Re: Column storage (EXTERNAL/EXTENDED) settings for bytea/text column

On Mon, Apr 11, 2011 at 03:19:23PM -0700, Joel Stevenson wrote:
> I'm trying to do some comparisons between the EXTERNAL and the EXTENDED storage methods on a bytea column and from the outside the setting doesn't appear to affect the value stored on initial insert, but perhaps I'm looking at the wrong numbers. If I create two new tables with a single bytea column and set one of them to external storage, then insert an existing bytea value from another table into each one, they appear to be of exactly the same size. This is using PG 9.0.3 on Debian Lenny, using the backports-sloppy deb package of PG 9.
>
> (I've verified that the first table has "extended" storage via pg_attribute and that the second table has external.)
>
> create table obj1 ( object bytea );
> create table obj2 ( object bytea );
> alter table obj2 alter column object set storage external;
> insert into obj1 ( object ) select object from serialized_content where id = 12345;
> insert into obj2 ( object ) select object from obj1;

If the value that shows up for insertion is already compressed, EXTERNAL storage
will not decompress it. Change this line to

insert into obj2 ( object ) select object || '' from obj1;

to observe the effect you seek.

Given the purpose of EXTERNAL storage, this might qualify as a bug.

> select pg_total_relation_size('obj1') as o1, pg_total_relation_size( (select reltoastrelid from pg_class where relname = 'obj1' ) ) as otoast1, pg_total_relation_size('obj2') as o2, pg_total_relation_size( (select reltoastrelid from pg_class where relname = 'obj2' ) ) as otoast2;
> o1 | otoast1 | o2 | otoast2
> -------+---------+-------+---------
> 65536 | 57344 | 65536 | 57344

> Can I use the relation size like this to determine whether or not compression is happening for these toast columns? If not, is there a way that I can confirm that it is or isn't active? The results appear to be similar for text columns.

Yes; the sizes you're seeing through that method should be accurate.

nm