Random Sampling in Google BigQuery

Question

I just discovered that the RAND() function, while undocumented, works in BigQuery. I was able to generate a (seemingly) random sample of 10 words from the Shakespeare dataset using:

SELECT word FROM
(SELECT rand() as random,word FROM [publicdata:samples.shakespeare] ORDER BY random)
LIMIT 10

My question is: Are there any disadvantages to using this approach instead of the HASH() method defined in the "Advanced examples" section of the reference manual? https://developers.google.com/bigquery/query-reference

Answer 1

Great to know RAND() is available!

In my case I needed a predefined sample size. Instead of needing to know the total number of rows and do the division sample size over total rows, I'm using the following query:

SELECT word, rand(5) as rand
FROM [publicdata:samples.shakespeare]
order by rand
#Sample size needed = 10
limit 10

Summarizing, I use ORDER BY + LIMIT to ramdomize and then extract a defined number of samples.

Answer 2

One additional tip to make it even simpler: You can order by the function it self, ie:

select x from y order by rand() limit 100

=> Sample of 100

Answer 3

For stratified sampling, check https://stackoverflow.com/a/52901452/132438

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Good job finding it :). I requested the function recently, but it hasn't made it to documentation yet.

I would say the advantage of RAND() is that the results will vary, while HASH() will keep giving you the same results for the same values (not guaranteed over time, but you get the idea).

In case you want the variability that RAND() brings while still getting consistent results - you can seed it with an integer, as in RAND(3).

Notice though that the example you pasted is doing a full sort of the random values - for sufficiently big inputs this approach won't scale.

A scalable approach, to get around 10 random rows:

SELECT word
FROM [publicdata:samples.shakespeare]
WHERE RAND() < 10/164656

(where 10 is the approximate number of results I want to get, and 164656 the number of rows that table has)

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standardSQL update:

#standardSQL
SELECT word
FROM `publicdata.samples.shakespeare`
WHERE RAND() < 10/164656

or even:

#standardSQL
SELECT word
FROM `publicdata.samples.shakespeare`
WHERE RAND() < 10/(SELECT COUNT(*) FROM `publicdata.samples.shakespeare`)