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IDS Forum

Re: More Smart Large Object Problems

Posted By: Andreas Legner
Date: Saturday, 3 December 2016, at 7:04 p.m.

In Response To: More Smart Large Object Problems (DAVID GROVE)

Hi David,

it's not quite as you think: if two rows, in same or in different=20
tables, refer to the same sblob, this typically would be the result of a=20
copy from first row to second row, either whole row or only the sblob=20
field. Alternatively you'd have created the sblob using specific API and =

then assigned it the two rows. If you just inserted the same photo into=20
two rows, you'd naturally have got two separate copies.

Now, given an sblob referred to by two rows/tables, if you update this=20
sblob in one of these rows/tables, you't not modify that specific sblob,=20
but instead you'd create a new sblob, update that specific row/table's=20
sblob pointer to point to the new sblob, and decrement the old, unchanged=20
sblob's reference count.
So the problem you're referring to is not new and not introduced by the=20
duplication of what used to be single sblobs.

I think the idea of 1-to-n here really only was storage savings, but never =

any guaranteed data sharing.

HTH,
Andreas

From: "DAVID GROVE" <david.grove@alaska.gov>
To: ids@iiug.org
Date: 02.12.2016 23:46
Subject: More Smart Large Object Problems [38222]
Sent by: ids-bounces@iiug.org

Informix 12.10=20
Solaris 10=20

Until recently, I have not been too involved with sblobs. Now that I have=20
to=20
deal with them, I am finding continual problems-- both theoretical and=20
administrative/management (as in "lack of")-- with these things. Basic=20
violations of fundamental Relational Theory appears to be rampant.=20

Recently, I wanted to replace some drives and as a result, needed to=20
replace a=20
smaller sbspace with a larger one on a different drive.=20

I had an existing 8GB sbspace. I created a new 30GB sbspace. Then, using=20
the=20
query helpfully supplied by Mr. Renaut (=20
http://members.iiug.org/forums/ids/index.cgi/read/38162 ) to identify all=20
tables with sblobs, and then applying the method described in the IBM=20
Technote=20
"How to move a tables's sblobs" to all the tables identified by the query, =

I=20
moved all the sblobs to the new sbspace.=20

The sblobs, which occupied just a little less than 8 GB in the old=20
sbspace,=20
now require 12GB in the new sbspace.=20

I encountered this expansion before when doing dbimport and dbexport, and=20
the=20
explanation is that when multiple tables reference the same sblob (i.e.,=20
have=20
a row and column with the same LO HANDLE), the actual sblob is exported=20
(and=20
imported) twice.=20

Now, I did not do an export/import to move the sblobs. Just used the=20
method=20
described in the Technote (it uses the LOCOPY function.)=20

Could it be that this is the exact same problem as when using=20
dbexport/dbimport?=20

If so, this is, IMO, grossly wrong. A Relational Database should not=20
denormalize data, of its own accord. If that is what happened, then data=20
identity and referential integrity for those sblobs is destroyed. That=20
means=20
there WILL BE deletion anomalies (and insertion anomalies and update=20
anomalies), as dexplicated historically at great length and depth by Codd=20
&=20
Date. (OK, developed by Codd, explained by Date.) When someone deletes one =

of=20
these photos, another table that references its own copy of the same photo =

will not have its photo deleted. There will be multiple photos for the=20
same=20
individual. Users won't know which is correct. Etc. Etc.=20

Someone please tell me I am totally misunderstanding this and that there=20
is=20
some other explanation (and a remedy).=20

What can I do to make Informix honor referential integrity for sblobs. I=20
fear=20
that, since the deed is done, I have a situation that is not repairable. I =

mean, if one LO HANDLE (being used in multiple tables to reference a=20
single=20
sblob) has now become multiple LO HANDLEs, each referencing a different=20
instantiation of the same sequence of bits, then it can't be "walked=20
back". I=20
fear big trouble.=20

I truly hope I am going off half cocked, and am misunderstanding the=20
situation. Someone slap me down by telling me I am totally off base. But,=20
if=20
not, what is the solution to the problem of the same object being=20
replicated=20
with a different LO HANDLE, when using the Informix-supplied, built-in=20
functions? Is there any way to return our multiple tables that reference=20
the=20
same object to having the same LO HANDLE, referencing a SINGLE instance of =

the=20
object?=20

Thank you.=20

DG=20

***************************************************************************=
****=20

Forum Note: Use "Reply" to post a response in the discussion forum.=20

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