NYCPHP Meetup

[Jeff Siegel]

Thanks for pointing this out. Definitely food for thought.

Jeff
====

Ophir Prusak wrote:

> I hear what you're saying, but I'm still not convinced.
> 
> Any select query with an ORDER BY clause requires the database to do 
> some sorting before returning results.
> 
> I did a quick Google search and found this thread on the subject:
> http://lists.mysql.com/mysql/161338
> 
> The short answer is that if you're doing this for a table with a very 
> large number of rows, there are more efficient ways than ORDER BY RAND().
> Read the above thread for details.
> 
> Ophir
> 
> 
> David Sklar wrote:
> 
>> Ophir Prusak wrote:
>>
>>>
>>> I've never seen this solution before. Very simple.
>>> I wondering though about the efficiency of this solution.
>>>
>>> It seems to me that this assigns a random value to each row and then 
>>> orders by that row.
>>> Does the DB do a full table scan using this method?
>>>
>>> If yes, this would be a very inefficient method for very large tables.
>>
>>
>>
>>
>> It is inefficient if you are retrieving the whole table ordered by 
>> RAND(), but with the LIMIT clause, the table scan is short-circuited 
>> after one row is found. So,
>>
>> SELECT * FROM table ORDER BY RAND()
>>
>> can be slow if there are a lot of rows in the table, but
>>
>> SELECT * FROM table ORDER BY RAND() LIMIT 1
>>
>> is OK.
>>
>> David
>>
>>
>>
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> 
> 
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