In SQL Server, I could update a table's definition by first making
some checks on the system tables e.g.

IF NOT EXISTS (SELECT 1 FROM syscolumns WHERE ID = OBJECT_ID
('mytable') AND Name = 'newcolumn')
UPDATE mytable ADD newcolumn INTEGER

Can I do the same somehow in Firebird?

Also, what is PSQL? I see this mentioned a lot in the release notes.

Thanks in advance.