Postgres Corrupted Index Error: ‘Index is not a btree’

Understanding the Error

PostgreSQL is highly acclaimed for its robustness and reliability. However, on rare occasions, due to hardware failure, file system issues or abrupt termination of the database service, certain indexes might get corrupted. A common manifestation of a corrupted index is the ‘Index is not a btree’ error, which you encounter when trying to access the corrupted index.

This error says, basically, that PostgreSQL expected a B-Tree index (the default index type), but the index structure did not meet the expected format. This error can severely impact the performance of your database and can also prevent you from accessing data altogether in the worst cases.

Possible Causes

• Unexpected system crashes
• Hardware malfunctions
• Filesystem corruption events
• Improper shutdown of the database server

Rebuild the Index

Rebuilding a corrupted index is the most straightforward way to fix the error. PostgreSQL comes with the REINDEX command which can rebuild an index using the data stored in the table.

Steps to follow:

1. Connect to your database using psql or any other PostgreSQL client.
2. Identify the corrupted index. The error message normally specifies which one is corrupted.
3. Run the REINDEX command targeting the corrupted index.

Example:

REINDEX INDEX your_corrupted_index_name;

No additional code is needed as the REINDEX command is fully capable of fixing the index in isolation.

Advantages:

• Simple and direct approach.
• No need for complex operations if the index is the only corrupted object.

Limitations:

• For large tables, the REINDEX operation can take a significant amount of time.
• Requires exclusive lock on the specific index, which may cause downtime for operations that require that index.

Restore From Backup

If reindexing does not solve the issue, or if the corrupted index has led to data inconsistency, restoring from a backup can be considered. This involves replacing the corrupted index with one from a backup, assuming the backup is not corrupted.

Steps to implement:

1. Identify the most recent backup before corruption occurred.
2. Restore the index from the backup to your database.

Restoring from a backup typically uses tools like pg_restore which are beyond SQL scripting.

Advantages:

• Ensures data consistency with a clean copy of the index.
• Good practice to have backups readily available for such scenarios.

Limitations:

• Not all setups may have a usable backup.
• Depending on the backup and recovery tool used, you might incur downtime.
• If the backup is too old, some recent data might be lost.