Subject: Re: Extremely slow DELETE with cascade foreign keys
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-performance@lists.postgresql.org
References: <20171205174914.7q4x56gii3f626zg@alvherre.pgsql>
From: Rodrigo Rosenfeld Rosas <rr.rosas@gmail.com>
Message-ID: <833f6794-3a02-ca8a-5c95-3f1bcaf686b4@gmail.com>
Date: Tue, 5 Dec 2017 16:15:14 -0200
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Em 05-12-2017 15:49, Alvaro Herrera escreveu:
> Rodrigo Rosenfeld Rosas wrote:
>> Em 05-12-2017 15:25, Tom Lane escreveu:
>>>> Normally this is because you lack indexes on the referencing columns, so
>>>> the query that scans the table to find the referencing rows is a
>>>> seqscan.
>>> Actually though ... the weird thing about this is that I'd expect to
>>> see a separate line in the EXPLAIN output for time spent in the FK
>>> trigger.  Where'd that go?
>> Yes, I was also hoping to get more insights through the EXPLAIN output :)
> It normally does.  Can you show \d of the table containing the FK?

\d highlighted_text
                                          Tabela "public.highlighted_text"
     Coluna    |            Tipo             | Collation | Nullable 
|                   Default
--------------+-----------------------------+-----------+----------+----------------------------------------------
  id           | integer                     |           | not null | 
nextval('highlighted_text_id_seq'::regclass)
  date_created | timestamp without time zone |           | not null |
  last_updated | timestamp without time zone |           | not null |
  reference_id | integer                     |           | not null |
  highlighting | text                        |           | |
Índices:
     "highlighted_text_pkey" PRIMARY KEY, btree (id)
     "highlighted_text_reference_id_idx" btree (reference_id)
Restrições de chave estrangeira:
     "fk_highlighted_text_reference" FOREIGN KEY (reference_id) 
REFERENCES "references"(id) ON DELETE CASCADE

The highlighted_text_reference_id_idx was previously missing.

begin; explain analyze delete from "references" where id=966539; rollback;
BEGIN
Tempo: 0,466 ms
                                                              QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
  Delete on "references"  (cost=0.43..8.45 rows=1 width=6) (actual 
time=2.683..2.683 rows=0 loops=1)
    ->  Index Scan using references_pkey on "references" 
(cost=0.43..8.45 rows=1 width=6) (actual time=2.609..2.612 rows=1 loops=1)
          Index Cond: (id = 966539)
  Planning time: 0.186 ms
  Trigger for constraint fk_highlighted_text_reference: time=0.804 calls=1
  Execution time: 3.551 ms
(6 registros)

Tempo: 4,791 ms
ROLLBACK
Tempo: 0,316 ms

drop index highlighted_text_reference_id_idx;
DROP INDEX
Tempo: 35,938 ms

begin; explain analyze delete from "references" where id=966539; rollback;
BEGIN
Tempo: 0,494 ms
                                                              QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
  Delete on "references"  (cost=0.43..8.45 rows=1 width=6) (actual 
time=0.112..0.112 rows=0 loops=1)
    ->  Index Scan using references_pkey on "references" 
(cost=0.43..8.45 rows=1 width=6) (actual time=0.071..0.074 rows=1 loops=1)
          Index Cond: (id = 966539)
  Planning time: 0.181 ms
  Trigger for constraint fk_highlighted_text_reference: time=2513.816 
calls=1
  Execution time: 2513.992 ms
(6 registros)

Time: 2514,801 ms (00:02,515)
ROLLBACK
Tempo: 0,291 ms

It displayed the spent on the trigger this time. How about deleting the 
field values?

begin; explain analyze delete from field_values where 
transaction_id=2479; rollback;
BEGIN
Tempo: 0,461 ms
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------
  Delete on field_values  (cost=0.43..364.98 rows=453 width=6) (actual 
time=4.732..4.732 rows=0 loops=1)
    ->  Index Scan using index_field_values_on_transaction_id on 
field_values (cost=0.43..364.98 rows=453 width=6) (actual 
time=0.137..0.949 rows=624 loops=1)
          Index Cond: (transaction_id = 2479)
  Planning time: 0.210 ms
  Trigger for constraint field_value_booleans_field_value_id_fkey on 
field_values: time=7.953 calls=624
  Trigger for constraint field_value_currencies_field_value_id_fkey on 
field_values: time=5.548 calls=624
  Trigger for constraint field_value_jurisdictions_field_value_id_fkey 
on field_values: time=6.376 calls=624
  Trigger for constraint fk_field_value_date_range_field_value_id on 
field_values: time=5.735 calls=624
  Trigger for constraint fk_field_value_dates_field_value_id on 
field_values: time=6.316 calls=624
  Trigger for constraint fk_field_value_numerics_field_value_id on 
field_values: time=6.368 calls=624
  Trigger for constraint fk_field_value_options_field_value_id on 
field_values: time=6.503 calls=624
  Trigger for constraint fk_field_value_strings_field_value_id on 
field_values: time=6.794 calls=624
  Trigger for constraint fk_field_value_time_spans_field_value_id on 
field_values: time=6.332 calls=624
  Trigger for constraint fk_references_field_value_id on field_values: 
time=7.382 calls=624
  Trigger for constraint fk_highlighted_text_reference on references: 
time=644994.047 calls=390
  Execution time: 645065.326 ms
(16 registros)

Time: 645066,726 ms (10:45,067)
ROLLBACK
Tempo: 0,300 ms

Yeah, for some reason, now I got the relevant trigger hints :) Go figure 
out why it didn't work the last time I tried before subscribing to this 
list :)

Glad it's working now anyway :)

Thanks,
Rodrigo.

>
> alvherre=# begin; explain analyze delete from pk where a = 505; rollback;
> BEGIN
> Duración: 0,207 ms
>                                                      QUERY PLAN
> ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────
>   Delete on pk  (cost=0.00..8.27 rows=1 width=6) (actual time=0.023..0.023 rows=0 loops=1)
>     ->  Index Scan using pk_pkey on pk  (cost=0.00..8.27 rows=1 width=6) (actual time=0.012..0.013 rows=1 loops=1)
>           Index Cond: (a = 505)
>   Trigger for constraint fk_a_fkey: time=201.580 calls=1
>   Total runtime: 201.625 ms
> (5 filas)
>
> alvherre=# \d fk
>           Tabla «public.fk»
>   Columna │  Tipo   │ Modificadores
> ─────────┼─────────┼───────────────
>   a       │ integer │
> Restricciones de llave foránea:
>      "fk_a_fkey" FOREIGN KEY (a) REFERENCES pk(a) ON DELETE CASCADE
>
>