Is distinct faster than GROUP BY Postgres?


Is distinct faster than 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 better 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.

What is the difference between GROUP BY and distinct?

GROUP BY lets you use aggregate functions, like AVG , MAX , MIN , SUM , and COUNT . On the other hand DISTINCT just removes duplicates. This will give you one row per department, containing the department name and the sum of all of the amount values in all rows for that department.

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.

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.

What is the difference between distinct and unique?

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.

Why is Count distinct so slow?

It’s slow because the database is iterating over all the logs and all the dashboards, then joining them, then sorting them, all before getting down to real work of grouping and aggregating.

Is distinct costly?

Why you shouldn’t use SELECT distinct?

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.

Is distinct bad?

If you’re querying a table that is expected to have repeated values of some field or combination of fields, and you’re reporting a list of the values or combinations of values (and not performing any aggregations on them), then DISTINCT is the most sensible thing to use.