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Comment by lmm | Hacker News Reader

There is always an alternative to writing SQL queries by hand, and it's usually a better one IME. Any ORM worth its salt will let you do the query in your blog post via the ORM, as a single query.

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> There is always an alternative to writing SQL queries by hand, and it's usually a better one IME.

I spent years writing code using Spring/Hibernate, and I can state with certainty that both of those statements are demonstrably false.

Every application starts with good intentions, a simple CRUD webapp, and an ORM, then at some point the business requirements yield an N+1 problem in ORMs or several non-trivial left joins into records that don't map the shape of the entities. At that point it's far easier to write the query in straight SQL and produce a straightforward mapping into the record structure, which doesn't play well with the ORM because that bypasses its entity cache, which causes another huge set of problems on its own. So now not only is there ORM maintenance and SQL maintenance, there is now a problem with the conjunction of the two technologies.

I agree but I think Hibernate is an extreme. There is a middleground, query builders, which enable dynamic query construction (e.g. dynamically appending filter predicates) with reduced cognitive load of something like Hibernate.

I personally was heavily in the camp of "write raw queries ideally with code generation for statically typed/generated code" (as exist in Rust, Go, TypeScript, etc.), but I have since tempered my position since it does become a bit brittle and repetitive. Lately I've been playing with Jooq and it seems great.

There are tradeoffs everywhere though, so with Jooq you still aren't 1-to-1 with raw SQL, there is a bit to learn, but I consider it a worthwhile investment (and a minor one relative to an actual ORM).

A pure query builder is just writing SQL on syntax tree level, more or less. This makes sense for the same reason why you want your macros to operate on ASTs and not raw text. But I would argue that it's still much closer to plain text SQL in the code than to any ORM.

Right, but it lives with your application code and has the same syntax as the application code. That's probably preferable to SQL stored procedures (which often live outside source control).

I’ve found that all this does is make the query less readable. SQL is purpose made for writing queries, and avoids unnecessary syntax noise you get when trying to fit the query into a host language based dsl.

That really depends on the language - specifically, on whether it already has constructs that can map nicely (e.g. LINQ in C#), or macros to define them, or syntax that is generally amenable to DSLs even without macros in the picture (e.g. Lisps).

SQL itself is also not a particularly well-designed query language. E.g. the order of the query doesn't reflect the natural data flow (SELECT .. FROM .. is reversed - compare to XQuery's FLWOR, for example), there are warts like WHERE vs HAVING etc. A good DSL can do much better.

SQL is powerful. A DSL that "fixes" things in this area getting all the other language feature interactions right isn't trivial, all the while users have to learn yet another language. Take PRQL for example: https://prql-lang.org. It looks nice, but the examples are very basic. What about window functions, grouping sets, lateral, DML, recursive SQL, pattern matching, pivot/unpivot etc. Might be doable, but perhaps, they've already made a decision that won't enable one of those features without adding new kludges.

Besides, every single "fix" will be a proprietary solution, while SQL is an ISO/IEC standard that's here to stay and universally adopted.

> A good DSL can do much better.

Stonebraker's QUEL was "better", before SQL, and yet, where is QUEL today?

> Besides, every single "fix" will be a proprietary solution, while SQL is an ISO/IEC standard that's here to stay and universally adopted.

And yet in practice the fixes end up more portable. How many of the things on your list of non-basic SQL have consistent syntax across databases, yet alone consistent behaviour?

All of them

A good DSL is not easy to implement, of course.

But the point here isn't just that it can be more regular than SQL. Integrating with the syntax of the host language is also a considerable advantage, ideally with static type checking.

[PRQL core-dev here]

Thanks for the PRQL shout-out!

> Take PRQL for example: https://prql-lang.org. It looks nice, but the examples are very basic. What about window functions, grouping sets, lateral, DML, recursive SQL, pattern matching, pivot/unpivot etc.

Window functions are very much supported! Check out the examples on the home page & in the docs.

The others aren't yet, but not because of a policy — we've started with the most frequently used features and adding features as they're needed.

In a statically typed language, what you get from a good query builder is that "malformed SQL statements" blow up at compile-time instead of at run-time.

Some languages also provide this for SQL strings (e.g. the sqlx library in Rust) will compile-time check raw SQL strings.

    That's probably preferable to SQL stored 
    procedures (which often live outside source control).

Stored procs definitely have some big pros and big cons, but I don't think this is one of them -- any ORM with a decent set of tools to manage migrations (ActiveRecord is one) makes this objection a non issue IMO.

I explicitly do not want to manage stored procedures in the same way as typical migrations - if I did, I would wind up with many, many versions of the procedure in my code base as it evolved over time. This would make grepping or locating the latest version pretty annoying.

