On 9 Jan 2012, at 18:57, Radosław Smogura wrote:
> In real world BLOBs are transfered as references, and those references are
> managed in way as the trigger does. Nacked PG doesn't support deletion, Oid
> is
> universal type so it can't be used by GC approach, unles collector will know
> which Oid is LOB oid.
What do you mean by "nacked"?
You can unlink lob's, there's your support for deletion.
> Oid is like void*, it's abstarct pointer. If you get void* you don't know if
> data referenced by it represent person row, or car row, you don't know if
> void* is even reference or just 64 bit number. Current implementation is not
> type safe. You can't just write UPDATE TABLE x SET blob = 'aadfasfasfda'
> which
> in current times should be supported, but you may write (if are not fully
> familiar with db) UPDATE table X set varchar_d = blob_column;
That's easy to remedy, similar to how most implementations in C don't use
straight void pointers. In C you'd just typedef them to something meaningful:
typedef blob oid;
Similarly you can wrap them in a domain in PG:
create domain blob as oid;
It would be cool if that would allow to add an FK-constraint to the oid in
pg_largeobject to that domain, but alas, that isn't possible in my version (I'm
a bit behind with pg 8.4).
I agree that it would be nice if PG provided a built-in type for lobs (blob's
are a subdivision of those), especially if that would also handle the reference
to pg_largeobject.
> In fact LOB's id may be stored even as varchar. So true is that PG supports
> LOBs, but due to missing functionality LOBs are quite hard to manage. It's
> like car withot steering wheel - you may drive, but it's little bit hard.
That's probably just because PG knows to cast that varchar to something
compatible with oid's. I suspect that in recent versions that cast may not be
allowed anymore though.
And remember, SELECT 'my explicit string value'; does not in fact denote a
string value, but a literal. While the query is still in SQL notation (meaning
until the query parser is done with it), everything is text.
The way I understand it, a literal gets a meaningful type once it is compared
to a value of a known type (typically from a column) or once it gets cast to a
type explicitly. If that never happens, I expect that the literal will not be
converted to any type and stay the text value that it was in the SQL query
string.
This is probably documented, but I don't have time to dig into the manuals
right now.
Alban Hertroys
--
Screwing up is an excellent way to attach something to the ceiling.
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