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

> How much does the function-ed UPDATE statement take?

INSERTs into the parent table are taking ~0.1 ms/INSERT at the beginning
of the transaction and this increases to ~0.43 ms after 30k INSERTs.
Without the trigger, INSERTs into the parent table consistently take
around 0.05ms.

Regards,
Tobias

>
>
> Regards,
> Ninad Shah
>
> On Sat, 17 Jul 2021 at 02:57, Tobias Gierke
> <tobias(dot)gierke(at)code-sourcery(dot)de
> <mailto: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
>