Re: Linear slow-down while inserting into a table with an ON INSERT trigger ?

In addition to what Justin was saying, another possible consideration may
be the transaction isolation level that you're running at. I don't recall
what Postgres' default is off hand but "Serializable" is the most strict
and would likely impose some overhead on what you described. Check out
https://www.postgresql.org/docs/12/transaction-iso.html for details. If
your particular use case can loosen up some of the strictness in the
context of that transaction it might possibly result in a speed
improvement. Just make sure you don't trade off data integrity for speed or
else you'll get invalid data quickly!

-- Ben Scherrey

On Sat, Jul 17, 2021 at 4:27 AM Tobias Gierke <
tobias(dot)gierke(at)code-sourcery(dot)de> wrote:

> Hi,
>
> Probably my google-foo is weak today but I couldn't find any (convincing)
> explanation for this.
>
> I'm running PostgreSQL 12.6 on 64-bit Linux (CentOS 7, PostgreSQL compiled
> from sources) and I'm trying to insert 30k rows into a simple table that
> has an "ON INSERT .. FOR EACH STATEMENT" trigger.
>
> Table "public.parent_table"
>
> Column | Type | Collation | Nullable | Default
> -------------+------------------------+-----------+----------+--------------------------------------------
> id | bigint | | not null | nextval('parent_table_id_seq'::regclass)
> name | character varying(64) | | not null |
> enabled | boolean | | not null |
> description | character varying(255) | | |
> deleted | boolean | | not null | false
> is_default | boolean | | not null | false
>
> Indexes:
> "parent_pkey" PRIMARY KEY, btree (id)
> "uniq_name" UNIQUE, btree (name) WHERE deleted = false
>
> Referenced by:
> TABLE "child" CONSTRAINT "child_fkey" FOREIGN KEY (parent_id) REFERENCES parent_table(id) ON DELETE CASCADE
>
> Triggers:
> parent_changed BEFORE INSERT OR DELETE OR UPDATE OR TRUNCATE ON parent_table FOR EACH STATEMENT EXECUTE FUNCTION parent_table_changed();
>
>
> This is the trigger function
>
> CREATE OR REPLACE FUNCTION parent_table_changed() RETURNS trigger LANGUAGE plpgsql
> AS $function$
> BEGIN
> UPDATE data_sync SET last_parent_table_change=CURRENT_TIMESTAMP;
> RETURN NEW;
> END;
> $function$
>
>
> I'm trying to insert 30k rows (inside a single transaction) into the
> parent table using the following SQL (note that I came across this issue
> while debugging an application-level performance problem and the SQL I'm
> using here is similar to what the application is generating):
>
> BEGIN;
> -- ALTER TABLE public.parent_table DISABLE TRIGGER parent_changed;
> PREPARE my_insert (varchar(64), boolean, varchar(255), boolean, boolean) AS INSERT INTO public.parent_table (name,enabled,description,deleted,is_default) VALUES($1, $2, $3, $4, $5);
> EXECUTE my_insert ($$035001$$, true, $$null$$, false, false);
> EXECUTE my_insert ($$035002$$, true, $$null$$, false, false);
> ....29998 more lines
>
>
> This is the execution time I get when running the script while the trigger
> is enabled:
>
> ~/tmp$ time psql -q -Upostgres -h dbhost -f inserts.sql test_db
>
> real 0m8,381s
> user 0m0,203s
> sys 0m0,287s
>
>
> Running the same SQL script with trigger disabled shows a ~4x speed-up:
>
>
> ~/tmp$ time psql -q -Upostgres -h dbhost -f inserts.sql test_db
>
> real 0m2,284s
> user 0m0,171s
> sys 0m0,261s
>
>
> Defining the trigger as "BEFORE INSERT" or "AFTER INSERT" made no
> difference.
>
> I then got curious , put a "/timing" at the start of the SQL script,
> massaged the psql output a bit and plotted a chart of the statement
> execution times.
> To my surprise, I see a linear increase of the per-INSERT execution times,
> roughly 4x as well:
>
> While the execution time per INSERT remains constant when disabling the
> trigger before inserting the data:
>
> What's causing this slow-down ?
>
> Thanks,
> Tobias
>