Question

I want to get some random records from db. There is two solution for this :

1- Using TABLESAMPLE for getting data from db directly.

2- Write a method In my application for doing this. In this method we generate multiple random number and get data like this :

select * from db where ID = @RandomNumber

if this ID does not exist, I pass a new number.

Now which one has better performance?

Answer 1

According to the documentation for TABESAMPLE you shouldn’t use it if you “really want a sample of individual rows”:

If you really want a random sample of
individual rows, modify your query to
filter out rows randomly, instead of
using TABLESAMPLE. For example, the
following query uses the NEWID
function to return approximately one
percent of the rows of the
Sales.SalesOrderDetail table:

 SELECT  * 
 FROM    Sales.SalesOrderDetail
 WHERE   0.01 >= CAST(CHECKSUM(NEWID(), SalesOrderID) & 0x7fffffff AS float)
         / CAST (0x7fffffff AS int)

The SalesOrderID column is included in
the CHECKSUM expression so that
NEWID() evaluates once per row to
achieve sampling on a per-row basis.
The expression CAST(CHECKSUM(NEWID(),> SalesOrderID) & 0x7fffffff AS float / CAST(0x7fffffff AS int) evaluates to a random float value between 0 and 1.

Either way, given the potentially endless number of requests you could make by passing in @RandomNumber (in theory the first 1000 requests you make might return nothing), the better approach is to limit the resultset on the server.