Add generic dbIsReadOnly

[anhqle]

From Issue #189

Users that use a read-only mode would likely want a way to periodically check whether their connection is truly read only.

For my own backend, I've added this generic, but want to suggest this feature here. I can contribute a PR if there is interest.

Closes #189

[codecov-io]

Codecov Report

Merging #190 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   43.34%   43.34%           
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  Files          16       16           
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  Hits          202      202           
  Misses        264      264

Impacted Files	Coverage Δ
R/DBObject.R	0% <ø> (ø)	⬆️

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[krlmlr]

Thanks. I wonder in which situations the read-only status of an existing connection can change?

[anhqle]

I agree that it's unlikely for a read-only connection to change its read-only status.

The generic is more of an UX improvement, e.g. confirming the read-only status whenever the user print(connection).

Given that an user may initiate a connection and work with it over hours, such a reminder is re-assuring.

(Tagging @wlattner, in case this DBI feature is something civis-r still uses).

[krlmlr]

Thanks, I'm happy to include it. How do you open a read-only connection to a database? Does this method also cover situations where the read-only status is due to missing write permissions?

[anhqle]

An user opens a read-only connection with CivisConnection(read_only = TRUE, ...).

Then, whenever the user sends a query (via dbSendQuery) using this read-only connection, we prepend the query with BEGIN READ ONLY (as instructed by Amazon Redshift documentation, which we use).

Our R package doesn't deal with permission. It simply sends users' queries from R to their databases hosted on our cloud. We check permissions in the cloud.

[krlmlr]

I wonder if we should provide a default method for "DBIConnection"?

Could you please also update the documentation?

[krlmlr]

Thanks! Let's avoid a default method for now.