Create an SQL tbl (abstract)
Generally, you should no longer need to provide a custom
method you you can default
tbl_sql(subclass, src, from, ..., vars = NULL)
name of subclass
needed for agreement with generic. Not otherwise used.
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.