Re: Matview size - space increased on concurrently refresh

P.S.: I am on postgres 11.3

Il giorno ven 12 lug 2019 alle ore 16:32 Nicola Contu <
nicola(dot)contu(at)gmail(dot)com> ha scritto:

> Hello,
> we noticed with a simple matview we have that refreshing it using the
> concurrently item the space always increases of about 120MB .
> This only happens if I am reading from that matview and at the same time I
> am am refreshing it.
>
> cmdv3=# SELECT
> pg_size_pretty(pg_relation_size('public.matview_nm_connections'::regclass));
> pg_size_pretty
> ----------------
> 133 MB
> (1 row)
>
> cmdv3=# refresh materialized view matview_nm_connections;
> REFRESH MATERIALIZED VIEW
> cmdv3=# SELECT
> pg_size_pretty(pg_relation_size('public.matview_nm_connections'::regclass));
> pg_size_pretty
> ----------------
> 133 MB
> (1 row)
>
> cmdv3=# \! date
> Fri Jul 12 13:52:51 GMT 2019
>
> cmdv3=# refresh materialized view matview_nm_connections;
> REFRESH MATERIALIZED VIEW
> cmdv3=# SELECT
> pg_size_pretty(pg_relation_size('public.matview_nm_connections'::regclass));
> pg_size_pretty
> ----------------
> 133 MB
> (1 row)
>
>
> Let's try concurrently.....
>
> cmdv3=# refresh materialized view CONCURRENTLY matview_nm_connections;
> REFRESH MATERIALIZED VIEW
> cmdv3=# SELECT
> pg_size_pretty(pg_relation_size('public.matview_nm_connections'::regclass));
> pg_size_pretty
> ----------------
> 261 MB
> (1 row)
>
>
> So the matview is not really used and it does not have anything strange
> but that matview growth to 12GB as we refresh it once an hour.
> It had the free percent at 97%.
> I understand with concurrenlty it needs to take copy of the data while
> reading, but this seems to be too much on the space side.
>
> Is this a bug? Or is there anyone can help us understanding this?
>
> Thanks a lot,
> Nicola
>