Node selection mental model

[jtcohen6]

It feels like there's endless appetite to provide explanation and example for when resource selection feels counterintuitive. I think we have a lot to gain from laying out exactly the way that node selection does work:

1. dbt selects any resource that is matched by one or more of the --models/--select criteria, in the order of selection methods (e.g. tag:), then graph operators (e.g. +), then set operators (,). The selected resources may be models, sources, seeds, snapshots, tests. IFF the command is dbt test, dbt also selects any test that directly depends on a selected resource.
2. dbt de-selects any resource that is matched by one or more of the --exclude criteria. IFF the command is dbt test, dbt also de-selects any test that directly depends on an excluded resource.
3. dbt now has a list of still-selected resources of varying types. As a final step, it tosses away any resource that does not match the resource type of the current invocation. (Only seeds are kept for dbt seed, only models for dbt run, only tests for dbt test, and so on.)

Things like test selection and the source: selection method feel tricky because they intersect with this process is ways that can feel quite different from dbt run -m my_model. Rather than write more about each edge case, I think we need to build the mental model, so that they seem less like "exceptions" to rote-memorize and more like tricky cases that are still consistent with the overall schematic.

(Note that step 2 works a bit different for YAML selectors because the exclude: key subtly differs from --exclude, in order to power more complex set logic. That's briefly documented here.)

[runleonarun]

@joellabes and @jtcohen6 is this request still valid?

[joellabes]

Yes please! Happy to help work with someone as a technical resource