Re: Largeobject Access Controls (r2460)

Takahiro Itagaki wrote:
> KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
>
>> The attached patch fixes these matters.
>
> I'll start to check it.

Thanks,

>> We have to reference pg_largeobject_metadata to check whether a certain
>> large objct exists, or not.
>
> What is the situation where there is a row in pg_largeobject_metadata
> and no corresponding rows in pg_largeobject? Do we have a method to
> delete all rows in pg_largeobject but leave some metadata?

It is a case when we create a new large object, but write nothing.

postgres=# SELECT lo_create(1234);
lo_create
-----------
1234
(1 row)

postgres=# SELECT * FROM pg_largeobject_metadata WHERE oid = 1234;
lomowner | lomacl
----------+--------
10 |
(1 row)

postgres=# SELECT * FROM pg_largeobject WHERE loid = 1234;
loid | pageno | data
------+--------+------
(0 rows)

In this case, the following two queries are not equivalent.
* SELECT oid FROM pg_largeobject_metadata
* SELECT DISTINCT loid FROM pg_largeobject

The second query does not return the loid of empty large objects.

The prior implementation inserted a zero-length page to show here is
a large object with this loid, but it got unnecessary with this
enhancement.
If we need compatibility in this level, we can insert a zero-length
page into pg_largeobject on LargeObjectCreate().
It is harmless, but its worth is uncertain.

Thanks,
--
OSS Platform Development Division, NEC
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>