Re: Improving EXPLAIN's display of SubPlan nodes

Hi,

> EXPLAIN has always been really poor at displaying SubPlan nodes
> in expressions: you don't get much more than "(SubPlan N)".
> This is mostly because every time I thought about it, I despaired
> of trying to represent all the information in a SubPlan accurately.
> However, a recent discussion[1] made me realize that we could do
> a lot better just by displaying the SubLinkType and the testexpr
> (if relevant). So here's a proposed patch. You can see what
> it does by examining the regression test changes.
>
> There's plenty of room to bikeshed about exactly how to display
> this stuff, and I'm open to suggestions.
>
> BTW, I was somewhat depressed to discover that we have exactly
> zero regression coverage of the ROWCOMPARE_SUBLINK code paths;
> not only was EXPLAIN output not covered, but the planner and
> executor too. So I added some simple tests for that. Otherwise
> I think existing coverage is enough for this.

I reviewed the code and tested the patch on MacOS. It looks good to me.

Although something like:

```
+ Filter: (ANY (base_tbl.a = $1) FROM SubPlan 1 (returns $1))
+ SubPlan 1 (returns $1)
```

... arguably doesn't give much more information to the user comparing
to what we have now:

```
- Filter: (SubPlan 1)
- SubPlan 1
```

... I believe this is the right step toward more detailed EXPLAINs,
and perhaps could be useful for debugging and/or educational purposes.
Also the patch improves code coverage.

--
Best regards,
Aleksander Alekseev