[web-devel] Sql join feature in persistent

[Michael Snoyman]

Max: Let's address your desire for sharding in a separate thread: I
think it's a good idea, and it's a feature I would like to implement.
I also think that your approach seems fairly feasible and not too
disruptive. I want to make sure that introducing sharding code doesn't
negatively affect users not taking advantage of it.

As for joins (and other relational features): we should definitely add
some helpers to Persistent. The main questions are:

* What should their type signatures be?
* Should we offer optimizations for the SQL backends to use proper JOINs?

For the first, I think we still need to clarify a bit. Anton's code is
a good start, but it certainly won't be amenable to SQL-backend
optimizations. And I think we *should* strive to produce some
efficient SQL code, though I want to avoid pouring too much effort
into such an endeavor. Perhaps if we can write some code to handle the
most common cases (eg, a 1-to-many relation between two tables) we'll
be adhering well to the 80/80 rule.

My ideal would be having some nice high-level functions that handle
joins, optimize those to proper SQL joins where possible, and for
backends where that is not possible simply do the join logic in
Haskell.

I definitely won't have time to get started on this coding for the
next few weeks, but if anyone is interested in trying to implement any
of the features under discussion, let me know, and I'll be happy to
offer any assistance/code review I can.

Michael

On Tue, Feb 22, 2011 at 4:39 PM, Greg Weber <greg at gregweber.info> wrote:
> The basic Persistent backend is designed without joins ("web-scale"), and
> with the idea that it should be SQL agnostic. App-level join functionality
> is definitely needed.
> If someone wanted to design a layer on top of it with other SQL features
> that just the SQL backends can use that is fine. However, this might start
> re-inventing the wheel. For those interested in SQL joins and otherwise
> advanced SQL queries, it would be great if they started trying out a better
> tool for the job, like the recently released DSH [1] or (bitrotting?)
> HaskellDB. Relational Algebra sql query libraries can be very productive to
> use for 95% of use cases, even if they aren't "web-scale".
> [1] http://hackage.haskell.org/package/DSH
> [2] http://hackage.haskell.org/package/TableAlgebra
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