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
