What's faster, SELECT DISTINCT or GROUP BY in MySQL?

Question

If I have a table

CREATE TABLE users (
  id int(10) unsigned NOT NULL auto_increment,
  name varchar(255) NOT NULL,
  profession varchar(255) NOT NULL,
  employer varchar(255) NOT NULL,
  PRIMARY KEY  (id)
)

and I want to get all unique values of profession field, what would be faster (or recommended):

SELECT DISTINCT u.profession FROM users u

or

SELECT u.profession FROM users u GROUP BY u.profession

?

Answer 1

I think MySQL is smart enough to do the same for both of them.

Answer 2

They are essentially equivalent to each other (in fact this is how some databases implement DISTINCT under the hood).

If one of them is faster, it's going to be DISTINCT (especially in case you have an index on profession). This is because, although the two are the same, an optimizer would have to catch the fact that your group-by is not taking advantage of any group members, just their keys. DISTINCT makes this explicit, so you can get away with a slightly dumber optimizer.

When in doubt, test.

Answer 3

If you don't have to do any group functions (sum, average etc in case you want to add numeric data to the table), use SELECT DISTINCT. I suspect it's faster, but i have nothing to show for it.

In any case, if you're worried about speed, create an index on the column.

Answer 4

A DISTINCT and GROUP BY usually generate the same query plan, so performance should be the same across both query constructs.

GROUP BY should be used to apply aggregate operators to each group. If all you need is to remove duplicates then use DISTINCT.

If you are using sub-queries execution plan for that query varies so in that case you need to check the execution plan before making decision of which is faster.

Answer 5

It seems that the queries are not exactly the same. At least for MySQL.

Compare:

1. describe select distinct productname from northwind.products
2. describe select productname from northwind.products group by productname

The second query gives additionally "Using filesort" in Extra.

Answer 6

I Am a bit of a "Stack Overflow" Newbie so I don't yet have the reputation or points to vote up or comment, so I will just say this - Real world examples... go for the simplest and shortest if you can -- DISTINCT seems to be more what you are looking for only because it will give you EXACTLY the answer you need and only that!

];o)

Answer 7

If you have an index on profession, these two are synonyms.

If you don't, then use DISTINCT.

GROUP BY in MySQL sorts results. You can even do:

SELECT u.profession FROM users u GROUP BY u.profession DESC

and get your professions sorted in DESC order.

DISTINCT creates a temporary table and uses it for storing duplicates. GROUP BY does the same, but sortes the distinct results afterwards.

So

SELECT DISTINCT u.profession FROM users u

is faster, if you don't have an index on profession.

Answer 8

SELECT DISTINCT will always be the same, or faster, than a GROUP BY. On some systems (i.e. Oracle), it might be optimized to be the same as DISTINCT for most queries. On others (such as SQL Server), it can be considerably faster.