Re: simplifying foreign key/RI checks

On Tue, Jan 19, 2021 at 3:01 AM Pavel Stehule <pavel.stehule@gmail.com> wrote:
> po 18. 1. 2021 v 13:40 odesílatel Amit Langote <amitlangote09@gmail.com> napsal:
>> I started with the check that's performed when inserting into or
>> updating the referencing table to confirm that the new row points to a
>> valid row in the referenced relation.  The corresponding SQL is this:
>>
>> SELECT 1 FROM pk_rel x WHERE x.pkey = $1 FOR KEY SHARE OF x
>>
>> $1 is the value of the foreign key of the new row.  If the query
>> returns a row, all good.  Thanks to SPI, or its use of plan caching,
>> the query is re-planned only a handful of times before making a
>> generic plan that is then saved and reused, which looks like this:
>>
>>               QUERY PLAN
>> --------------------------------------
>>  LockRows
>>    ->  Index Scan using pk_pkey on pk x
>>          Index Cond: (a = $1)
>> (3 rows)
>
>
> What is performance when the referenced table is small? - a lot of codebooks are small between 1000 to 10K rows.

I see the same ~2x improvement.

create table p (a numeric primary key);
insert into p select generate_series(1, 1000);
create table f (a bigint references p);

Unpatched:

insert into f select i%1000+1 from generate_series(1, 1000000) i;
INSERT 0 1000000
Time: 5461.377 ms (00:05.461)


Patched:

insert into f select i%1000+1 from generate_series(1, 1000000) i;
INSERT 0 1000000
Time: 2357.440 ms (00:02.357)

That's expected because the overhead of using SPI to check the PK
table, which the patch gets rid of, is the same no matter the size of
the index to be scanned.

--
Amit Langote
EDB: http://www.enterprisedb.com