Change column type

I want to change a column type from VARCHAR to BIGINT. All values
currently stored in the column are integers. Saving column data,
recreating the column and restoring data seems to be too convoluted.
Besides, I have multiple constraints declared on this column, which I
will have to recreate once the column is dropped. I wonder if the
easiest way to do this would be just to change the atttypid in
pg_attribute?

I also wonder why it is possible to declare an FK constraint on a column
of type VARCHAR to reference a PK column of type BIGINT?

Thanks.

Oleg

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