Postgresql Index Only Scan Doesnt Properly Work On Group By

Question

I have a table like:

CREATE TABLE summary
(
    id serial NOT NULL,
    user_id bigint NOT NULL,
    country character varying(5),
    product_id bigint NOT NULL,
    category_id bigint NOT NULL,
    text_id bigint NOT NULL,
    text character varying(255),
    product_type integer NOT NULL,
    event_name character varying(255),
    report_date date NOT NULL,
    currency character varying(5),
    revenue double precision,
    last_event_time timestamp
);

My table size is 1786 MB (except index). In here, I've created index like below:

CREATE INDEX "idx_as_type_usr_productId_eventTime"
    ON summary USING btree
    (product_type, user_id, product_id, last_event_time)
    INCLUDE(event_name);

And my simple query looks like below:

select 
   event_name, 
   max(last_event_time) 
from summary s 
where s.user_id = ? and s.product_id = ? and s.product_type = ? 
   and s.last_event_time > '2020-03-01' and s.last_event_time < '2020-03-25'
group by event_name;

When I explain it, it looks like;

HashAggregate  (cost=93.82..96.41 rows=259 width=25) (actual time=9187.533..9187.536 rows=10 loops=1)
  Group Key: event_name
  Buffers: shared hit=70898 read=10579 dirtied=22650
  I/O Timings: read=3876.367
  ->  Index Only Scan using "idx_as_type_usr_productId_eventTime" on summary s  (cost=0.56..92.36 rows=292 width=25) (actual time=0.485..9153.812 rows=87322 loops=1)
        Index Cond: ((product_type = 2) AND (product_id = ?) AND (product_id = ?) AND (last_event_time > '2020-03-01 00:00:00'::timestamp without time zone) AND (last_event_time < '2020-03-25 00:00:00'::timestamp without time zone))
        Heap Fetches: 35967
        Buffers: shared hit=70898 read=10579 dirtied=22650
        I/O Timings: read=3876.367
Planning Time: 0.452 ms
Execution Time: 9187.583 ms

In here, everything looks fine. But when I execute it, it takes more than 10 seconds, sometime it takes more than 30 seconds.

1. In here, if I execute it without Group By, it returns so quickly like less than 2 seconds. What can be the effect of Group By? The select part is so little (like a 500 rows).
2. This table has insert/update operations with 30/per second. Can this be related with this indexing problem?

Updated:

Query Without - GroupBy:

select 
   event_name, 
   last_event_time 
from summary s 
where s.user_id = ? and s.product_id = ? and s.product_type = ? 
   and s.last_event_time > '2020-03-01' and s.last_event_time < '2020-03-25';

Explain Without - Group By:

Index Only Scan using "idx_as_type_usr_productId_eventTime" on summary s  (cost=0.56..92.36 rows=292 width=25) (actual time=0.023..79.138 rows=87305 loops=1)
  Index Cond: ((product_type = ?) AND (user_id = ?) AND (product_id = ?) AND (last_event_time > '2020-03-01 00:00:00'::timestamp without time zone) AND (last_event_time < '2020-03-25 00:00:00'::timestamp without time zone))
  Heap Fetches: 22949
  Buffers: shared hit=37780 read=12143 dirtied=15156
  I/O Timings: read=4418.930
Planning Time: 0.639 ms
Execution Time: 4625.213 ms

Answer 1

There are several problems:

• PostgreSQL had to set hint bits, which dirty the pages and cause writes.

• PostgreSQL has to fetch table rows from disk to fetch their visibility.

• PostgreSQL has to scan 80000 pages to get 87000 rows, so the index must be totally bloated.

The first two can be taken care of by running

VACUUM summary;

which is always a good idea after a bulk load, and the bloat can be cured by

REINDEX INDEX "idx_as_type_usr_productId_eventTime";