On Mon, 9 Jan 2012 00:40:08 +0100, Stefan Keller wrote:
2012/1/8 Radosław Smogura <rsmogura@xxxxxxxxxxxxxxx> wrote:
Not quite, PostgreSQL doesn't have LOB, nor OID type that is only
reference> to LOB. In fact, BLOB behaviour in JDBC> is just thin
wrapper for this what is missing in PostgreSQL - BLOBs.
I can't follow: PostgreSQL has bytea and LO. I'm ok with bytea too
but
I'd like to get the job done with LO too.
JDBC offers both:
http://jdbc.postgresql.org/documentation/head/binary-data.html
I tried to summarize the characteristics here:
http://www.gis.hsr.ch/wiki/PostgreSQL_-_Binary_Large_Objects
There's an important principle that the code which "allocates"
resources is also responsible to release it.
AFAIK in this case it's JDBC which choses to use LO (which creates
pg_largeobjects entries) and it's therefore also JDBC which has to
clean up.
Hmm we have really different point of view. JDBC driver is just
interface which translates JDBC "commands" to database commands. JDBC
driver is not database. So responsible for this is database, but
database doesn't have "real" BLOBs, this what is made in PG JDBC driver
is just "not perfect" way to add this functionality to PostgreSQL. To
make complete system You need PostgreSQL, JDBC driver, and this trigger.
From reasons I mentioned before it's quite hard for JDBC driver to
implement deallocation, because driver will be in need to ask and to
"know" for OIDs to unlink. This could made from driver database which
need to parse queries, manages db structure etc, and this will be
executed twice in driver and in database - this is ineffective. Just for
example driver may support this for "DELETE FROM", but what will be if
deletion will be result of calling stored procedure? What if stored
procedure will be created in language different then pgSQL (driver still
may try to download it), but if this will be C, or any other? JDBC
driver must be universal and fully "compatible" with PG functionality,
as it may be used only as "helper" for presentation layer, true
processing logic may be somewhere e. g. legacy system for which we add
web interface.
Because of this JDBC driver can't expose desired behavior, even if
everyone agrees that current behavior is unperfect.
Only PG server may do this, or Hibernate for example by new annotation
@MangedPgLob.
Regards,
Radek
Yours, Stefan
P.S. Just in order get some more insight I also tried to forward this
to the JDBC list (which currently seems to have problems accepting
new
subscriptions).
2012/1/8 Radosław Smogura <rsmogura@xxxxxxxxxxxxxxx>:
On Sun, 8 Jan 2012 21:57:37 +0100, Stefan Keller wrote:
Thanks, Radosław, for the clarification.
2012/1/8 Radosław Smogura <rsmogura@xxxxxxxxxxxxxxx> wrote:
3. pg_largeobjects is system table, hibernate do not creates it.
4. Trigger based approach is good for this, but You need to do
this
mannualy
That's exactly my point:
Since JDBC manages creation of OID and pg_largeobjects it's also
JDBC
which is responsible for update/delete of rows in pg_largeobjects.
As the user expects, the only thing JDBC would have to do is to
issue
lo_unlink() when rows are updated or deleted.
Thus, it's currently a bug in the JDBC driver. And whatever the
solution is, it needs to be mentioned in the JDBC docs.
Not quite, PostgreSQL doesn't have LOB, nor OID type that is only
reference
to LOB. In fact, BLOB behaviour in JDBC
is just thin wrapper for this what is missing in PostgreSQL - BLOBs.
It was
build form available parts. In addition OID type may be used and
it's used
as the system id or may be used as just some kind of row id - all
types,
tables, sequences etc has OID. You may create table with "WITH OIDS"
clause.
You may use OID data type just as replacement for (unsigned) int, so
JDBC
can create LOB but it can't decide if given field in row is
reference to LOB
or e.g. table and what with statements "DELETE WHERE date > "?
In fact JDBC driver is so "stupid" that if you will call getBytes or
getBlob
on any column with Oid it will ask for LOB.
Hibernate knows this and it does what is best - calls standard BLOB
interface, and creates table with Oid column.
And here again round trip, in case of deletion only Hibernate may
delete
given LOB because only Hibernate and You knows that any value in Oid
column
will reflect LOB - JDBC driver doesn't "knows " this, but... oids
may be
shared because those are only numbers, not all tables may be covered
by
Hibernate, and assuming huge imagination, someone may encode OID by
adding
1, may store it as long, etc. I know it's quite popular that DB
schema comes
from entities, but not always You have such flexibility and
sometimes You
create Entities for particular schema.
So, as You see only this trigger approach is currently (universally)
the
best way. If this is that module I think about it's just trigger
which calls
unlink on replace or deletion - in fact You may write own without
any
problems. Those are few lines only.
5. If you want to use bytea use
@Type(type="org.hibernate.type.PrimitiveByteArrayBlobType") (I
think you
should remove @Lob too) on your field.
In fact, this annotation syntax usage looks like the Hibernate
mapping
for PostgreSQL could be enhanced.
