[database] how to emulate "insert ignore" and "on duplicate key update" (sql merge) with postgresql?

Looks like PostgreSQL supports a schema object called a rule.

http://www.postgresql.org/docs/current/static/rules-update.html

You could create a rule ON INSERT for a given table, making it do NOTHING if a row exists with the given primary key value, or else making it do an UPDATE instead of the INSERT if a row exists with the given primary key value.

I haven't tried this myself, so I can't speak from experience or offer an example.