When executed, is there any difference at between the following two sql queries:
SELECT name, count(*) FROM mytable GROUP BY name HAVING count(*) > 1
And:
SELECT * from (SELECT name, count(*) cnt FROM mytable GROUP BY name) x where cnt > 1
In other words, is having more a "convenience" clause to simplify having to do subselect, or does the query engine fundamentally performance different when a having statement is used vs the second approach? Currently in mysql:
Create table:
CREATE TABLE `mytable` (
`name` varchar(20) NOT NULL DEFAULT ''
) ENGINE=InnoDB DEFAULT CHARSET=utf-8;
In almost any other database, the two would be equivalent. For conciseness, HAVING is usually a better choice.
At least historically, MySQL materialized subqueries. So, this query:
SELECT *
FROM (SELECT name, count(*) as cnt
FROM mytable
GROUP BY name
) x
WHERE cnt > 1;
suggests that it is going to write out the derived table, and then re-scan it for the final WHERE. However, this makes little difference to performance because the GROUP BY is already reading and writing the data.
So, these queries are probably quite similar in performance on MySQL. And, they would have the same execution plan on almost any other database. The HAVING clause results in the simpler query.
I am trying to query a large table something like 70 million records. The hard thing is that the group by query based on three fields does not respond. Is there a clever way of doing that? Is use of VID in group by section better rather than verb field?
This is the query I run :
SELECT
VID, s_name, verb, o_name, count(*) as total
FROM
table_name
group by verb, s_name, o_name;
This is a bit long for a comment. Doing aggregations in MySQL on 70 million rows is an expensive operation.
Can you try the following to see if performance is better?
create index idx_table_name_3 on table_name(s_name, verb, o_name);
select distinct s_name, verb, o_name
from table_name;
If this finishes in a finite amount of time, it might be possible to do the aggregation more efficiently.
Then try this:
select s_name, verb, o_name,
(select count(*)
from table_name t2
where t2.s_name = t.s_name and
t2.verb = t.verb and
t2.o_name = t.o_name
) as cnt
from (select distinct s_name, verb, o_name
from table_name
) t;
The subquery should be materialized using the index. The aggregation should be calculated from the index as well. If so, then this will trick MySQL into not using file sort for the group by -- and the performance may improve.
I've got a query:
select count(*) from `table` where `something`>123
If the table has few million records, the query runs really slow even though there's an index on column something. However, in fact I'm interested in value of:
min(100000, count(*))
So is there any way to prevent MySQL from counting rows when it already found 100k? I've found something like:
select count(*) from (select 1 from `table` where `something`>123 limit 100000) as `asd`
It's much faster than count(*) if the table has a few million matching entries, but count(*) runs much faster when there are less than 100000 matches.
Is there any way to do it faster?
I don't have the points to comment, so I am posting this as an answer...
Have you tried using EXPLAIN to see if your index on something is actually being used? It sounds like this query is doing a Table Scan. Ideally, you will want to see something like "Extra: Using where; Using index".
Out of curiosity, is something a nullable field?
As an aside, perhaps the query optimizer would do better with the following:
select count(*) as cnt
from table
where something > 123
having count(*) > 100000
Might help to make better use of value range limitation.
select count(*) - (select count(*) from t where something <= 123) as cnt
from t
The other thing might be to have an update trigger counting.
I have a select query like this
select count(distinct id)*100/totalcount as freq, count (distinct id) from
<few joins, conditions, gorup by here> .....
Will this result in 2 calculations of count under MySql 5.0? I can calculate the frequency in my appliation as well if this is a problem. I am aware of the solutions presented in Adding percentages to multiple counts in one SQL SELECT Query but I just want to avoid nested queries
select count(distinct id)*100/totalcount as freq, count (distinct id) from
<few joins, conditions, gorup by here> .....
Yes, it will result in several evaluations.
Each recordset on DISTINCT id will be built separately for each function
Note that if not for DISTINCT, MySQL would use each record only once (though in multiple function calls).
Since COUNT is very cheap, function calls add almost nothing to overall query time.
You can benefit from rewriting your query as this:
SELECT COUNT(id) * 100 / totalcount AS freq,
COUNT(id)
FROM (
SELECT DISTINCT id
FROM original_query
) q
BTW, why do you need both GROUP BY and DISTINCT in one query? Could you please post your original query as it is?
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
?
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. This is because, although the two are the same, a query 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!
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.
All of the answers above are correct, for the case of DISTINCT on a single column vs GROUP BY on a single column.
Every db engine has its own implementation and optimizations, and if you care about the very little difference (in most cases) then you have to test against specific server AND specific version! As implementations may change...
BUT, if you select more than one column in the query, then the DISTINCT is essentially different! Because in this case it will compare ALL columns of all rows, instead of just one column.
