VACUUM FULL vs dropped columns
While doing some testing I noticed this, which seems somewhat perverse:
create table t();
insert into t select from generate_series(1,10000);
select 'alter table t ' || string_agg(' add column c'||x::text||' int
default ' ||x::text,',')
from generate_series(1,1000) x \gexec
create table t_dropped();
insert into t_dropped select from generate_series(1,10000);
select 'alter table t_dropped ' || string_agg(' add column
c'||x::text||' int default ' ||x::text,',')
from generate_series(1,1000) x \gexec
alter table t_dropped drop column c900;
select pg_total_relation_size('t') as size_t,
pg_total_relation_size('t_dropped') as size_t_dropped;
size_t | size_t_dropped
----------+----------------
40960000 | 40960000
(1 row)
vacuum full t;
vacuum full t_dropped;
select pg_total_relation_size('t') as size_t,
pg_total_relation_size('t_dropped') as size_t_dropped;
size_t | size_t_dropped
----------+----------------
40960000 | 81920000
(1 row)
Why does VACUUM FULL cause the size of this table with a single
dropped column (1 out of 1000) cause the table size to double?
cheers
andrew
--
Andrew Dunstan https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Andrew Dunstan <andrew.dunstan@2ndquadrant.com> writes:
Why does VACUUM FULL cause the size of this table with a single
dropped column (1 out of 1000) cause the table size to double?
VACUUM FULL will rewrite the tuples with a null bitmap where they
had none before (cf reform_and_rewrite_tuple). That's only a rather
marginal increase in the tuple size, but in this particular example,
it pushes the tuples from just under half a page to just over, so
that you go from 2 tuples/page to 1.
regards, tom lane
On Sun, Mar 11, 2018 at 9:49 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Andrew Dunstan <andrew.dunstan@2ndquadrant.com> writes:
Why does VACUUM FULL cause the size of this table with a single
dropped column (1 out of 1000) cause the table size to double?VACUUM FULL will rewrite the tuples with a null bitmap where they
had none before (cf reform_and_rewrite_tuple). That's only a rather
marginal increase in the tuple size, but in this particular example,
it pushes the tuples from just under half a page to just over, so
that you go from 2 tuples/page to 1.
Aha! Thanks for the explanation.
cheers
andrew
--
Andrew Dunstan https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services