The distinct clause is used in the select statement to remove duplicate rows from a result set. Ability to generate queries with distinctuniquegroup by. So, couple of days ago, some guy, from periscope company wrote a blogpost about getting number of distinct elements, per group, faster using subqueries. In the first, for each set of rows that have a distinct col1,col2 value its taking one of those rows and using its col3 value. The group by clause follows the where clause in a select statement and precedes the order by clause. I have a query where i want to select the usertable records that have a matching entry in an event table. But i want to confirm is the group by faster because it doesnt have to sort results, whereas distinct must produce sorted results. Almost a year ago, i wrote a custom experimental aggregate replacing countdistinct. Postgresql is an object relational database management system ordbms whereas mysql is a community driven dbms system.
Is there any difference on performance when choosing distinct. Browse other questions tagged postgresql performance index groupby count or ask your own question. Distinct is used to filter unique records out of the records that satisfy the query criteria. The problem with the native countdistinct is that it forces a sort on the input relation, and when the amount of data is significant say, tens of millions rows, that may be a significant performance drag. Im building this query generatively based on user input, and that second example is easily doable. Slow query on large table with group by and order by. Actually, i think i answered my own question already. Pg supports two comparison statements is distinct from and is not distinct from, these essentially treat null as if it was a known value, rather than a special case for unknown. The talk will cover postgresql grouping and aggregation facilities and best practices of using them in fast and efficient manner. A distinct and group by usually generate the same query plan, so performance should be the same across both query constructs. The domain column being aggregated has around 16k distinct values, and there are 780k rows in total for the entire table, not the slice being selected in these queries. I am trying to get a distinct set of rows from 2 tables.
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. Use distinct for dedupping thats what it tells the reader. Almost a year ago, i wrote a custom experimental aggregate replacing count distinct. This is more important than the rest of this answer. I believe the only exception to this is in regards to parallel query, as currently only group bys may be parallelised, not distinct. 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. By the way, this is yet another example of how twitter can be used in a good and positive way within the work environment and within. Count distinct performance compared on top 4 sql databases. The cost estimate seems similar to the group by, but the actual cost is much higher. Yet performance was excellent compared to mysql and postgres despite the naive plans.
This is done to eliminate redundancy in the output andor compute aggregates that apply to these groups. Apr 20, 2020 postgresql is an object relational database management system ordbms whereas mysql is a community driven dbms system. The postgresql group by clause is used in collaboration with the select statement to group together those rows in a table that have identical data. Distinct, distinct on and all it is not uncommon to have duplicate data in the results of a query. Is there any dissadvantage of using group by to obtain a unique list. Execution time is always a very important factor considering performance as one of the major factors is teradata warehouse. Oct 01, 2014 the task because slightly more verbose and daunting when joining a table, because there are no shorthands for the is not distinct from form. But i hope that these examples will serve to illustrate that distinct does add an addtional load on the sql server.
Thing is, the queries used in the article are not simple. There is no difference in your 2 queries for oracle versions up to 10. If the percentage of null values in the column method is high more than 20 percent, depending. Your second example was the syntax i was trying to understand. Jul 19, 2017 not sure if this should be implemented, by allowing distinct to be applied to any column unrestricted clients could potentially ddos a database ive bumped into a slow distinct query in postgresql a while ago and solved it by using a group by instead of distinct, remember distinct generating a more expensive seq scan, i dont have the details anymore but a quick googling suggest the problem. After looking at someone elses query i noticed they were doing a group by to obtain the unique list.
Now im wondering if something similar might be lurking in postgresql. Jan 26, 2017 the biweekly newsletter keeps you up to speed on the most recent blog posts and forum discussions in the sql server community. Difference between distinct and group by charles nagy. Oct 25, 2010 the problem comes into picture when we use group by or distinct to find it. Then, the original authors submitted second blogpost comparing speed between four different db engines. Since in group by it has to group and then provide the result but this is not the case in distinct. The biweekly newsletter keeps you up to speed on the most recent blog posts and forum discussions in the sql server community. Itzik is a tsql trainer, a cofounder of solidq, and blogs about tsql. And distinct on is a postgres extension from way back thats a bit of a performance hack. Improve performance of countgroup by in large postgressql table. So any ideas whats going on here if they all are using the same naive plan on the first query. Distinct or group by which one is better performer oracle.
