Tag: postgresql

Test driven development for PostgreSQL

I have a mixed love/hate relationship with tests.

I hate writing them. I hate remembering to add them when I'm in the zone, and application code is flowing freely from the tips of my fingers.

But when I do add them, I absolutely love the ability to twist and replace the most core innards of application, and be able to tell that at least the sanity check of the code passes.

I love them even more when they prevent me for repeating some mistake/bug – i.e. when there is a bug, and I have tests (which clearly failed, as they didn't catch the bug), I add test for this specific bug, so I know that in future it will not happen again.

For a long time I've been fan of stored procedures (or functions) – of course not for everything, but where it makes sense.

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What mistakes you can avoid when looking for help on IRC?

Today, there was this one person on IRC, which asked question and provided some data. While working on helping him (her?), I noticed some things, that bugged me before in other cases, but this time i decided to write about it – it's kind of rant, and if you (the reader) are the person that I'm basing my example on – please do not feel “punished" – it just so happens, that you exhibited some things that make helping others more difficult than it could be – so: you're not special, although I would really prefer if you were 🙂

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Waiting for 9.0 – pg_upgrade

On May, 12ve, Bruce Momjian committed new contrib module for 9.0 – pg_upgrage.

As I understand – this is what was available before as pg-migrator.

If you're not familiar with it – it's a tool that allows upgrade of $PGDATA from some version to some version. What's the use case? Let's assume you have this 200GB database working as 8.3, and you'd like to go to 8.4 (or 9.0). Normal way is pg_dump + pg_restore – which will take some time. With pg-migrate/pg_upgrade it should be faster, and easier. So, let's play with it.

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Stupid tricks – hiding value of column in select *

One of the most common questions is “how do I get select * from table, but without one of the column".

Short answer is of course – name your columns, instead of using *. Or use a view.

But I decided to take a look at the problem.

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Getting unique elements

Let's assume you have some simple database with “articles" – each article can be in many “categories". And now you want to get list of all articles in given set of categories.

Standard approach:

SELECT
    a.*
FROM
    articles AS a
    JOIN articles_in_categories AS aic ON a.id = aic.article_id
WHERE
    aic.category_id IN (14,62,70,53,138)

Will return duplicated article data if given article is in more than one from listed categories. How to remove redundant rows?

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Tips n’ Tricks – using “wrong” index

More than once I've seen situation when there is a table, with serial primary key, and rows contain also some kind of creation timestamp, which is usually monotonic, or close to monotonic.

Example of such case are for example comments or posts in forums – each get it's ID, but they also have creation timestamp. And it usually is so that higher ids were added later than the lower ids.

So, let's assume you have such table, and somebody asks you to make a report on data from last month. How?

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