Dear Members,

In this thread I asked an doubt , that create table
with Foreign key does not works when there are
multiple connections. Then we concluded that it is
standard behaviour of firebird that "when there are
multiple connection meta data update is not allowed ,
even there is no "interesting" transaction.

But today when I kept working with firebird , I
found that drop table (including tables which has
foreign key) got dropped successfully when there are
multiple connection. I hope even this involves meta
data updates. Unlike create table this does not thrown
any errror. So please do clarify me whether is this
the standard behaviour. Since I feel this
functionality as deviation from create table meta data
update functionality , I have raised this in the
forum.

S.Karthick

--- Helen Borrie <helebor@...> wrote:

> At 11:06 PM 29/08/2004 -0700, you wrote:
> >Dear Team ,
> >
> > We are using mysql in our oraganisation. We are
> >anlysing the migration to firebird. But one
> blocking
> >factor is meta data update. As of now when we have
> >multiple connection and when we try to create
> table
> >with "foreign key" it throws meta data error.
> > On seraching the knowledge base we found the
> >below link,
> >
> >
>
>http://www.ibphoenix.com/main.nfs?a=ibphoenix&l=;KNOWLEDGEBASE;ID='6'
> >
> > Can anyone please let us know why Firebird has
> >such a behavior?
>
> The knowledgebase links you found explain it. You
> don't see it in MySQL
> because MySQL doesn't have referential integrity
> support or transactions.
> Its ability to protect itself from assault on its
> metadata is pretty flimsy.
>
> To put it simply, you can't change the metadata of
> an object if it is in use.
>
> [...]
>
> >But still I'm unable to understand the exact reason
> >for the Firebird to have this constraint.
>
> Firebird metadata is stored in tables inside the
> database; changing
> metadata changes the data stored in these tables.
> Since objects that are
> using that metadata have to hold a consistent view
> of it during a
> transaction, a metadata update will be refused if it
> affects any
> "interesting" transactions involving the affected
> objects and their
> dependencies. ("interesting" = any transaction that
> is in any state except
> Committed).
>
> You think that opening a table and looking at its
> data is not "accessing"
> it. But, yes, it is "accessing" it. Everything you
> do in Firebird happens
> in transactions. So, if one transaction has
> selected some data from an
> object, that object, and any objects dependent on
> it, are in use. That
> situation won't change until that transaction is
> committed.
>
> Many of the third-party tools, by default, run their
> "browser-style"
> queries in transactions that are never subjected to
> a "hard" commit. This
> mechanism, known as "Commit Retaining", commits work
> but doesn't release
> the objects involved.
>
> >The other databases we used like MYSQL , Oracle
> supports this functionality.
>
> Apples and oranges. MySQL doesn't implement
> transaction isolation or
> referential integrity, so online metadata changes
> are possible (and so is
> the resulting database corruption...) Oracle and
> Firebird do have
> transactions and RI. Oracle supports deferred
> metadata changes under some
> conditions and blocks new transactions involving
> objects that have pending
> deferred metadata changes; Firebird (mostly)
> doesn't supported deferred
> metadata changes (for stored procedures it can,
> under some conditions).
>
> > I believe this will add more value to firebird
> to support this
> > behavior. Firebird team: is there any plan to
> support this
>
> Deferred metadata changes have been considered from
> time to time. It's not
> a priority currently...nice to have for some,
> perhaps, but it's difficult
> to see any great "value" in it. It's not in the
> plans for Firebird 2 or 3,
> that I'm aware of.
>
> > or is there any workaround to support
> multiconnection meta data
> > update?.
>
> No. Keep metadata updates separated from user work
> and all will be sweet.
>
>
> > I'd appreciate any links/document of firebird
> >architecture/design that explains this behavior.
>
> Search the knowledgebases for "transaction
> isolation" and "referential
> integrity"; also "multi-generational architecture",
> which will help
> understanding of how it all hangs together. For
> someone not aware of them,
> another keyword to search would be "dependencies".
>
> ./heLen
>
>

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