Improve default implementation of dbQuoteString() and dbQuoteIdentifier()

[krlmlr]

• Single quotes for Strings: http://stackoverflow.com/a/1992331/946850, escape by repetition~~, escape backslash, escape single quote with backslash~~
• Double quotes for Database stuff, escape by repetition
• Vectorize dbQuoteString()
• Add dots in dbQuoteIdentifier()
• Return "SQL" object

[hadley]

I'm not sure we should provide too much by default - most database APIs provide explicit quoting/escaping functions which should be used preferentially to doing this in R.

[imanuelcostigan]

@krlmlr dbQuoteIdentifier should not return a vector which the current implementation does (fails DBItest quote_identifier_not_vectorized) which I assume is based on the reference here but which isn't clear from ?DBI::dbQuoteIdentifier()

[krlmlr]

@imanuelcostigan: Yes, that's the plan. I'll look into making the documentation clearer. (Could you please fix the reference link?)

[krlmlr]

I'm now convinced that dbQuoteIdentifier() should be vectorized, because this seems to be the standard with most existing backends. We'll figure out another way to provide schema information.

[hadley]

I think something that creates a schema object is probably the way forward: dbQuoteIdentifer(con, schema("x", "y"))

[krlmlr]

@hadley: This looks complicated. I'd like to keep the vectorized nature and avoid using new classes if at all possible.

Please see #24 (comment) for the current design. Happy to take your feedback there.