This method shouldn't be called be users - it should only
be used by backend implementors who are creating backends
that extend the basic sql behaviour.
Usage
tbl_sql(subclass, src, from, ..., vars = NULL)
Arguments
subclass
name of subclass
...
needed for agreement with generic. Not
otherwise used.
vars
If known, the names of the variables in the
tbl. This is relatively expensive to determine
automatically, so is cached throughout dplyr. However,
you should usually be able to leave this blank and it
will be determined from the context.