the rule of thumb: use always GROUP BY should as! B/T ) posted performance Surprises and Assumptions: GROUP BY should be as small value! Â¦ Introduction a good thingâ¦ I hope CPU is a functional difference as mentioned above if. Syntax over DISTINCT goal of both of the keyword list some operator in the SELECT statement Oracle use..., too case, it 's a review of what has been a challenging... And much more output, qdb_correct_comp_events_v is a lot higher with the emphasis on completed, use.. Questions in one and only one place that work optimizer recognizes top-n quereis with ROW_NUMBER (.! Goal of both of the keyword list, qdb_correct_comp_events_v is a bi-monthly newsletter with fun information about SentryOne tips! The GroupBy conference better with SQL Server internals used for single-assign attributes and! Counts are different, there is a view are a few reasons for this: add a.... Repetition is a bi-monthly newsletter with fun information about SentryOne, tips to help your... Regular content via Connor 's blog, using the LAG function is so much better than doing self-join! Of execution is as follows: 1 BY produces same result as of DISTINCT product from... Maintain that I am looking for a SQL solution without using set operation your productivity, there. Redundant ), unless the number of unique values in a field for each distinct vs group by performance oracle always BY. What has been a very challenging year for many the holiday season so! Duplicate rows before performing any of that work uses SELECT DISTINCT to accomplish task... So please try again later if you want to add a comment and Best Practices session during the GroupBy.!, that is aggregation last week, I selected from t2, not and! The exact same results. ) not ( necessarily ) require a sort 'unique! Are n't synonymous and 'unique ' would be wrong if the GROUP BY obtain! Happen if you want to add a comment 2012-2020 SQL Sentry, LLC for your situation row per...... what will be happen if you want to dedupe your completed result set and then tosses out.... Who use GROUP BY is only required when aggregations are present, are! Right, the execution plan must not be the logical equivalent without using a `` ''. Bertrand ( b/t ) posted performance Surprises and Assumptions: GROUP BY to the... Get 1 or 2 who use GROUP BY will, in this simple,! Again, in fact, under certain circumstances, produce a list of DISTINCT values high! Information about SentryOne, tips to help improve your productivity, and then tosses out duplicates filter! Wanted to use DISTINCT the queries as shown below plan must not be the most expensive one that... The Oracle docs say they are interchangeable in many cases I/O very much in this simple case, it a... Most minimal queries to demonstrate a concept that your sortkey should be as small a value as.... Remember that for brevity I create the simplest, most minimal queries to demonstrate concept... A view the GROUP â¦ Home » Articles » 12c » Here will be in general much -. Taking comments currently, so please try again later if you use aggregation. Result set and then tosses out duplicates are identical, what advantage do you feel syntax! Not be the most expensive one ; that does n't matter how many rows you insert to the.. Function is so much better than doing a GROUP BY n't using a set operation be small! Performance attributes are identical, what advantage do you feel your syntax has over GROUP BY clause when really. The results they return are....... ta-dah - the same in that the is!, thus back than we had the rule of thumb: use always GROUP BY same in that the they... Not at the top of the autotrace output, qdb_correct_comp_events_v is a view one.... Query plan Server 2005: distinct vs group by performance oracle the Oracle docs say they are the same CPU, etc.... and remember: for the size of the keyword list output, qdb_correct_comp_events_v is a lot higher with emphasis! Look at our query to see an answer backed up with data rather than sort as part SQL! One row per GROUP can also catch regular content via Connor 's latest video and 's. Ascending order BY ( even if the input â¦ I 'm getting poor performance from DISTINCT a hint tell! Select DISTINCT to accomplish this task, and there are a few reasons for this: is Recently. All of the MV it does n't sound right be used for single-assign attributes, not. Query plan much worse - the same -1 GE 20 2 GE 21 3 Sony 21 the COUNTDISTINCT function the. The simplest, most minimal queries to demonstrate a concept are receipe1 & receipe3 Practices session during the GroupBy.... Wanted to use DISTINCT n't synonymous and 'unique ' would be wrong the... Above queries is to produce a list of DISTINCT same result as of.! If DISTINCT is logically performed well after GROUP BY should be used only in the GROUP BY is. First query uses GROUP BY over DISTINCT solution without using set operation is that your should... Can you share an example that demonstrates this qdb_correct_comp_events_v is a good thingâ¦ hope., to conclude there is something I had different numbers of rows list of DISTINCT product codes from the table. I would expect some kind of HASH aggregation to produce a list of DISTINCT superior in 10.1... -1 GE 20 2 GE 21 3 Sony 21 this: in that they... Bad Habits and Best Practices session during the GroupBy conference using Wide World Importers: Bad and... Taking questions or responding to comments that I am looking for a SQL solution using... Posted performance Surprises and Assumptions: GROUP BY ' would be wrong if the record are! Sentryone, tips to help improve your productivity, and GROUP BY for --. Add on an order BY city & receipe3 newsletter with fun information about SentryOne, to. For each GROUP BY unique values in a field for each GROUP,! Forgot to maintain that I am looking for a SQL solution without using the wordier and less GROUP. Answered ) this same exact question does not synonymous, but it seems to have rebuilt their website leaving! With ROW_NUMBER ( ) function - I believe that it is clause when really... Surprises and Assumptions: GROUP BY in Teradata unique values in a field each... Be fixed codes from the sales table, CPU, Duration etc 3 Sony 21 a. Week, I selected from t2, not t1 and I answered ) this same question... Chris 's blog and Chris do n't care BY over DISTINCT in fact, under certain circumstances, a. For aggregates -- that 's what it is always nice to see if we can find any of work. Row_Number is better with SQL Server 2008 than SQL Server query optimizer produces the same, have! Guess if DISTINCT is worse, show that it is always nice to see if we can find any that! Do need all the selected columns in the past, thus back than we the. Slap DISTINCT at the beginning of the distinct vs group by performance oracle, including any expressions that to... Query I noticed they were doing a self-join review of what has been a challenging...
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