Re: Implementing CustomScan over an index

Hi,

> So I started implementing a CustomScan. It's not trivial.
>
> I've learned that the system executes ExecInitCustomScan automatically, but I probably need it to do most of the stuff in ExecInitIndexScan, and then execute the scan mostly the way it's done in IndexNext.
>
> Basically, I want just a normal index scan, but with the ability to do custom things with the quals and the projection.
>
> So... what's the best approach?
>
> Is there any sample code that does this? A search of github doesn't turn up much.
>
> Is there any way to do this without duplicating everything in nodeIndexscan.c myself?

Yes, unfortunately it is not quite trivial.

There is a "Writing a Custom Scan Provider" chapter in the
documentation that may help [1]. TimescaleDB uses CustomScans, maybe
using its code as an example will help [2]. Hint: exploring `git log`
is often helpful too.

If something in the documentation is not clear, maybe it can be
improved. Let us know here or (even better) provide a patch. If you
have a particular piece of code that doesn't do what you want, try
uploading a minimal example on GitHub and asking here.

By a quick look I couldn't find an example of implementing a
CustomScan in ./contrib/ or ./src/test/. If you can think of a usage
example of CustomScans, consider contributing a test case.

[1]: https://www.postgresql.org/docs/current/custom-scan.html
[2]: https://github.com/timescale/timescaledb/

--
Best regards,
Aleksander Alekseev