Correct sqlite-to-postgres column mapping

[eth3lbert]

The backend column was added later to the pstat_series table.
This PR reorders the column order to comply with the schema.

Closes #1827

[Kobzol]

That's interesting. The backend column was indeed added later, but after this migration, backend should be before metric. So it probably depends on which version of the SQLite table you have when doing the export.

[eth3lbert]

Interesting! What would you expect in this situation? Perhaps adding some columns mapping strategy?

[eth3lbert]

Oh wait! In SQLite, the backend column is placed before the metric column. However, in PostgreSQL, it remains after the metric column.

[Kobzol]

Oh, good find, I made this error when writing the last migration. It's a PITA to keep these two DB backends in sync.

[eth3lbert]

Yep, and it seems somewhat inevitable. I think it would be reasonable to introduce some column mapping to avoid these kinds of issues in the future.

[Kobzol]

Yeah, the conversion script should probably be able to support different column order in both backends.

[eth3lbert]

Or, we could try passing columns to functionsqlite_attributes and return a mapped and ordered columns back.
The column names are available through

select name from PRAGMA_TABLE_INFO('table_name') order by cid

[Kobzol]

This is a bit outside of my current SQL-fu, but if it works, let's go for it :)

[Kobzol]

This looks reasonable. I assume that the conversion now works for you, after the latest commit?

[eth3lbert]

Yes, works as expected.

P.S. I just made some minor wording changes and added information about the panics.