The significant time for group by was to talk to the storage engine sending data and for the distinct it was creating the temporary table copying to tmp table. I have a table with a large number of rows 10k in the example below, but 1m in some databases. Postgres has caught up in terms of performance of linux vs windows, however linux is still preferred because of the internal architecture surrounding key components like threading. I have always used distinct to filter duplication, reserving group by for aggregations counting, etc. Postgresql support modern applications feature like json, xml etc. Performance tuning queries in postgresql january 20, 2016. I happen to be one that enjoys it and want to share some of the techniques ive been. We provide you with a 3page postgresql cheat sheet in pdf format. If all you need is to remove duplicates then use distinct. Ive tried comparing the execution plans, but they seem to be the same for both queries. Distinct or group by which one is better performer.
Pgbench provides a convenient way to run a query repeatedly and collect statistics about performance. Jul 24, 2009 these are really trivial examples of how distinct can make a difference in a query plan and thus the performance of a query. I happen to be one that enjoys it and want to share some of the techniques ive been using lately to tune poor performing queries in postgresql. Demonstrated optimized solution to get the first record for each group by group in postgresql using distinct on and lateral subqueries. I would like to find the distinct values for one of the columns. The table is insertonly and was analyzed before running these queries. Sometimes, people get confused when to use distinct and when and why to use group by in sql queries. So, couple of days ago, some guy, from periscope company wrote a blogpost about getting number of distinct elements, per group, faster using subqueries this was then submitted to hacker news and rprogramming on reddit then, the original authors submitted second blogpost comparing speed between four different db engines. Group by should be used to apply aggregate operators to each group. Really this will help to people of postgresql community. Ive bumped into a slow distinct query in postgresql a while ago and solved it by using a group by. Select distinct vs group by in proc sql posted 01282015 2468 views i just spent a heck of a time debugging a sas program today, only to discover the root cause to be the difference between select distinct and group by inside a proc sql procedure.
With 500 000 records in hsqldb with all distinct business keys, the performance of distinct is now better 3 seconds, vs group by which took around 9 seconds. The distinct clause can be used on one or more columns of a table. Historically with older now unsupported versions of postgresql pre. In general distinct on in that fashion is most usable when combined with an order by so that you can get a particular row. Distinct on in postgresql noel herrick joining tables is a common practice when writing a sqlbased application, and i can writing a join in my sleep, but its always frustrating when you have a table and you want to join it to another, only once, and you realize that sql doesnt have a builtin way of expressing that.
The group by clause follows the where clause in a select statement and precedes the order by. Always add on an order by even if it is redundant, unless you really dont care. The following illustrates the syntax of the distinct clause. No write operations that would effect the visibility map since the last vacuum and all columns in the query have to be covered by the index. This was then submitted to hacker news and rprogramming on reddit. Mar 29, 2007 a distinct and group by usually generate the same query plan, so performance should be the same across both query constructs. In 40 minutes the audience will learn several techniques to optimise queries containing group by, distinct or distinct on keywords. Sql server difference between distinct and group by. The distinct clause keeps one row for each group of duplicates. If its true, then i could save considerable time by using group by where i have been using distinct in the past. Or does it have to do with the complexity of the query.
In this case, the distinct applies to each field listed after the distinct keyword, and therefore returns distinct combinations. Both return same number of rows, but with some execute time difference between them. Once again putting my architect hat on, i want linux and windows oses to be on equal footing not it runs ok on windows. I would like to know if there is any difference concerning performance when choosing distinct or group by to bring distinct rows from a query. From what ive read on the net, these should be very similar, and should generate equivalent plans, in such cases. Is there any difference on performance when choosing. Id be interested to know if you think there are any scenarios where distinct is better than group by, at least in terms of. Why is postgresql taking 384 seconds while sql server takes only 4. As far as i known, columns in group by could be reordered without loss of correctness. But if i understand correctly, you are saying that group by should be preferred even for the simpler use. 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.
Ill test the other queries for performance later and see if i can use them. The table has an index on clicked at time zone pst. Do not use the distinct phrase, unless the number of distinct values is high. 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. The postgresql cheat sheet provides you with the common postgresql commands and statements that enable you to work with postgresql quickly and effectively. Dec 21, 2007 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. Postgresql cheat sheet download the cheat sheet in pdf.
In performance wise distinct is good or group by is good. Select distinct x from mytable select x from mytable group by x however, in my case postgresql server8. 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. Huge performance difference when using group by vs distinct. Performance tuning queries in postgresql geeky tidbits. So which is more efficient distinct or group by since distinct redistributes the rows immediately, more data may move between the amps, where as group by that only sends unique values between the amps. Performance wise distinct is more effective than group by. Jan 22, 2016 the talk will cover postgresql grouping and aggregation facilities and best practices of using them in fast and efficient manner. Getting count of distinct elements, per group, in postgresql. Oracle introduced hash group by and hash distinct execution plans in 10. A distinct and group by usually generate the same query plan, so performance should be the same across both query.
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