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

Try to do an UPDATE. If it doesn't modify any row that means it didn't exist, so do an insert. Obviously, you do this inside a transaction.

You can of course wrap this in a function if you don't want to put the extra code on the client side. You also need a loop for the very rare race condition in that thinking.

There's an example of this in the documentation: http://www.postgresql.org/docs/9.3/static/plpgsql-control-structures.html, example 40-2 right at the bottom.

That's usually the easiest way. You can do some magic with rules, but it's likely going to be a lot messier. I'd recommend the wrap-in-function approach over that any day.

This works for single row, or few row, values. If you're dealing with large amounts of rows for example from a subquery, you're best of splitting it into two queries, one for INSERT and one for UPDATE (as an appropriate join/subselect of course - no need to write your main filter twice)

As @hanmari mentioned in his comment. when inserting into a postgres tables, the on conflict (..) do nothing is the best code to use for not inserting duplicate data.:

query = "INSERT INTO db_table_name(column_name)
         VALUES(%s) ON CONFLICT (column_name) DO NOTHING;"

The ON CONFLICT line of code will allow the insert statement to still insert rows of data. The query and values code is an example of inserted date from a Excel into a postgres db table. I have constraints added to a postgres table I use to make sure the ID field is unique. Instead of running a delete on rows of data that is the same, I add a line of sql code that renumbers the ID column starting at 1. Example:

q = 'ALTER id_column serial RESTART WITH 1'

If my data has an ID field, I do not use this as the primary ID/serial ID, I create a ID column and I set it to serial. I hope this information is helpful to everyone. *I have no college degree in software development/coding. Everything I know in coding, I study on my own.

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.

On bulk, you can always delete the row before the insert. A deletion of a row that doesn't exist doesn't cause an error, so its safely skipped.