Re: Simple Atomic Relationship Insert

The loop to run it twice handles that yes. I don't think that buys
you anything over a more traditional non-cte method though. I'd run
them a few thousand times to see if there's any difference in runtimes
but my guess is the CTE version would be slightly slower here.

v_id integer;
BEGIN;
select id into v_id from hometowns where name = hometown_name;
BEGIN
insert into hometowns (name)
select hometown_name where v_id is null
returning id into v_id;
EXCEPTION WHEN unique_violation
THEN
select id into v_id from hometowns where name = hometown_name;
END;
insert into users (name, hometown_id)
values ('Robert', v_id);
END;

On Tue, Jan 13, 2015 at 5:06 PM, Robert DiFalco
<robert(dot)difalco(at)gmail(dot)com> wrote:
> This seems to get rid of the INSERT race condition.
>
> CREATE OR REPLACE FUNCTION select_hometown_id(hometown_name VARCHAR) RETURNS
> INTEGER AS $
> DECLARE hometown_id INTEGER;
> BEGIN
> LOOP
> BEGIN
> WITH sel AS (
> SELECT id FROM hometowns WHERE name = hometown_name
> ), ins AS (
> INSERT INTO hometowns (name)
> SELECT hometown_name
> WHERE NOT EXISTS(SELECT 1 FROM sel)
> RETURNING id
> )
> SELECT id INTO hometown_id FROM ins UNION ALL SELECT id FROM sel;
> RETURN hometown_id;
>
> EXCEPTION WHEN unique_violation
> THEN
> END;
> END LOOP;
> END;
> $ LANGUAGE plpgsql;
>
>
> On Tue, Jan 13, 2015 at 1:53 PM, Brian Dunavant <brian(at)omniti(dot)com> wrote:
>>
>> With the single CTE I don't believe you can do a full upsert loop. If
>> you're doing this inside of a postgres function, your changes are
>> already atomic, so I don't believe by switching you are buying
>> yourself much (if anything) by using a CTE query instead of something
>> more traditional here.
>>
>> The advantages of switching to a CTE would be if this code was all
>> being done inside of the app code with multiple queries.
>>
>> On Tue, Jan 13, 2015 at 4:39 PM, Robert DiFalco
>> <robert(dot)difalco(at)gmail(dot)com> wrote:
>> > Well, traditionally I would create a LOOP where I tried the SELECT, if
>> > there
>> > was nothing I did the INSERT, if that raised an exception I would repeat
>> > the
>> > LOOP.
>> >
>> > What's the best way to do it with the CTE? Currently I have the
>> > following
>> > which gives me Duplicate Key Exceptions when two sessions try to insert
>> > the
>> > same record at the same time.
>> >
>> > CREATE OR REPLACE FUNCTION select_hometown_id(hometown_name VARCHAR)
>> > RETURNS
>> > INTEGER AS $
>> > DECLARE hometown_id INTEGER;
>> > BEGIN
>> > WITH sel AS (
>> > SELECT id FROM hometowns WHERE name = hometown_name
>> > ), ins AS (
>> > INSERT INTO hometowns (name)
>> > SELECT hometown_name
>> > WHERE NOT EXISTS(SELECT 1 FROM sel)
>> > RETURNING id
>> > )
>> > SELECT id INTO hometown_id FROM ins UNION ALL SELECT id FROM sel;
>> > RETURN hometown_id;
>> > END;
>> > $ LANGUAGE plpgsql;
>> >
>> > And that is no bueno. Should I just put the whole thing in a LOOP?
>
>