[Tutorial] Compound indexing and query optimization

[HeinrichvonStein]

This PR adds a compound indexing and query optimization tutorial.

[benitav]

is this last note meant to be here?

[HeinrichvonStein]

Whoops, copy pasta error. Thanks for catching

[benitav]

I'd suggest to link to the SDK reference in docs since it's the higher-level starting point (and also includes link to source code): https://docs.powersync.com/client-sdk-references/javascript-web

[rkistner]

I think this tutorial should be reworked a bit.

The guide focuses on compound indexes, but does not explain (1) what a compound index is, or (2) when to use a compound index versus a single-column index.

The performance benchmarks show the difference between using the index vs a table scan, but (1) does not give any details of how the benchmark was performed (e.g. number of table rows, number of results, the index used). The difference displayed us also quite small (206ms vs 556ms) - indexes can make a much bigger difference in some cases. And once again it doesn't show the difference compared to a single-column index, which could be a very small in this case.

The guide mentions order of indexed columns affects query performance, and skipping leading columns affect performance, but it doesn't clarify what this means, or how it affects performance. The examples don't clarify much here.

Overall, I'd recommend:

1. Start with a guide focusing on single-column indexes, maybe just mentioning compound indexes as a footnote.
2. Add detail on when the index can be used or not, perhaps also mentioning operator other than =. But keep it simple here - see the point below on the SQLite docs.
3. Add a guide on testing query performance and viewing the query plain (the EXPLAIN QUERY PLAN bit). The performance timeline may also be useful here.
4. SQLite already has some great and thorough docs on how the query planner and indexes work here and here. Maybe it's better to just include some basic examples in the doc here, specifically noting how indexes are defined in PowerSync, then link to those for the rest.

[michaelbarnes]

Thanks for the review @rkistner we're going to rework this guide.

[HeinrichvonStein]

This needs to be revisited. Closing