drop table if exists tbl;
create table tbl(a serial, b int not null, c text not null);
insert into tbl (b,c) select i, random()::text from generate_series(1, 100000000) i;
alter table tbl add primary key (a);
analyze tbl;
checkpoint;
\timing

-- new code
--\copy (delete from tbl where a <= 5000000 returning *) to foo.dat

-- old code
--SET work_mem TO '1GB';
--\copy (with t as (delete from tbl where a <= 5000000 returning *) select * from t) to foo.dat