NewPostgresDB make the connection to the postgres db with a singleton way.

NewPsqlInvoice return a new ppointer of PsqlInvoice.

NewPsqlInvoiceHeader return a new pointer of PsqlInvoiceHeader // Constructor.

NewPsqlInvoiceItem return a new pointer of PsqlInvoiceItem // Constructor.

NewPsqlProduct return a new pointer of PsqlProduct // Constructor.

Pool return a unique instance of db.

PsqlInvoice used for work with postgres - invoice.

PsqlInvoiceHeader used for work with postgres - invoiceHeader.

PsqlInvoiceItem used for work with postgres - invoiceItem.

PsqlProduct used for work with postgres - product.