Friday, June 19, 2015

Rotfang, the random data generator

The other day I gave a lightning talk at pgCon about a Foreign Data Wrapper called Rotfang, which is for generating arbitrary amounts of random data. This is intended for generating test cases. The software supports a small number of data types natively, but also allows you to use user-supplied functions to generate data. It's available on bitbucket. A short example:

CREATE FUNCTION random_tstz (typmod int)
RETURNS timestamptz
    SELECT now() - (random() * 20.0) * interval '1 year'

CREATE FOREIGN TABLE rand2 (b boolean, ts timestamptz)
SERVER rotfang
OPTIONS (maxrows '10', func_ts 'random_tstz');

SELECT * FROM rand2;

Sunday, June 14, 2015

What is a Set type?

Yesterday I proposed an Unconference talk about Set types, and I've had a couple of people ask me about what Set types are and what they would be for, so here is a brief rundown.

Say you have a table of things that you want to be able to apply some set of tags to. It might be blog posts, for example. The tags might be subject classifications, or reader ratings. Let's say for the sake of argument that it's going to be subject classifications, and that we actually have a (large) enum type to standardize that.

The classic way to do this is to have a table of {blog_post_id, tag} which will be unique on the combination. Up to now adding a tag is a nuisance - you need to make sure the tag doesn't already exist or you'll get a uniqueness violation. In 9.5 that's improved with INSERT ... ON CONFLICT IGNORE. Even then it's a bit more work that I really want to do. What I really want is something like this:
UPDATE blog_posts
SET subject_tags = subject_tags + 'my_new_subject'
WHERE post_id = 12345
and then I'll be able to do
SELECT * FROM blog_posts
WHERE subject_tags ? 'some subject'
It's also possible to do this, somewhat inefficiently, by using hstore or jsonb fields (in jsonb you'd just have a top level object). In both cases you would use dummy values - say make everything have a value of 1. But that's ugly, and error prone, and rather hackish.

Now I don't know if there is enough extra value here to justify the work and code maintenance involved. I've managed to live without it all these years without attacking what little hair I have left. But it's something where I have occasionally thought "I could do this so much more simply and elegantly if I had a set type." So I'd like to see if there is enough interest to make it into a real project.

Wednesday, June 3, 2015

Blackhole FDW updated

There have been some new things added to the FDW API in 9.5, so I have just updated the blackhole Foreign Data Wrapper to reflect the changes, so it now has documented skeleton functions for every FDW callback.