transaction ID wrap limit is 2147484146 Start Postgresql with error in Logfile

Hi,
we are new-new on PostgreSQL,
we are making a benchmark on a Database of 300GB, and postgresql 8.1 on
AIX 5.3
after having loading the Database, creating indexes , executing the
analyze commands
we have launched a select query, the elapsed time for this query was 13
hours,
after this execution we fech the logfile and we have found the following
message :
transaction ID wrap limit is 2147484146, limited by database "postgres"

these parameters are set
export PGDATA=/INST/PG81
export PGUSER=pg_810
export pg_810=/INST/TMP

We have 3GB of memory and put these settings :

shared_buffers = 12288 # min 16 or max_connections*2,
8KB each
work_mem = 12288 # min 64, size in KB
#maintenance_work_mem = 49152 # min 1024, size in KB
maintenance_work_mem = 262144 # min 1024, size in KB, 256M

We stopped the instance and started again and the message still appears:

We don't know if it is an informatio message or an error message .
We don't understand why the message is like we will be on another PGDATA
"postgres" ,
but we are on the right one.

Someone can help us ?
Thanks
regards