Re: can't shrink relation

Sabin Coanda wrote:
> sorry for the previous incomplete post. I continue with the log:

Not really a performance question, this. Perhaps general/admin lists
would be better next time. No matter...

> NOTICE: relation "pg_shdepend" TID 11/1: DeleteTransactionInProgress
> 2657075 --- can't shrink relation
> NOTICE: relation "pg_shdepend" TID 11/2: DeleteTransactionInProgress
> 2657075 --- can't shrink relation
> .....
> NOTICE: relation "pg_shdepend" TID 36/93: DeleteTransactionInProgress
> 2658105 --- can't shrink relation
>
> What happen ? What I have to do ?

This is where having a copy of the source pays off. cd to the top-level
of your source and type:
find . -type f | xargs grep 'shrink relation'
Amongst the translation files you'll see .../backend/commands/vacuum.c

A quick search in there reveals...

case HEAPTUPLE_DELETE_IN_PROGRESS:
/*
* This should not happen, since we hold exclusive lock on
* the relation; shouldn't we raise an error? (Actually,
* it can happen in system catalogs, since we tend to
* release write lock before commit there.)
*/
ereport(NOTICE,
(errmsg("relation \"%s\" TID %u/%u: DeleteTransactionInProgress %u
--- can't shrink relation",
relname, blkno, offnum, HeapTupleHeaderGetXmax(tuple.t_data))));
do_shrinking = false;

So - it's wants to shrink a table but there is a delete in progress so
it can't do so safely. This shouldn't happen unless it's a system table,
and checking your error message, we're looking at pg_shdepend which is
indeed a system table.

> I notice that I don't get such messages when I run just VACUUM without FULL
> option.

That's because VACUUM doesn't reclaim space, it just marks blocks as
available for re-use. If you insert 2 million rows and then delete 1
million, your table will have 1 million gaps. A vacuum will try and
track those gaps (see your "free space map" settings in postgresql.conf)
whereas a vacuum-full will actually move rows around and then shrink the
size of the file on-disk once all the gaps are together at the end of
the file.

A vacuum full needs to lock the table, since it's moving rows around.

HTH

--
Richard Huxton
Archonet Ltd