1
The PostgreSQL
uses the model MVCC (Multiversion Concurrency Control)
for control of transactions and in it the Multixact Ids are used for lock control records. It is implemented as a 32-bit integer. Because of this limit a server running many transactions can suffer the "transaction Wraparound ID" when the counter returns to zero and all transactions effectively preceding are seen as future.
The solution to prevent this is to evaluate when it is necessary to run a VACUUM (https://www.postgresql.org/docs/current/app-vacuumdb.html). Postgresql also implements an optional but recommended Feature called autovacuum (https://www.postgresql.org/docs/current/runtime-config-autovacuum.html) which periodically and automatically carries out the assessment, by enabling it.
Just to remind: version 9.1 has stopped receiving support and updates since 2016, evaluate upgrading to a newer version, the current version is 12.0.
Look at this post talking about --- http://avds.eti.br/rediscover-of-computers/linux/como-instalar-o-postgre-e-agadmin-no-linux/217/
– Luara Amaral