Compare versions in PostgreSQL

November 25, 2021 — Ulysse

Yesterday, one of my colleagues asked me for a version comparison trick in SQL. I gave him a very basic one, and looked a bit the internet for a simple pure SQL solution to do easily a bit more than that. Nothing convinced me really, hence this blog post and implementation.

When using semantic versioning (MAJOR.MINOR.PATCH), there are some nice tools to compare a given version with a requirement, for instance, ruby’s Gemfile syntax goes like:

# Minor and patch version can only be greater or
# equal to MAJOR.3.0.
# Major must be 2.
gem "fast-polylines", "~> 2.3"
# Only the patch version can be greater
# than the expected one.
gem "hiredis", "~> 0.6.3"
# Only patch version may vary.
gem "oj", "= 3.13"
# Any version greater or equal to 3.
gem "rgeo", ">= 3"

This can also be used in code directly:

Gem::Version("10.0.1") > Gem::Version("10.0.0") # true

See the ruby documentation for a more in-depth explanation of those operators.

But what if you had a table with a version (text) column in your SQL? Well, fortunately in PostgreSQL, there is a nice string_to_array function which lets us split our version easily:

SELECT string_to_array('4.2.0', '.'); -- {4,2,0}

And we can easily convert our resulting array to integers to simplify later computation:

SELECT ARRAY['4', '2']::int[];

Since in PostgreSQL array comparison is done by comparing each pair consecutively, we are already very close to a complete solution:

SELECT ARRAY[2, 1] > ARRAY[2, 0]; -- ture
SELECT ARRAY[2, 1] > ARRAY[2]; -- true

combining what we have already, it is quite easy to do basic comparison between two versions now:

SELECT string_to_array('6.6.6', '.')::int[] > string_to_array('6.5', '.')::int[]

However, what if we wanted to compare using ruby like ~> comparison? We’ll have to dig a tiny bit deeper.

So ~> X.Y.Z basically means that the expected version should be:

  • >= X.Y.Z
  • < X.Y.(Z+1)

And this apply to a comparison without patch as well, ~> X.Y implies:

  • >= X.Y
  • < X.(Y+1)

Hence we could just write:

SELECT string_to_array('6.6.6', '.')::int[] >= string_to_array('6.6.6', '.')::int[]
AND string_to_array('6.6.6', '.')::int[] < string_to_array('6.6.7', '.')::int[]

However, I’d like to be able to express this comparison in a single line:

SELECT semver_match(ver, req)
FROM (VALUES ('6.6.6', '~> 6.6.6')) AS _ (ver, req);

So how can we can we compar our ver with both >= 6.6.6 and < 6.6.7? Feel free to pause there and find your own solution!

The first comparison is trivial, we just have to parse our 6.6.6 text. The second one is a bit harder since we have to somehow increment our array. There is no postgres operator that lets us mutate an array in place, except when setting a value. Hence I went for a more functional way: consider the n - 1 first elements of my array, and using array_append to concatenate the last value, incremented:

SELECT array_append(
	(ARRAY[4,2,1])[1:(array_length(ARRAY[4,2,1], 1) - 1)], -- X.Y
	(ARRAY[4,2,1])[array_length(ARRAY[4,2,1], 1)] + 1 -- Z + 1

Once we have that, we can use a CASE statement to see which operator was used, and the rest is just a bit of syntax:

-- example usage: semver_match('4.2.1', '~> 4.2.0').
CREATE OR REPLACE FUNCTION semver_match(version text, req text) RETURNS boolean
    AS $$
    WHEN req LIKE '~>%' THEN
		string_to_array(version, '.')::int[] >= string_to_array(substring(req from 4), '.')::int[]
		string_to_array(version, '.')::int[] <
		-- increment last item by one. (X.Y.Z => X.Y.(Z+1))
			(string_to_array(substring(req from 4), '.')::int[])[1:(array_length(string_to_array(req, '.'), 1) - 1)], -- X.Y
			(string_to_array(substring(req from 4), '.')::int[])[array_length(string_to_array(req, '.'), 1)] + 1 -- Z + 1
    WHEN req LIKE '>%' THEN string_to_array(version, '.')::int[] > string_to_array(substring(req from 3), '.')::int[]
    WHEN req LIKE '<%' THEN string_to_array(version, '.')::int[] < string_to_array(substring(req from 3), '.')::int[]
    WHEN req LIKE '>=%' THEN string_to_array(version, '.')::int[] >= string_to_array(substring(req from 4), '.')::int[]
    WHEN req LIKE '<=%' THEN string_to_array(version, '.')::int[] <= string_to_array(substring(req from 4), '.')::int[]
    WHEN req LIKE '=%' THEN
		(string_to_array(version, '.')::int[])[1:array_length(string_to_array(substring(req from 3), '.'), 1)] =
		string_to_array(substring(req from 3), '.')::int[]
    END $$;

Now we only have left to test our function:

-- tests.
	CASE WHEN semver_match(ver, req) = expected
	THEN '✅' ELSE '❌' END AS test_passed
	('2.3.1', '> 2.3', TRUE),
	('2.3.1', '< 2.3.2', TRUE),
	('2.3.1', '~> 2.3.2', FALSE),
	('2.4.3', '~> 2.3.2', FALSE),
	('2.3.2', '~> 2.3.2', TRUE),
	('2.3.2', '= 2.3.2', TRUE),
	('2.3.2', '= 2.3', TRUE),
	('2.3.2', '= 2.4', FALSE)
) AS _ (ver, req, expected);

The whole code is also available on StackOverflow, feel free to improve or ping me in the comments for suggestions!