hanahmily commented on pull request #10:
URL:
https://github.com/apache/skywalking-banyandb/pull/10#issuecomment-866431221
> > From your screenshots, the `Scan` is the first operation even fields
exist. From the design, `Select` which should be the first one parsed ChunkIDs
from `index`, then the results will be passed to `FetchEntity`(I didn't see
this step in the parsed plan). The `Scan` will be used only there's no any
`Selection` input, which means it will be picked rarely.
>
> As I understood, the logical plan does not care about the indexes, it just
prepares the metadata and resolves fields so that the existence of these fields
which are referenced can be guaranteed.
>
> As we've discussed in the last PR, we can select the indexes while
generating physical plans based on cost-first consideration.
Not that. cost-based optimization only takes place when a field belongs to
more than one index. Supposing `service_id` is indexed by `service_id` +
`instance_id` and `service_id` + `endpoint_id`, we should leverage cost-based
statistics to determine which one should be used. In this case, the combination
of `service_id` and `instance_id` has fewer cardinalities, which will get less
cost. Based on that, the query physical optimizer should pick up it instead of
`service_id` + `endpoint_id`.
As I mentioned in another PR, we don't support the composite index for now.
We don't have to implement the above plan optimization.
> the logical plan does not care about the indexes
That's the convention of a traditional SQL database due to the complex index
type, for example, single, composite, unique, group by and etc. Such a database
can't determine query paths without statistics. But the query of BanyanDB
doesn't, which knows how to access single fields index based on the criteria.
We also remove `tags` from query criteria, which's due to the huge cost of
`Scan` filtering. As I mentioned, the query plan picks up `Scan` when no fields
are input.
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