Postgres INSERT ON CONFLICT DO UPDATE vs INSERT or UPDATE

Query 3 is the Postgres syntax for "UPSERT" (= UPDATE or INSERT), introduced in Postgres 9.5.

From the documentation:

ON CONFLICT DO UPDATE guarantees an atomic INSERT or UPDATE outcome; provided there is no independent error, one of those two outcomes is guaranteed, even under high concurrency. This is also known as UPSERT – “UPDATE or INSERT”.

This is the best practice for what you are trying to achieve.

I noticed/tested that is much faster for INSERTS (have yet to test UPSERTS) to use a WHERE NOT EXISTS in addition to ON CONFLICT. Typically about 3x faster than just allowing the ON CONFLICT to handle existence checks. I think this may carry over into UPSERTS, making it likely faster to do an INSERT and then and UPDATE. Here is my test for inserts only...

    --so i can keep rerunning    DROP TABLE if exists temp1;    DROP TABLE if exists temp2;    --create a billion rows    SELECT GENERATE_SERIES AS id INTO TEMP temp1    FROM GENERATE_SERIES(1, 10000000);    CREATE UNIQUE INDEX ux_id  ON temp1(id);    ALTER TABLE temp1 CLUSTER ON ux_id;    --create a second table to insert from, with the same data    SELECT * INTO TEMP temp2     FROM temp1;    CREATE UNIQUE INDEX ux_id2  ON temp2(id);    ALTER TABLE temp2 CLUSTER ON ux_id2;    --test inserting with on conflict only    INSERT INTO temp1(id)    SELECT id    FROM temp2 ON conflict DO nothing;    --execution time: 14.71s (1million rows)    --test inserting with not exists and on conflict    INSERT INTO temp1(id)    SELECT t2.id    FROM temp2 t2    WHERE NOT EXISTS (SELECT 1 FROM temp1 t1 WHERE t2.id = t1.id)     ON conflict DO nothing;    --execution time: 5.78s (1million rows)