[petergeoghegan]

> The core idea of the project is to get us the convenience of specifying the "what question I want answered," but without the inconvenience of "how is the answer computed / with which specific set of queries / where did the data come from?"

So...exactly like SQL, then?

[ghusbands]

The post you're replying to directly addresses that. When you write SQL:

> It's no longer a simple "just works" query language the moment you have to worry about predicate flattening, join decomposition, CTEs that introduce optimization barriers, and "IN()" being faster than equivalent "JOINs".

Though it doesn't seem to address how to optimize things using the GraphQL compiler, when there's a need, without massaging the queries, as with SQL.

[obi1kenobi]

Just like how GCC and Clang/LLVM know all the quirks of various CPUs and can optimize accordingly, GraphQL compiler aims to know the quirks of various databases (down to individual database versions: e.g., in Postgres 12 certain kinds of CTEs are no longer an optimization barrier) and optimize accordingly.

This is clearly a massive challenge, but one made easier by the fact that GraphQL compiler queries (unlike SQL queries) operate at a much higher level of abstraction. In SQL, you write "here's a CTE, now recursively JOIN X.foo to Y.bar" where X and Y could be just about anything, even something where a recursive JOIN is nonsensical. If you want to put a WHERE clause, you have to decide whether it goes in the recursive CTE itself, in a separate CTE that is ordered before the recursive CTE, or if you want to wrap the recursive CTE into another SELECT and put the WHERE there. The correct answer varies from database to database, and as a function of the size, layout, and index coverage of your data.

In GraphQL compiler, your queries are much more declarative in comparison: your query would say "find all subgraphs where vertex A's field 'foo' has value 123, and where A has a recursively-expanded edge (i.e. 0+ hops along that edge) to a vertex with field 'bar' with value 456". It's then the compiler's job to figure out which of the many equivalent SQL statements (or other db language queries, if you aren't using SQL) is going to be the best way to compute the result you asked for.

Here's an example from our test suite: input query: https://github.com/kensho-technologies/graphql-compiler/blob...

Microsoft SQL Server-flavored compiled SQL output: https://github.com/kensho-technologies/graphql-compiler/blob...

I'm writing a blog post about this with more detail, follow me on Twitter if you'd like to see it when it comes out.

[darksaints]

If you know all of the quirks of the various databases, why aren't you hacking on their optimizers? Why write a compiler that knows what's slow and what's not when you can just fix what's slow?

[obi1kenobi]

It's an "and" rather than an "either-or" :)

When the database is open-source, and I spot something that's broken that I know how to fix, I try to fix it. Here's a fix for a severe database query planner correctness bug I contributed to an open-source database called OrientDB: https://github.com/orientechnologies/orientdb/pull/7015

Unfortunately, Microsoft SQL Server, Oracle, and many other databases are not open-source, and I can't hack on their query planners. And even if they were, SQL is an absolutely massive language (the complete spec is 100,000+ pages). The GraphQL compiler query language is tiny in comparison, the spec is maybe 10 pages: https://graphql-compiler.readthedocs.io/en/latest/language_s...

It's a lot easier to intelligently map a small language to a big one than it is to optimize the big language outright.

In a sense, SQL is just not designed to be easy to optimize — it's too broad, and there are too many equivalent-ish ways of doing the same thing. This is why even after incredibly smart people cumulatively spent engineer-millennia on the query execution and optimization systems in SQL databases, we still keep having issues and there are still plenty of areas for improvement.

More info and more concrete examples of "why not just write SQL" in my upcoming blog post!

[petergeoghegan]

> In a sense, SQL is just not designed to be easy to optimize — it's too broad, and there are too many equivalent-ish ways of doing the same thing. This is why even after incredibly smart people cumulatively spent engineer-millennia on the query execution and optimization systems in SQL databases, we still keep having issues and there are still plenty of areas for improvement.

The main reason is the inherent difficulty of cardinality estimation, as the paper says.

Not every optimization is worth having. There is typically a distributed cost, paid in extra planner cycles for queries that don't benefit from the optimization. This is one of the main reasons why it's hard to contribute new optimizations to the Postgres planner. Naturally, it's possible that a marginal optimization will be incredibly important to one particular query or user. It's a value judgement in each case.

Frankly, I find the suggestion that SQL is not designed to be easy to optimize baffling.

[obi1kenobi]

I think we agree more than may seem apparent at first glance. In a sense, you are also making the same point I was trying and probably failed to make. Please bear with me as I give it another shot.

The difficulty of cardinality estimation is a function of the expressiveness of the language. Imagine a new query language, SQL2, that only has the SELECT and FROM keywords -- no WHERE, no JOIN, nothing else. Cardinality estimation in SQL2 is trivial: just store the counts for each table, and you can estimate everything trivially. Optimal query plans are trivial by extension as well.

Now let's add the WHERE keyword and a few operators to this SQL2. Cardinality estimation and good query planning got much harder now! For example, if the WHERE predicate touches two columns, we need to know about correlations between the two columns, or we might make incorrect estimates and therefore get worse plans. And since the plan space got bigger, we spend more cycles on planning. If we continue to add more features to SQL2 to bring it to parity with SQL proper, all these problems get harder as we go.

The language semantics behind GraphQL compiler aim to get sufficient expressiveness for many (hopefully, most) use cases, while limiting the scope so that the problems of cardinality estimation and good planning don't become too hard to solve effectively. In comparison, SQL is significantly more expressive, and as a result also much more difficult to execute and optimize.

[random_savv]

I think the parent's point might have been: SQL tried with mixed success. What makes the GraphQL compiler different?

[coolgeek]

The user you're replying to presumably understands a bit about the cited inconveniences, as well as SQL, in general

> user: petergeoghegan > about: PostgreSQL major contributor, committer.