How does postgres sort large strings?

Hi!

I thought that the sorting values are stored entirely in work_mem, but in fact it works somehow differently.
Can anyone suggest how this works?

For example, I created this 512MB incompressible file and test table:

tr -dcs '[:print:]' '[:print:]' < /dev/urandom | tr -d '"'\' | dd bs=1K count=512K of=asciidump

create unlogged table t1 (v text);
insert into t1 select pg_read_file('asciidump') from generate_series(1, 10);

select pg_column_size(v), octet_length(v) from t1 limit 1;
pg_column_size | octet_length
----------------+--------------
536870912 | 536870912

set work_mem to '64MB';

Now I think that 64MB is not enough to sort such large values and postgres will use temp files,
but in fact it does not.

select temp_files, temp_bytes from pg_stat_database where datname = current_catalog;
temp_files | temp_bytes
------------+------------
0 | 0

explain (analyze,verbose,buffers) select v from t1 order by v;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------
Sort (cost=94.38..97.78 rows=1360 width=32) (actual time=6433.138..6433.140 rows=10 loops=1)
Output: v
Sort Key: t1.v
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=543881 read=679794 written=118012
-> Seq Scan on public.t1 (cost=0.00..23.60 rows=1360 width=32) (actual time=0.007..0.009 rows=10 loops=1)
Output: v
Buffers: shared hit=1
Planning Time: 0.035 ms
Execution Time: 6433.155 ms

> Sort Method: quicksort Memory: 25kB

select temp_files, temp_bytes from pg_stat_database where datname = current_catalog;
temp_files | temp_bytes
------------+------------
0 | 0

WOW! How does it work?! :-)

--
Sergey Burladyan