4 Answers. Yes, as using DISTINCT will (sometimes according to a comment) cause results to be ordered. Sorting hundreds of records takes time. Try GROUP BY all your columns, it can sometimes lead the query optimiser to choose a more efficient algorithm (at least with Oracle I noticed significant performance gain).
Contents
- 1 Does distinct improve performance?
- 2 Is it bad to use distinct?
- 3 Does distinct make query slower?
- 4 Is it better to use distinct or GROUP BY?
- 5 Is distinct fast?
- 6 Why is distinct slow?
- 7 Is distinct bad SQL?
- 8 Is select distinct bad practice?
- 9 Should you use distinct?
- 10 Is distinct expensive?
- 11 What can I use instead of distinct in SQL?
- 12 What is faster distinct or GROUP BY Postgres?
- 13 Is distinct slower than GROUP BY?
- 14 Does GROUP BY affect performance?
- 15 What is difference between unique and distinct?
Does distinct improve performance?
Yes, basically it has to sort the results and then re-processed to eliminate the duplicates. This cull could also be being done during the sort, but we can only speculate as to how exactly the code works in the background. You could try and improve the performance by creating an index composed of all three (3) fields.
Is it bad to use distinct?
Well, improper use of “distinct” not only hides the real problem (duplicate entries in the tables, lack of condition in the on clause) as discussed above but also degrades query performance. This will bring the IO Cost (logical reads) of the query goes up significantly.
Does distinct make query slower?
Very few queries may perform faster in SELECT DISTINCT mode, and very few will perform slower (but not significantly slower) in SELECT DISTINCT mode but for the later case it is likely that the application may need to examine the duplicate cases, which shifts the performance and complexity burden to the application.
Is it better to use distinct or GROUP BY?
In MySQL, DISTINCT seems a bit faster than GROUP BY if theField is not indexed. DISTINCT only eliminate duplicate rows but GROUP BY seems to sort them in addition.
Is distinct fast?
DISTINCT is much faster with large amount data. I tested this, and found that on an indexed column, mysql, group by was about 6x slower than distinct with a fairly complicated query.
Why is distinct slow?
Why DISTINCT queries are slow on PostgreSQL Why are DISTINCT queries slow on PostgreSQL when they seem to ask an “easy” question? It turns out that PostgreSQL currently lacks the ability to efficiently pull a list of unique values from an ordered index.
Is distinct bad SQL?
If you are improving your SQL skills, then learn to use explicit JOIN clauses. Never use commas in the from clause. Using DISTINCT in your query when you intend to get distinct information is a good indication of a programmer using common sense.
Is select distinct bad practice?
As a general rule, SELECT DISTINCT incurs a fair amount of overhead for the query. Hence, you should avoid it or use it sparingly. The idea of generating duplicate rows using JOIN just to remove them with SELECT DISTINCT is rather reminiscent of Sisyphus pushing a rock up a hill, only to have it roll back down again.
Should you use distinct?
The distinct keyword is used in conjunction with select keyword. It is helpful when there is a need of avoiding duplicate values present in any specific columns/table. When we use distinct keyword only the unique values are fetched.
Is distinct expensive?
In a table with million records, SQL Count Distinct might cause performance issues because a distinct count operator is a costly operator in the actual execution plan.
What can I use instead of distinct in SQL?
6 Answers. GROUP BY is intended for aggregate function use; DISTINCT just removes duplicates (based on all column values matching on a per row basis) from visibility. If TABLE2 allows duplicate values associated to TABLE1 records, you have to use either option.
What is faster distinct or GROUP BY Postgres?
From experiments, I founded that the GROUP BY is 10 + times faster than DISTINCT. They are different. So what I learned is: GROUP-BY is anyway not worse than DISTINCT, and it is better sometimes.
Is distinct slower than GROUP BY?
DISTINCT is used to filter unique records out of all records in the table. It removes the duplicate rows. SELECT DISTINCT will always be the same, or faster than a GROUP BY.
Does GROUP BY affect performance?
Despite what MSDN documentation says, no, it doesn’t matter for GROUP BY queries. it does make a difference and it does produce different execution plans (and different result sets of course).
What is difference between unique and distinct?
The main difference between unique and distinct is that UNIQUE is a constraint that is used on the input of data and ensures data integrity. While DISTINCT keyword is used when we want to query our results or in other words, output the data.
