Pseudo Random Repeatable Sort in SQL Server (not N

问题:

I would like to randomly sort a result in a repeatable fashion for purposes such as paging. For this NEWID() is too random in that the same results cannot be re-obtained. Order by Rand(seed) would be ideal as with the same seed the same random collection would result. Unfortunately, the Rand() state resets with every row, does anyone have a solution?

declare @seed as int;
set @seed = 1000;

create table temp (
id int,
date datetime)

insert into temp (id, date) values (1,'20090119')
insert into temp (id, date) values (2,'20090118')
insert into temp (id, date) values (3,'20090117')
insert into temp (id, date) values (4,'20090116')
insert into temp (id, date) values (5,'20090115')
insert into temp (id, date) values (6,'20090114')

-- re-seeds for every item
select *, RAND(), RAND(id+@seed) as r from temp order by r
--1 2009-01-19 00:00:00.000 0.277720118060575   0.732224964471124
--2 2009-01-18 00:00:00.000 0.277720118060575   0.732243597442382
--3 2009-01-17 00:00:00.000 0.277720118060575   0.73226223041364
--4 2009-01-16 00:00:00.000 0.277720118060575   0.732280863384898
--5 2009-01-15 00:00:00.000 0.277720118060575   0.732299496356156
--6 2009-01-14 00:00:00.000 0.277720118060575   0.732318129327415
-- Note how the last column is +=~0.00002

drop table temp

-- interestingly this works:
select RAND(@seed), RAND()
--0.732206331499865 0.306382810665955

Note, I tried Rand(ID) but that just turns out to be sorted. Apparently Rand(n) < Rand(n+1)

回答1:

Building off of gkrogers hash suggestion this works great. Any thoughts on performance?

declare @seed as int;
set @seed = 10;

create table temp (
id int,
date datetime)

insert into temp (id, date) values (1,'20090119')
insert into temp (id, date) values (2,'20090118')
insert into temp (id, date) values (3,'20090117')
insert into temp (id, date) values (4,'20090116')
insert into temp (id, date) values (5,'20090115')
insert into temp (id, date) values (6,'20090114')

-- re-seeds for every item
select *, HASHBYTES('md5',cast(id+@seed as varchar)) r
from temp order by r
--1 2009-01-19 00:00:00.000 0x6512BD43D9CAA6E02C990B0A82652DCA
--5 2009-01-15 00:00:00.000 0x9BF31C7FF062936A96D3C8BD1F8F2FF3
--4 2009-01-16 00:00:00.000 0xAAB3238922BCC25A6F606EB525FFDC56
--2 2009-01-18 00:00:00.000 0xC20AD4D76FE97759AA27A0C99BFF6710
--3 2009-01-17 00:00:00.000 0xC51CE410C124A10E0DB5E4B97FC2AF39
--6 2009-01-14 00:00:00.000 0xC74D97B01EAE257E44AA9D5BADE97BAF

drop table temp

EDIT: Note, the declaration of @seed as it's use in the query could be replace with a parameter or with a constant int if dynamic SQL is used. (declaration of @int in a TSQL fashion is not necessary)

回答2:

You can use a value from each row to re-evaluate the rand function:

Select *, Rand(@seed + id) as r from temp order by r

adding the ID ensures that the rand is reseeded for each row. But for a value of seed you will always get back the same sequence of rows (provided that the table does not change)

回答3:

Creating a hash can be much more time consuming than creating a seeded random number.

To get more variation in the ourput of RAND([seed]) you need to make the [seed] vary significantly too. Possibly such as...

SELECT
    *,
    RAND(id * 9999)    AS [r]
FROM
   temp
ORDER BY
   r

Using a constant ensures the replicability you asked for. But be careful of the result of (id * 9999) causing an overflow if you expect your table to get big enough...

回答4:

SELECT *, checksum(id) AS r FROM table ORDER BY r

This kind of works. Although the output from checksum() does not look all that random to me. The MSDN Documentation states:

[...], we do not recommend using CHECKSUM to detect whether values have changed, unless your application can tolerate occasionally missing a change. Consider using HashBytes instead. When an MD5 hash algorithm is specified, the probability of HashBytes returning the same result for two different inputs is much lower than that of CHECKSUM.

But may be it faster.

回答5:

After doing some reading this is an accepted method.

Select Rand(@seed) -- now rand is seeded

Select *, 0 * id + Rand() as r from temp order by r

Having id in the expression causes it to be reevaluated every row. But multiplying it by 0 ensures that it doesnt not affect the outcome of rand.

What a horrible way of doing things!

回答6:

This has worked well for me in the past, and it can be applied to any table (just bolt on the ORDER BY clause):

SELECT *
FROM MY_TABLE
ORDER BY  
  (SELECT ABS(CAST(NEWID() AS BINARY(6)) % 1000) + 1);

回答7:

create table temp (
id int,
date datetime)

insert into temp (id, date) values (1,'20090119')
insert into temp (id, date) values (2,'20090118')
insert into temp (id, date) values (3,'20090117')
insert into temp (id, date) values (4,'20090116')
insert into temp (id, date) values (5,'20090115')
insert into temp (id, date) values (6,'20090114')

-- re-seeds for every item
select *, NEWID() r
from temp order by r

drop table temp