[Discuss] Add struct `Query` for datafusion

[jackwener]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I'm try to implement InSubquery.

I found that there isn't Query struct in datafusion. I think that it's because there isn't subquery handle in datafusion.

In the roadmap, datafusion include subquery implementation. I think Query struct is necessary just like datafusion-expr.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[jackwener]

@alamb @yahoNanJing @yjshen @mingmwang @tustvold How do you think about it?

[jackwener]

I guess that reason why use datafusion-expr instead of sqlparser is sqlparser need to adapt to various sql types and a little complex.

Maybe we also need to use customized Query and other.

[alamb]

@jackwener I am not sure about the need for Query -- if there is one, perhaps we can combine it with what @andygrove is working on in #2172

I was imagining that a query like this

SELECT f.x, sq.a FROM foo as f JOIN (select a from bar) as sq ON (f.x = sq.a),

results in a logical plan like this:

Projection(f.x, sq.a)
  --> Join(Inner f.x = sq.a)
    --> TableScan(foo)
    --> Projection(a)  <--- Here is where the "subquery"'s LogicalPlan is attached 
      --> TableScan(bar)

Similarly for

SELECT f.x  FROM foo as f WHERE f.x IN  (select a from bar) as sq;

results in a logical plan like this:

Projection(f.x, sq.a)
  --> Join(Semi f.x = sq.a)
    --> TableScan(foo)
    --> Projection(a)  <--- Here is where the "subquery"'s LogicalPlan is attached 
      --> TableScan(bar)

[alamb]

See #1209 for some more thinking on this matter

[xudong963]

Here is a framework #1649 for implementing subqueries that I wrote some time ago. After this framework is completed, I believe that many kinds of subqueries can be easily added, such as in, scalar subquery, and this framework takes decorrelated into account at the beginning.

Currently this ticket encounters some problems because datafusion currently records columns by name, not by index. So some work need to do before continuing.

Since I've been busy with changing jobs and adjusting to a new life, I haven't had much time to move forward with this ticket, so it's a bit delayed :(

[jackwener]

Here is a framework #1649 for implementing subqueries that I wrote some time ago. After this framework is completed, I believe that many kinds of subqueries can be easily added, such as in, scalar subquery, and this framework takes decorrelated into account at the beginning.

I also see this PR. I think it's not good choice to embed a logicplan to Expr.

[xudong963]

I think it's not good choice to embed a logicplan to Expr.

Why

[mingmwang]

Here is a framework #1649 for implementing subqueries that I wrote some time ago. After this framework is completed, I believe that many kinds of subqueries can be easily added, such as in, scalar subquery, and this framework takes decorrelated into account at the beginning.

I also see this PR. I think it's not good choice to embed a logicplan to Expr.

I also have the same doubt.

[alamb]

There are two kinds of subqueries

• Uncorrelated (aka subqueries that appear in the from clause that don't have references to the outer query)
• Correlated (subqueries that appear in the from clause or IN / NOT IN /etc and that refer to the outer query)

For correlated queries that that refer to the outer query using = predicates they can be transformed into SEMI-JOIN / ANTI-JOIN typically

Correlated queries that use something other than = (non equijoins) eventually have to be run once per each row in the outer query and would likely be expressed as a LogicalPlan in an Expr.

that being said, I think we can get very far in DataFusion (maybe forever) by handling only uncorrelated and equi-join correlated subqueries and avoid LogicalPlan in Expr

[jackwener]

Correlated queries that use something other than = (non equijoins) eventually have to be run once per each row in the outer query and would likely be expressed as a LogicalPlan in an Expr.

Maybe, we can use Query in an Expr (this is why I bring up this discussion).

Because the process is

| AST and binder --> Logical plan --> Execute plan |

Query is in AST and binder and will be transferred to LogicalPlan.

[xudong963]

Subquery processing is a complex matter, and I highly recommend that we figure out how to do it at the beginning (how to handle correlated and non-correlated), otherwise there will be a lot of bugs, and hard to cover areas later.

I recommend reading this article by materialize: https://materialize.com/decorrelation-subquery-optimization/, which mentions three articles, especially the tum one, which are very informative.

My previous framework was implemented according to this, and I will close it first. I'll continue when I'm free later, if others haven't done complete support :).

If anyone else has a better idea, feel free to ping me for a review!

[andygrove]

I just wanted to add a note here to say that I have dedicated time now to help with this effort. I think we can break this work down into phases and the logical starting point (pun intended) would be to decide on how we want to represent the various types of subquery in the LogicalPlan (unoptimized). Once we have that we can create separate issues and PRs for SQL query planning, DataFrame API, logical plan optimizations, and implementing the physical plans.

I am going to start by reviewing Apache Spark's logical plan for subqueries and then will put a proposal up. I'm not sure if this will be in the form of a PR or a Google document yet but I will share the proposal here, probably early next week.

[alamb]

See dask-contrib/dask-sql#474 and its subtasks such as #2238

[andygrove]

So I started prototyping and ran into an issue right away. As others have pointed out, we sometimes need to have an Expr that refers to a LogicalPlan, such as:

enum Expr {
  Exists {
    subquery: Box<LogicalPlan>
  }
  ...
}

However, the LogicalPlan enum is in the datafusion crate which depends on the datafusion-expr crate so I cannot reference LogicalPlan from Expr.

I was not paying much attention when this refactoring happened. Does it make sense to now move LogicalPlan as well to datafusion-expr crate?