Return rows from INSERT with ON CONFLICT without needing to update

问题

I have a situation where I very frequently need to get a row from a table with a unique constraint, and if none exists then create it and return. For example my table might be:

CREATE TABLE names(
    id SERIAL PRIMARY KEY,
    name TEXT,
    CONSTRAINT names_name_key UNIQUE (name)
);

And it contains:

id | name
 1 | bob 
 2 | alice

Then I'd like to:

 INSERT INTO names(name) VALUES ('bob')
 ON CONFLICT DO NOTHING RETURNING id;

Or perhaps:

 INSERT INTO names(name) VALUES ('bob')
 ON CONFLICT (name) DO NOTHING RETURNING id

and have it return bob's id 1. However, RETURNING only returns either inserted or updated rows. So, in the above example, it wouldn't return anything. In order to have it function as desired I would actually need to:

INSERT INTO names(name) VALUES ('bob') 
ON CONFLICT ON CONSTRAINT names_name_key DO UPDATE
SET name = 'bob'
RETURNING id;

which seems kind of cumbersome. I guess my questions are:

What is the reasoning for not allowing the (my) desired behaviour?
Is there a more elegant way to do this?

回答1:

It's the recurring problem of SELECT or INSERT, related to (but different from) an UPSERT. The new UPSERT functionality in Postgres 9.5 is still instrumental.

WITH ins AS (
   INSERT INTO names(name)
   VALUES ('bob')
   ON     CONFLICT ON CONSTRAINT names_name_key DO UPDATE
   SET    name = NULL
   WHERE  FALSE      -- never executed, but locks the row
   RETURNING id
   )
SELECT id FROM ins
UNION  ALL
SELECT id FROM names
WHERE  name = 'bob'  -- only executed if no INSERT
LIMIT  1;

This way you do not actually write a new row version without need.

^{I assume you are aware that in Postgres every UPDATE writes a new version of the row due to its MVCC model - even if name is set to the same value as before. This would make the operation more expensive, add to possible concurrency issues / lock contention in certain situations and bloat the table additionally.}

Detailed explanation and how to wrap this into a function:

Is SELECT or INSERT in a function prone to race conditions?
How to use RETURNING with ON CONFLICT in PostgreSQL?
Return rows from INSERT with ON CONFLICT without needing to update

Why are "excluded" rows not included in the RETURNING clause?

Get Id from a conditional INSERT

If concurrent UPDATE or DELETE (from a different session) are not possible you don't need to lock the row and can simplify:

WITH ins AS (
   INSERT INTO names(name)
   VALUES ('bob')
   ON     CONFLICT ON CONSTRAINT names_name_key DO NOTHING  -- no lock needed
   RETURNING id
   )
SELECT id FROM ins
UNION  ALL
SELECT id FROM names
WHERE  name = 'bob'  -- only executed if no INSERT
LIMIT  1;

来源：https://stackoverflow.com/questions/40323799/return-rows-from-insert-with-on-conflict-without-needing-to-update

标签

sql

postgresql

upsert

postgresql-9.5