Flyway (migration tool in Java) has a notion of “repeatable” migrations, though, which would do the trick.

    This would make grepping or locating the latest version pretty annoying

Wouldn't this be an issue with any database object managed via migrations? Do any of them make this easy for any database object?

In ActiveRecord, you have your migrations folder(s) and then you have your `structure.sql` (essentially the raw output of mysqldump or pgdump) or the equivalent.

If I need to see the literal database definition of any database object I look it up in there. Not the slickest solution but works well enough - really just a few keystrokes in my editor.

I'd be curious how other migration tools handle (or fail to handle) this.

> Wouldn't this be an issue with any database object managed via migrations?

Like I mentioned, check out flyway repeatable migrations.

It’s not hard to get SQL DDL and stored price in source control with Liquibase or Flyway. I’ve even done TDD sproc unit (integration) tests in them. But I’m a webapp turned data engineer guy…

> There are tradeoffs everywhere though, so with Jooq you still aren't 1-to-1 with raw SQL

You're probably hinting at writing derived tables / CTEs? jOOQ will never keep you from writing views and table valued functions, though. It encourages you do so! Those objects play very well with code generation, and you can keep jOOQ for the dynamic parts, views/functions for the static parts.

> I spent years writing code using Spring/Hibernate, and I can state with certainty that both of those statements are demonstrably false.

> Every application starts with good intentions, a simple CRUD webapp, and an ORM, then at some point the business requirements yield an N+1 problem in ORMs or several non-trivial left joins into records that don't map the shape of the entities. At that point it's far easier to write the query in straight SQL and produce a straightforward mapping into the record structure, which doesn't play well with the ORM because that bypasses its entity cache, which causes another huge set of problems on its own. So now not only is there ORM maintenance and SQL maintenance, there is now a problem with the conjunction of the two technologies.

I agree that that's often the end result, but in my experience 100% of cases are due to SQL fanboys who are unwilling to spend 5 minutes actually reading the ORM documentation and finding out how to do their N+1 query or complex join properly, which is actually easier than doing it in SQL if you try.

And don't get me started on "Hibernate is slow. The entity cache? Oh, our unnecessary custom SQL query made that inconsistent so we've disabled all caching".

I agree with you, however, some ORMs handle this fairly elegantly.

    At that point it's far easier to write the query in 
    straight SQL and produce a straightforward mapping 
    into the record structure, which doesn't play well 
    with the ORM because that bypasses its entity cache, 
    which causes another huge set of problems on its own

Rails' ActiveRecord ORM offers at least two ways to handle this.

1. ActiveRecord plays really nicely with views (including materialized views) in my experience. It treats them just like tables, basically, except you can't write to them. (note: there may actually be some cases where you can write to them; not sure)

2. You can supply your own handrolled SQL to ActiveRecord, e.g. `User.find_by_sql("select a,b,c from blahblahblah")`

YMMV obviously but I've been working with Rails since 2014 but this has covered all of my performance needs.

Plain old ActiveRecord default query generation is fine 99% of the time, and it's rather elegant/easy to sidestep it when I wish.

"Say now we want to display the album list sorted by album’s duration, shortest first."

How would we do this in an ORM like ActiveRecord or Django ORM, without generating multiple queries?

I haven't used Rails or ActiveRecord for a long time, but from memory it gives you an escape hatch in the form of Arel that lets you represent parts of your query as SQL while still using the ORM for the rest.

Edit: I think it's something like

    Album
      .select(:id, :name, "SUM(songs.length) AS total_length")
      .left_outer_joins(:songs)
      .group(:id)
      .order("SUM(songs.length) ASC")

I think you don't even need to explicitly wrap it in Arel.

I use rails every day. Looks close enough. You’d just need to specify the fields with the table name, I.e. ‘albums.id’.

Also, for anyone reading, a total_length method dynamically will be added objects returned. :)

I don't know about ActiveRecord or Django specifically, but: the obvious way in their query builder? Adding a joined collection, doing an aggregation on it, and sorting by it, are all easy in any ORM worth bothering with. What did you try and where did you get stuck?

In Django, you would write an annotation as documented at https://docs.djangoproject.com/en/4.1/topics/db/aggregation/...

So something like

  Album.objects.annotate(duration=Sum('track__length')).order_by('duration')

There are also libraries that enable you to define an annotation or aggregation inside a model, that you can then get with a call like select_properties, similar to the built-in select_related (which you use to get a foreign key in one query) and prefetch_related (which is select_related for many-to-many fields).

Thanks. I haven't used Django for ages, but do recall using annotate() in the past. At that time, I didn't need to worry about performance, so didn't look into what happens under the hood, i.e. whether the call generates more than one SQL query.

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