Re: INSERT ON CONFLICT and RETURNING

út 8. 9. 2020 v 12:34 odesílatel Konstantin Knizhnik <
k(dot)knizhnik(at)postgrespro(dot)ru> napsal:

>
>
> On 08.09.2020 12:34, Pavel Stehule wrote:
>
>
>
> út 8. 9. 2020 v 11:06 odesílatel Konstantin Knizhnik <
> k(dot)knizhnik(at)postgrespro(dot)ru> napsal:
>
>> I have performed comparison of different ways of implementing UPSERT in
>> Postgres.
>> May be it will be interesting not only for me, so I share my results:
>>
>> So first of all initialization step:
>>
>> create table jsonb_schemas(id serial, schema bytea primary key);
>> create unique index on jsonb_schemas(id);
>> insert into jsonb_schemas (schema) values ('some') on conflict(schema)
>> do nothing returning id;
>>
>> Then I test performance of getting ID of exitsed schema:
>>
>> 1. Use plpgsql script to avoid unneeded database modifications:
>>
>> create function upsert(obj_schema bytea) returns integer as $$
>> declare
>> obj_id integer;
>> begin
>> select id from jsonb_schemas where schema=obj_schema into obj_id;
>> if obj_id is null then
>> insert into jsonb_schemas (schema) values (obj_schema) on
>> conflict(schema) do nothing returning id into obj_id;
>> if obj_id is null then
>> select id from jsonb_schemas where schema=obj_schema into obj_id;
>> end if;
>> end if;
>> return obj_id;
>> end;
>> $$ language plpgsql;
>>
>
> In parallel execution the plpgsql variant can fail. The possible raise
> conditions are not handled.
>
> So maybe this is the reason why this is really fast.
>
>
> With this example I model real use case, where we need to map long key
> (json schema in this case) to short identifier (serial column in this
> case).
> Rows of jsonb_schemas are never updated: it is append-only dictionary.
> In this assumption no race condition can happen with this PLpgSQL
> implementation (and other implementations of UPSERT as well).
>

I am not sure, but I think this should be a design and behavior of MERGE
statement - it is designed for OLAP (and speed). Unfortunately, this
feature stalled (and your benchmarks show so there is clean performance
benefit).

Regards

Pavel

>
>
> --
> Konstantin Knizhnik
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
>