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Which one is faster distinct or GROUP BY?
DISTINCT creates a temporary table and uses it for storing duplicates. GROUP BY does the same, but sortes the distinct results afterwards. is faster, if you don’t have an index on profession .
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). It still uses the index though, at least in that SQL-Fiddle test.
Is GROUP BY faster than distinct Postgres?
GROUP-BY is anyway not worse than DISTINCT, and it is better sometimes.
What’s the difference between select distinct and group by?
Here is the Oracle query plan for the query using GROUP BY: The performance difference is probably due to the execution of the subquery in the SELECT clause. I am guessing that it is re-executing this query for every row before the distinct. For the group by, it would execute once after the group by.
Which is better distinct or group by in HSQLDB?
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. MySQL: 500 000 rows – 5 000 distinct business keys: Both queries: 0.5 second MySQL: 500 000 rows – all distinct business keys: SELECT DISTINCT – 11 seconds SELECT
Which is better group by or distinct execution plan?
The second method optimizes for speed, but potentially requires a large amount of memory if there are a lot of different keys. Since you either have enough memory or few different keys, the second method outperforms the first. It’s not unusual to see performance differences of 10x or even 100x between two execution plans.
How does distinct variation compare to group by variation?
The DISTINCT variation took 4X as long, used 4X the CPU, and almost 6X the reads when compared to the GROUP BY variation. (Remember, these queries return the exact same results.) We can also compare the execution plans when we change the costs from CPU + I/O combined to I/O only, a feature exclusive to Plan Explorer.