Thread: ERROR: "Database 'products', OID nnn, has disappeared from pg_database"

> Currently, we have been having a problem with some kind of corruption of
> the pg_database file.  We are using the database on a limited number of
> writes storage medium.  (Compact Flash Card)  So that we don't always
> write to the flash, and because we don't care about some of the data, part
> of the database is kept in RAM on a ram file system.  We have two
> databases on disk.  One database called config, the other the standard
> template1.  These are kept in /var/lib/pgsql_base_slave.  We have a
> properly init'ed database in /var/lib/pgsql/.  When postgresql starts up
> via the init scripts, we create a RAM file system on /var/lib/pgsql/base/,
> and then we create symlinks to the config and template1 databases.
>
>     The next set of steps is where we sometimes run into trouble.  At
> this point in the init script, we start up the postmaster.  Next we do a
> createdb products, where products is the actual name of the database.
> This works fine.  Next, we attempt to access the database using psql to
> create a set of tables.  This is where sometimes we run into a problem.
> Normally, this works fine, but sometimes we get the error
>
> "Database 'products', OID nnn, has disappeared from pg_database"
>
> where nnn is some number, and forever afterwards we can't access a
> database with that name.  Removing and recreating the database doesn't
> help.  Neither does vacuuming it.  If we delete the whole pgsql directory,
> and recreate it, then use initdb on it, everything is fresh and working
> again, but obviously this isn't a good solution, since we have to recreate
> all the databases, tables, and data.
>
>     On the shutdown side, we do a deletedb to remove the products
> database before we shutdown the postmaster.  Once that is complete, we do
> a shutdown.
>
> If you need a short list of the startup procedure that is above, here it
> is in a psuedocode fashion.
>
> mount ram drive.
> make symlinks to config and template1 databases
> startup postmaster
> createdb products
> create some tables in products
>
> ... normal operation ...
>
> ... shutdown process
> dropdb products
> shutdown postmaster.
>
>
>     I am assuming that some kind of corruption has occured within
> pg_database file, but I don't have the expertise to find where the problem
> is occuring.  Any help you can provide would be greatly appreciated.
>
>     I am including the init script used.
>  <<postgresql.txt>>
>
> Sandy Barnes
> Honeywell
> Business & General Aviation Engineering
> One Technology Center
> 23500 West 105th Street
> Olathe, KS.     66061
> tel         913.712.2027
> fax        913.712.1347
> email    sandy.barnes@honeywell.com
>

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"Barnes, Sandy (Sandra)" <Sandy.Barnes@Honeywell.com> writes:
>> Normally, this works fine, but sometimes we get the error
>>
>> "Database 'products', OID nnn, has disappeared from pg_database"
>>
>> where nnn is some number, and forever afterwards we can't access a
>> database with that name.  Removing and recreating the database doesn't
>> help.  Neither does vacuuming it.

Try vacuuming pg_database (after connecting to template1 or the other
always-there db).  7.1 will be a little smarter about this, but the
setup you describe is still pretty risky IMHO.  You have a nonvolatile
reference in pg_database to a volatile database in the RAM filesystem,
so a system crash will leave you with a dangling reference...

            regards, tom lane