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Understanding the behavior of autovacuum with invalid databases

A new value -2 is introduced into the datconnlimit column in the pg_database catalog to indicate databases that have been interrupted in the middle of the DROP DATABASE operation as invalid.

This new value is available from the following RDS for PostgreSQL versions:

Invalid databases do not affect autovacuum's ability to freeze functionality for valid databases. Autovacuum ignores invalid databases. Consequently, regular vacuum operations will continue to function properly and efficiently for all valid databases in your PostgreSQL environment.

Monitoring transaction ID

The age(datfrozenxid) function is commonly used to monitor the transaction ID (XID) age of databases to prevent transaction ID wraparound.

Since invalid databases are excluded from autovacuum, their transaction ID (XID) counter can reach the maximum value of 2 billion, wrap around to - 2 billion, and continue this cycle indefinitely. A typical query to monitor Transaction ID wraparound might look like:

SELECT max(age(datfrozenxid)) FROM pg_database;

However, with the introduction of the -2 value for datconnlimit, invalid databases can skew the results of this query. Since these databases are not valid and should not be part of regular maintenance checks, they can cause false positives, leading you to believe that the age(datfrozenxid) is higher than it actually is.

Adjusting the monitoring query

To ensure accurate monitoring, you should adjust your monitoring query to exclude invalid databases. Follow this recommended query:

SELECT
    max(age(datfrozenxid))
FROM
    pg_database
WHERE
    datconnlimit <> -2;
     

This query ensures that only valid databases are considered in the age(datfrozenxid) calculation, providing a true reflection of the transaction ID age across your PostgreSQL environment.

Resolving invalid database issue

When attempting to connect to an invalid database, you may encounter an error message similar to the following:

postgres=> \c db1
connection to server at "mydb.xxxxxxxxxx.us-west-2.rds.amazonaws.com" (xx.xx.xx.xxx), port xxxx failed: FATAL:  cannot connect to invalid database "db1"
HINT:  Use DROP DATABASE to drop invalid databases.
Previous connection kept
     

Additionally, if the log_min_messages parameter is set to DEBUG2 or higher, you may notice the following log entries indicating that the autovacuum process is skipping the invalid database:

       
2024-07-30 05:59:00 UTC::@:[32000]:DEBUG:  autovacuum: skipping invalid database "db6"
2024-07-30 05:59:00 UTC::@:[32000]:DEBUG:  autovacuum: skipping invalid database "db1"
       
     

To resolve the issue, follow the HINT provided during the connection attempt. Connect to any valid database using your RDS master account or a database account with the rds_superuser role, and drop invalid database(s).

SELECT
    'DROP DATABASE ' || quote_ident(datname) || ';'
FROM
    pg_database
WHERE
    datconnlimit = -2 \gexec