So if you have something like:
// This will NOT return unique by [id], but unique by (id,name)
SELECT DISTINCT id, name FROM some_query_with_joins
// This will select unique by [id].
SELECT id, name FROM some_query_with_joins GROUP BY id
It is a common mistake to think that DISTINCT keyword distinguishes rows by the first column you specified, but the DISTINCT is a general keyword in this manner.
So people you have to be careful not to take the answers above as correct for all cases... You might get confused and get the wrong results while all you wanted was to optimize!
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!
well distinct can be slower than group by on some occasions in postgres (dont know about other dbs).
tested example:
postgres=# select count(*) from (select distinct i from g) a;
count
10001
(1 row)
Time: 1563,109 ms
postgres=# select count(*) from (select i from g group by i) a;
count
10001
(1 row)
Time: 594,481 ms
http://www.pgsql.cz/index.php/PostgreSQL_SQL_Tricks_I
so be careful ... :)
Group by is expensive than Distinct since Group by does a sort on the result while distinct avoids it. But if you want to make group by yield the same result as distinct give order by null ..
SELECT DISTINCT u.profession FROM users u
is equal to
SELECT u.profession FROM users u GROUP BY u.profession order by null
It seems that the queries are not exactly the same. At least for MySQL.
Compare:
describe select distinct productname from northwind.products
describe select productname from northwind.products group by productname
The second query gives additionally "Using filesort" in Extra.
In MySQL, "Group By" uses an extra step: filesort. I realize DISTINCT is faster than GROUP BY, and that was a surprise.
After heavy testing we came to the conclusion that GROUP BY is faster
SELECT sql_no_cache
opnamegroep_intern
FROM telwerken
WHERE opnemergroep IN (7,8,9,10,11,12,13) group by opnamegroep_intern
635 totaal 0.0944 seconds
Weergave van records 0 - 29 ( 635 totaal, query duurde 0.0484 sec)
SELECT sql_no_cache
distinct (opnamegroep_intern)
FROM telwerken
WHERE opnemergroep IN (7,8,9,10,11,12,13)
635 totaal 0.2117 seconds ( almost 100% slower )
Weergave van records 0 - 29 ( 635 totaal, query duurde 0.3468 sec)
(more of a functional note)
There are cases when you have to use GROUP BY, for example if you wanted to get the number of employees per employer:
SELECT u.employer, COUNT(u.id) AS "total employees" FROM users u GROUP BY u.employer
In such a scenario DISTINCT u.employer doesn't work right. Perhaps there is a way, but I just do not know it. (If someone knows how to make such a query with DISTINCT please add a note!)
Here is a simple approach which will print the 2 different elapsed time for each query.
DECLARE #t1 DATETIME;
DECLARE #t2 DATETIME;
SET #t1 = GETDATE();
SELECT DISTINCT u.profession FROM users u; --Query with DISTINCT
SET #t2 = GETDATE();
PRINT 'Elapsed time (ms): ' + CAST(DATEDIFF(millisecond, #t1, #t2) AS varchar);
SET #t1 = GETDATE();
SELECT u.profession FROM users u GROUP BY u.profession; --Query with GROUP BY
SET #t2 = GETDATE();
PRINT 'Elapsed time (ms): ' + CAST(DATEDIFF(millisecond, #t1, #t2) AS varchar);
OR try SET STATISTICS TIME (Transact-SQL)
SET STATISTICS TIME ON;
SELECT DISTINCT u.profession FROM users u; --Query with DISTINCT
SELECT u.profession FROM users u GROUP BY u.profession; --Query with GROUP BY
SET STATISTICS TIME OFF;
It simply displays the number of milliseconds required to parse, compile, and execute each statement as below:
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 2 ms.
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.
This is not a rule
For each query .... try separately distinct and then group by ... compare the time to complete each query and use the faster ....
In my project sometime I use group by and others distinct
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.
If the problem allows it, try with EXISTS, since it's optimized to end as soon as a result is found (And don't buffer any response), so, if you are just trying to normalize data for a WHERE clause like this
SELECT FROM SOMETHING S WHERE S.ID IN ( SELECT DISTINCT DCR.SOMETHING_ID FROM DIFF_CARDINALITY_RELATIONSHIP DCR ) -- to keep same cardinality
A faster response would be:
SELECT FROM SOMETHING S WHERE EXISTS ( SELECT 1 FROM DIFF_CARDINALITY_RELATIONSHIP DCR WHERE DCR.SOMETHING_ID = S.ID )
This isn't always possible but when available you will see a faster response.
in mySQL i have found that GROUP BY will treat NULL as distinct, while DISTINCT does not.
Took the exact same DISTINCT query, removed the DISTINCT, and added the selected fields as the GROUP BY, and i got many more rows due to one of the fields being NULL.
So.. I tend to believe that there is more to the DISTINCT in mySQL.