Simple Random Samples from a Sql database

Question

How do I take an efficient simple random sample in SQL? The database in question is running MySQL; my table is at least 200,000 rows, and I want a simple random sample of about 10,000.

The "obvious" answer is to:

SELECT * FROM table ORDER BY RAND() LIMIT 10000 

For large tables, that's too slow: it calls RAND() for every row (which already puts it at O(n)), and sorts them, making it O(n lg n) at best. Is there a way to do this faster than O(n)?

Note: As Andrew Mao points out in the comments, If you're using this approach on SQL Server, you should use the T-SQL function NEWID(), because RAND() may return the same value for all rows.

EDIT: 5 YEARS LATER

I ran into this problem again with a bigger table, and ended up using a version of @ignorant's solution, with two tweaks:

• Sample the rows to 2-5x my desired sample size, to cheaply ORDER BY RAND()
• Save the result of RAND() to an indexed column on every insert/update. (If your data set isn't very update-heavy, you may need to find another way to keep this column fresh.)

To take a 1000-item sample of a table, I count the rows and sample the result down to, on average, 10,000 rows with the the frozen_rand column:

SELECT COUNT(*) FROM table; -- Use this to determine rand_low and rand_high    SELECT *     FROM table    WHERE frozen_rand BETWEEN %(rand_low)s AND %(rand_high)s ORDER BY RAND() LIMIT 1000 

(My actual implementation involves more work to make sure I don't undersample, and to manually wrap rand_high around, but the basic idea is "randomly cut your N down to a few thousand.")

While this makes some sacrifices, it allows me to sample the database down using an index scan, until it's small enough to ORDER BY RAND() again.

Answer 1

I think the fastest solution is

select * from table where rand() <= .3 

Here is why I think this should do the job.

• It will create a random number for each row. The number is between 0 and 1
• It evaluates whether to display that row if the number generated is between 0 and .3 (30%).

This assumes that rand() is generating numbers in a uniform distribution. It is the quickest way to do this.

I saw that someone had recommended that solution and they got shot down without proof.. here is what I would say to that -

• This is O(n) but no sorting is required so it is faster than the O(n lg n)

• mysql is very capable of generating random numbers for each row. Try this -

  select rand() from INFORMATION_SCHEMA.TABLES limit 10;

Since the database in question is mySQL, this is the right solution.

Answer 2

There's a very interesting discussion of this type of issue here: http://www.titov.net/2005/09/21/do-not-use-order-by-rand-or-how-to-get-random-rows-from-table/

I think with absolutely no assumptions about the table that your O(n lg n) solution is the best. Though actually with a good optimizer or a slightly different technique the query you list may be a bit better, O(m*n) where m is the number of random rows desired, as it wouldn't necesssarily have to sort the whole large array, it could just search for the smallest m times. But for the sort of numbers you posted, m is bigger than lg n anyway.

Three asumptions we might try out:

1. there is a unique, indexed, primary key in the table

2. the number of random rows you want to select (m) is much smaller than the number of rows in the table (n)

3. the unique primary key is an integer that ranges from 1 to n with no gaps

With only assumptions 1 and 2 I think this can be done in O(n), though you'll need to write a whole index to the table to match assumption 3, so it's not necesarily a fast O(n). If we can ADDITIONALLY assume something else nice about the table, we can do the task in O(m log m). Assumption 3 would be an easy nice additional property to work with. With a nice random number generator that guaranteed no duplicates when generating m numbers in a row, an O(m) solution would be possible.

Given the three assumptions, the basic idea is to generate m unique random numbers between 1 and n, and then select the rows with those keys from the table. I don't have mysql or anything in front of me right now, so in slightly pseudocode this would look something like:

 create table RandomKeys (RandomKey int) create table RandomKeysAttempt (RandomKey int)  -- generate m random keys between 1 and n for i = 1 to m   insert RandomKeysAttempt select rand()*n + 1  -- eliminate duplicates insert RandomKeys select distinct RandomKey from RandomKeysAttempt  -- as long as we don't have enough, keep generating new keys, -- with luck (and m much less than n), this won't be necessary while count(RandomKeys) &lt m   NextAttempt = rand()*n + 1   if not exists (select * from RandomKeys where RandomKey = NextAttempt)     insert RandomKeys select NextAttempt  -- get our random rows select * from RandomKeys r join table t ON r.RandomKey = t.UniqueKey 

If you were really concerned about efficiency, you might consider doing the random key generation in some sort of procedural language and inserting the results in the database, as almost anything other than SQL would probably be better at the sort of looping and random number generation required.