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.
Should I use distinct or GROUP BY?
If you want to group your results, use GROUP BY, if you just want a unique list of a specific column, use DISTINCT.
Does distinct improve performance?
Yes, the application needs to compare every record to the “distinct” records cache as it goes. You can improve performance by using an index, particularly on the numeric and date fields.
Does distinct reduce performance?
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).
Should I 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.
What is the difference between unique and distinct?
Unique and Distinct are two SQL constraints. The main difference between Unique and Distinct in SQL is that Unique helps to ensure that all the values in a column are different while Distinct helps to remove all the duplicate records when retrieving the records from a table.
Does distinct work with GROUP BY?
Well, GROUP BY and DISTINCT have their own use. GROUP BY cannot replace DISTINCT in some situations and DISTINCT cannot take place of GROUP BY. It is as per your choice and situation how you are optimizing both of them and choosing where to use GROUP BY and DISTINCT.
Is distinct an expensive operation?
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.
Why distinct is bad in SQL?
This is why I get nervous about use of ” distinct ” – the spraddr table may include additional columns which you should use to filter out data, and ” distinct ” may be hiding that. Also, you may be generating a massive result set which needs to be filtered by the “distinct” clause, which can cause performance issues.
How are distinct and GROUP BY similar?
The group by gives the same result as of distinct when no aggregate function is present. The SQL Server query optimizer produces the same plan for both the queries as shown below. Thus, to conclude there is a functional difference as mentioned above even if the group by produces same result as of distinct.
Which is better distinct or GROUP BY in Oracle?
DISTINCT implies you want a distinct set of columns. However, GROUP BY implies you want to compute some sort of aggregate value which you are not.
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.
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.
Which is faster distinct or GROUP BY in Teradata?
As a side-note; on Teradata 13.10 (and older versions), using DISTINCT can cause a sort operation, which is slower than GROUP BY. So from a performance perspective, one should prefer GROUP BY over DISTINCT wherever possible.
Is SELECT distinct faster than SELECT?
Most of the SELECT DISTINCT queries will perform exactly as fast as their simple SELECT counterparts, because the optimizer will do away with the step necessary for eliminating duplicates.
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.
Does distinct remove duplicates?
DISTINCT adds a plan operator to remove duplicates. DISTINCT and GROUP BY without an aggregate function result in the same plan.
Why distinct is used in SQL?
The SELECT DISTINCT statement is used to return only distinct (different) values. Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values.
