Let's write a database (part 4)

Hello, hello, hello. Welcome to part four of this adventure where we’re making a database by hand. In part three we did some joins, and today we’re doing to do some limiting.

If you’d like to look at the code, it’s right here.

No no, no no no no, no no no no, no no, there’s no limits

Limit is very straightforward, it’s used to control the amount of data we get back from a query.

select * from albums limit 10

…will get 10 items from the albums table. To help decide how we choose which 10, we’ll need ordering which will come next.

We add a new Limit to the Query type, containing the following:

pub struct Limit {
    pub from: Box<Query>,
    pub limit: u64,
}

Then we add this to our run_query function:

Query::Limit(Limit { limit, from }) => {
    let QueryStep {
        schema,
        mut rows,
    } = run_query(from)?;

let size: usize = (*limit).try_into().unwrap();

    rows.truncate(size);

    Ok(QueryStep { schema, rows })
}

That’s the whole thing. What a disappointing blog post.

What’s next?

Next time we’re doing ordering, which is marginally more exciting, but not that exciting tbh.

Make sense? If not, get in touch!