You have right, but Hibernate team will be in need to add auto
deletion for
bulk updates, too. PostgreSQL isn't still so popular it's worth of
it (and
they have no so much developers bug report may stand for months
without any
comment). Look how many peoples ask for Lobs. It means no one needs
true LOB
- true LOB large object stored outside table/row data space, to
allow out of
statements operation like streaming, partial updates etc. This is my
definition of LOB, because this is idea of LOB. I think only DB2
stores LOBs
in row, PostgreSQL do not make this but adverts this bytea ugly
approach.
Personally, I create app which captures images from WebCam (like
You) - just
frames not movies. From above reason I wanted to move to bytea, but
due to
changes (wired instability and leak of backward compatibility) I
still have
Oid. Because I have only two tables for Lobs I have garbage
collection
simple script.
Best regards
Radek
Yours, Stefan
2012/1/8 Radosław Smogura <rsmogura@xxxxxxxxxxxxxxx>:
On Sun, 8 Jan 2012 18:08:09 +0100, Stefan Keller wrote:
I'd like to backup my statement below regarding in JDBC driver
from
PostgreSQL:
When storing fields of type BLOB it inserts the binary string in
system table pg_largeobject (via user table). But when rows in
user
table get updated or deleted it does not update nor delete
corresponding rows in table pg_largeobject.
That's really a bug!
Fortunately there's a solution indicated in the official docs
(http://www.postgresql.org/docs/9.1/static/lo.html)
To me, something like this should be implemented before hand in
JDBC
driver.
And in any case there should be a bold note about this in the
JDBC
docs
(http://jdbc.postgresql.org/documentation/head/binary-data.html )
Yours, Stefan
2012/1/6 Stefan Keller <sfkeller@xxxxxxxxx>:
Hi,
I run into a nasty behavior of current PostgreSQL JDBC.
I maintain images (from Webcams). In the Java and Hibernate
(JPA) code
I specified a @Lob annotation on class MyData and a
attribte/data type
"byte[] mydata;". Hibernate then generates two tables in
PostgreSQL,
one called MyData with a column mydata of type oid and an
internal one
called pg_largobjects (which contain foreign keys to the oid).
That's
also explained in the JDBC docs [1], saying "PostgreSQL provides
two
distinct ways to store binary data. Binary data can be stored in
a
table using the data type bytea or by using the Large Object
feature
which stores the binary data in a separate table in a special
format
and refers to that table by storing a value of type oid in your
table."
Now, when replacing the images (few hundred) with new ones using
Java,
pg_largeobjects grew constantly until the file system run out of
space. So old image data did'nt get released! This is to me a
bug
because the user/programmer must (and should) assume that there
is a
strict 1:1 relationship between generated table MyData and its
LOB
column data (stored in pg_largeobjects).
=> I finally found the supplied module 'lo' [2] which releases
detached records. Is this the recommended way to resolve this
problem?
Searching for explanations I found a ticket HHH2244 [3] which
was
closed by the Hibernate team without action referring to the
JDBC
Spec. which says: "An SQL BLOB is a built-in type that stores a
Binary
Large Object as a column value in a row of a database table".
=> In other words: The PostgreSQL JDBC team should take action
on this
but didn't until now, right?
There is another issue about "PostgreSQL and BLOBs" [4]. First
it
cites PostgreSQL JDBC docs [1]. The thread [4] ends somehow too
in a
'deadlock' concluding "...the PostgreSQLDialect (as of 3.5.5)
needs
to change not to use MaterializedBlobType until the Postgres
(JDBC)
team changes their driver (which does not seem to have happened
in the
last 6 years)."
=> Any solutions or comments form insiders on this?
Yours, Stefan
[1]
http://jdbc.postgresql.org/documentation/head/binary-data.html
[2] http://www.postgresql.org/docs/current/interactive/lo.html
[3] https://hibernate.onjira.com/browse/HHH-2244
[4] http://relation.to/Bloggers/PostgreSQLAndBLOBs
This is common approach for PostgreSQL and some proxy of real life
Blobs.
One time I submitted bug about this to Hibernate. But step by
step.
1. BLOBs are... Large Objects, they are stored as reference
because those
objects are large, if you will store this objects as bytea then
select *
will return all large data. It may not be comfortable not only to
download
few GB of data, but to keep this on stack too. From your
perspective it
doesn't matters because you put it in byte[]. But if You will keep
e.g.
CD-ROM images then it's much more better to use streaming approach
then
bytea[]. More over due to some security JDBC driver will at least
double
memory consumed by bytea.
2. Specifying hibernate data type as bytea do not resolve problems
because
it will still use LOB approach.
3. pg_largeobjects is system table, hibernate do not creates it.
4. Trigger based approach is good for this, but You need to do
this
mannualy
5. If you want to use bytea use
@Type(type="org.hibernate.type.PrimitiveByteArrayBlobType") (I
think you
should remove @Lob too) on your field.
Regards,
Radosław Smogura
--
Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
