Support Existing Table Column Information

[ruxuez]

So far, we don't have any information about the DataFrame's
columns and their types. This patch supports retrieving column
information of an existing table in the database.
Moreover, adds a table check when creating a DataFrame from table,
so that the user can catch the name error early.

[xuebinsu]

This is a good start! Let's try not to break the API at the beginning so that some of the code can enter version 1.2.0 .

[xuebinsu]

How about exposing this functionality as DataFrame.describe(), so that this can be done lazily, i.e. only when user needs the info?

[ruxuez]

How about exposing this functionality as DataFrame.describe(), so that this can be done lazily, i.e. only when user needs the info?

Do you mean for now only do it lazily?

[xuebinsu]

How about exposing this functionality as DataFrame.describe(), so that this can be done lazily, i.e. only when user needs the info?

Do you mean for now only do it lazily?

Yes, when user asks for it.