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

Some SQL servers have a feature where INSERT is skipped if it would violate a primary/unique key constraint. For instance, MySQL has INSERT IGNORE.

What's the best way to emulate INSERT IGNORE and ON DUPLICATE KEY UPDATE with PostgreSQL?

With PostgreSQL 9.5, this is now native functionality (like MySQL has had for several years):

>INSERT ... ON CONFLICT DO NOTHING/UPDATE ("UPSERT") > >9.5 brings support for "UPSERT" operations. INSERT is extended to accept an ON CONFLICT DO UPDATE/IGNORE clause. This clause specifies an alternative action to take in the event of a would-be duplicate violation.

...

>Further example of new syntax:

INSERT INTO user_logins (username, logins)
VALUES ('Naomi',1),('James',1) 
ON CONFLICT (username)
DO UPDATE SET logins = user_logins.logins + EXCLUDED.logins;

Edit: in case you missed warren's answer, PG9.5 now has this natively; time to upgrade!

---

Building on Bill Karwin's answer, to spell out what a rule based approach would look like (transferring from another schema in the same DB, and with a multi-column primary key):

CREATE RULE "my_table_on_duplicate_ignore" AS ON INSERT TO "my_table"
  WHERE EXISTS(SELECT 1 FROM my_table 
                WHERE (pk_col_1, pk_col_2)=(NEW.pk_col_1, NEW.pk_col_2))
  DO INSTEAD NOTHING;
INSERT INTO my_table SELECT * FROM another_schema.my_table WHERE some_cond;
DROP RULE "my_table_on_duplicate_ignore" ON "my_table";

Note: The rule applies to all INSERT operations until the rule is dropped, so not quite ad hoc.

For those of you that have Postgres 9.5 or higher, the new ON CONFLICT DO NOTHING syntax should work:

INSERT INTO target_table (field_one, field_two, field_three ) 
SELECT field_one, field_two, field_three
FROM source_table
ON CONFLICT (field_one) DO NOTHING;

For those of us who have an earlier version, this right join will work instead:

INSERT INTO target_table (field_one, field_two, field_three )
SELECT source_table.field_one, source_table.field_two, source_table.field_three
FROM source_table 
LEFT JOIN target_table ON source_table.field_one = target_table.field_one
WHERE target_table.field_one IS NULL;

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)

To get the insert ignore logic you can do something like below. I found simply inserting from a select statement of literal values worked best, then you can mask out the duplicate keys with a NOT EXISTS clause. To get the update on duplicate logic I suspect a pl/pgsql loop would be necessary.

INSERT INTO manager.vin_manufacturer
(SELECT * FROM( VALUES
  ('935',' Citroën Brazil','Citroën'),
  ('ABC', 'Toyota', 'Toyota'),
  ('ZOM',' OM','OM')
  ) as tmp (vin_manufacturer_id, manufacturer_desc, make_desc)
  WHERE NOT EXISTS (
    --ignore anything that has already been inserted
	SELECT 1 FROM manager.vin_manufacturer m where m.vin_manufacturer_id = tmp.vin_manufacturer_id)
)

INSERT INTO mytable(col1,col2) 
    SELECT 'val1','val2' 
    WHERE NOT EXISTS (SELECT 1 FROM mytable WHERE col1='val1')

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.

This solution avoids using rules:

BEGIN
   INSERT INTO tableA (unique_column,c2,c3) VALUES (1,2,3);
EXCEPTION 
   WHEN unique_violation THEN
     UPDATE tableA SET c2 = 2, c3 = 3 WHERE unique_column = 1;
END;

but it has a performance drawback (see PostgreSQL.org):

> A block containing an EXCEPTION clause is significantly more expensive > to enter and exit than a block without one. Therefore, don't use > EXCEPTION without need.

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.

For data import scripts, to replace "IF NOT EXISTS", in a way, there's a slightly awkward formulation that nevertheless works:

DO
$do$
BEGIN
PERFORM id
FROM whatever_table;

IF NOT FOUND THEN
-- INSERT stuff
END IF;
END
$do$;