Distinct vs group by performance postgresql download

Is there any dissadvantage of using group by to obtain a unique list. A distinct and group by usually generate the same query plan, so performance should be the same across both query. Once again putting my architect hat on, i want linux and windows oses to be on equal footing not it runs ok on windows. In performance wise distinct is good or group by is good.

I believe the only exception to this is in regards to parallel query, as currently only group bys may be parallelised, not distinct. Historically with older now unsupported versions of postgresql pre. Im building this query generatively based on user input, and that second example is easily doable. Actually, i think i answered my own question already. A distinct and group by usually generate the same query plan, so performance should be the same across both query constructs. Why is postgresql taking 384 seconds while sql server takes only 4. Apr 20, 2020 postgresql is an object relational database management system ordbms whereas mysql is a community driven dbms system. But if i understand correctly, you are saying that group by should be preferred even for the simpler use. Performance wise distinct is more effective than group by. Improve performance of countgroup by in large postgressql table. Postgresql cheat sheet download the cheat sheet in pdf. 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.

Then, the original authors submitted second blogpost comparing speed between four different db engines. Ability to generate queries with distinctuniquegroup by. The group by clause follows the where clause in a select statement and precedes the order by clause. The problem with the native count distinct 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. 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. Oracle introduced hash group by and hash distinct execution plans in 10. The cost estimate seems similar to the group by, but the actual cost is much higher. Jan 22, 2016 the talk will cover postgresql grouping and aggregation facilities and best practices of using them in fast and efficient manner. Jan 20, 2016 performance tuning queries in postgresql january 20, 2016. Ive tried comparing the execution plans, but they seem to be the same for both queries.

The biweekly newsletter keeps you up to speed on the most recent blog posts and forum discussions in the sql server community. I would like to find the distinct values for one of the columns. In 40 minutes the audience will learn several techniques to optimise queries containing group by, distinct or distinct on keywords. 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. But i want to confirm is the group by faster because it doesnt have to sort results, whereas distinct must produce sorted results. As far as i known, columns in group by could be reordered without loss of correctness. But i hope that these examples will serve to illustrate that distinct does add an addtional load on the sql server.

Distinct or group by which one is better performer oracle. 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. Id be interested to know if you think there are any scenarios where distinct is better than group by, at least in terms of. Huge performance difference when using group by vs distinct. Ill test the other queries for performance later and see if i can use them. The distinct clause keeps one row for each group of duplicates. The talk will cover postgresql grouping and aggregation facilities and best practices of using them in fast and efficient manner. Yet performance was excellent compared to mysql and postgres despite the naive plans. The following illustrates the syntax of the distinct clause. Getting count of distinct elements, per group, in postgresql.

Almost a year ago, i wrote a custom experimental aggregate replacing count distinct. 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. 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. Postgresql support modern applications feature like json, xml etc. 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.

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. This was then submitted to hacker news and rprogramming on reddit. Do not use the distinct phrase, unless the number of distinct values is high. Slow query on large table with group by and order by. The table has an index on clicked at time zone pst.

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. I have a query where i want to select the usertable records that have a matching entry in an event table. In this case, the distinct applies to each field listed after the distinct keyword, and therefore returns distinct combinations. 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. Both return same number of rows, but with some execute time difference between them. Always add on an order by even if it is redundant, unless you really dont care. Postgresql is an object relational database management system ordbms whereas mysql is a community driven dbms system. If its true, then i could save considerable time by using group by where i have been using distinct in the past. Sometimes, people get confused when to use distinct and when and why to use group by in sql queries. Select distinct x from mytable select x from mytable group by x however, in my case postgresql server8. Ive bumped into a slow distinct query in postgresql a while ago and solved it by using a group by. 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.

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. From what ive read on the net, these should be very similar, and should generate equivalent plans, in such cases. I happen to be one that enjoys it and want to share some of the techniques ive been. Is there any difference on performance when choosing. The group by clause follows the where clause in a select statement and precedes the order by. If all you need is to remove duplicates then use distinct. Count distinct performance compared on top 4 sql databases. So, couple of days ago, some guy, from periscope company wrote a blogpost about getting number of distinct elements, per group, faster using subqueries. Difference between distinct and group by charles nagy.

Browse other questions tagged postgresql performance index groupby count or ask your own question. 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. 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. Mar 29, 2007 a distinct and group by usually generate the same query plan, so performance should be the same across both query constructs. Now im wondering if something similar might be lurking in postgresql. And distinct on is a postgres extension from way back thats a bit of a performance hack. We provide you with a 3page postgresql cheat sheet in pdf format. Really this will help to people of postgresql community. 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. The distinct clause is used in the select statement to remove duplicate rows from a result set.

Or does it have to do with the complexity of the query. The postgresql cheat sheet provides you with the common postgresql commands and statements that enable you to work with postgresql quickly and effectively. If the percentage of null values in the column method is high more than 20 percent, depending. The table is insertonly and was analyzed before running these queries. 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.

Distinct or group by which one is better performer. Performance tuning queries in postgresql january 20, 2016. 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. Since in group by it has to group and then provide the result but this is not the case in distinct. I am trying to get a distinct set of rows from 2 tables.

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. Execution time is always a very important factor considering performance as one of the major factors is teradata warehouse. 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. 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. The distinct clause can be used on one or more columns of a table. Group by should be used to apply aggregate operators to each group. Itzik is a tsql trainer, a cofounder of solidq, and blogs about tsql. Distinct is used to filter unique records out of the records that satisfy the query criteria. 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. There is no difference in your 2 queries for oracle versions up to 10. 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 have a table with a large number of rows 10k in the example below, but 1m in some databases.

Oct 25, 2010 the problem comes into picture when we use group by or distinct to find it. Performance tuning queries in postgresql geeky tidbits. So any ideas whats going on here if they all are using the same naive plan on the first query. After looking at someone elses query i noticed they were doing a group by to obtain the unique list. This is done to eliminate redundancy in the output andor compute aggregates that apply to these groups. Is there any difference on performance when choosing distinct. Sql server difference between distinct and group by.

Your second example was the syntax i was trying to understand. The effects of distinct in a sql query webbtech solutions. Almost a year ago, i wrote a custom experimental aggregate replacing countdistinct. 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. Thing is, the queries used in the article are not simple. Use distinct for dedupping thats what it tells the reader. 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. 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. Distinct, distinct on and all it is not uncommon to have duplicate data in the results of a query. Pgbench provides a convenient way to run a query repeatedly and collect statistics about performance. Demonstrated optimized solution to get the first record for each group by group in postgresql using distinct on and lateral subqueries. 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. In general distinct on in that fashion is most usable when combined with an order by so that you can get a particular row.