PostgreSQL Pg_upgrade Fails With Unlogged Table And Logged Sequence Bug 18562

by StackCamp Team 78 views

Hey everyone! Today, we're diving deep into a recurring bug that's been causing headaches for PostgreSQL users attempting to upgrade their databases. Specifically, we're talking about bug #18562, which surfaces when using pg_upgrade to migrate from older versions (like 14.12) to newer ones (such as 15.7 or 16.3). This issue arises in scenarios involving unlogged tables paired with logged sequences. Let's break down the problem, the reproduction steps, and what it all means for your PostgreSQL upgrades.

Understanding the Issue: The Unlogged Table and Logged Sequence Conundrum

So, what's the big deal with unlogged tables and logged sequences? In PostgreSQL, unlogged tables are exactly what they sound like – tables where data isn't written to the write-ahead log (WAL). This makes them faster for write operations, but also means they aren't crash-safe. If your database crashes, unlogged tables are truncated. On the flip side, logged sequences do write to the WAL, ensuring their state is preserved across crashes. Before PostgreSQL 15, sequences associated with unlogged tables were, by default, still logged. This combination, while functional in older versions, creates a snag during pg_upgrade in newer versions, specifically when using binary upgrade mode.

The core of the problem lies in how pg_upgrade handles the relfilenode, an internal identifier for database files. During a binary upgrade, PostgreSQL attempts to preserve these identifiers to minimize data movement. However, the presence of a logged sequence tied to an unlogged table triggers an unexpected request for a new relfilenode, leading to the upgrade process grinding to a halt. The error message, "ERROR: unexpected request for new relfilenode in binary upgrade mode," is your key indicator that you've stumbled upon this bug. This happens during the “Restoring database schemas in the new cluster” phase, which is a critical step in the upgrade process.

Reproducing the Bug: A Step-by-Step Guide

To truly grasp the issue, let's walk through the steps to reproduce it. Stephan Blakeslee, who initially reported the bug, provided a clear set of instructions. We'll adapt those here to make it even easier for you to follow along and confirm if you're facing the same problem.

  1. Set up your environment: You'll need both an older PostgreSQL version (14.12 in this case) and a newer one (15.7 or 16.3). You can use tools like Postgres.app on macOS, or your distribution's package manager on Linux. Ensure you have the binaries for both versions readily accessible.

  2. Initialize the old database: Create a data directory for your old version (14.12) and initialize a database cluster using the initdb command. Make sure to specify a user (like postgres) during initialization.

    /tmp/postgresql/14.12/bin/initdb -D /tmp/postgresql/14.12/data -U postgres

  3. Start the old database server: Fire up the PostgreSQL 14.12 server using pg_ctl. This will get your old database instance running and ready to accept connections.

    /tmp/postgresql/14.12/bin/pg_ctl -D /tmp/postgresql/14.12/data start

  4. Create the problematic schema: This is where the magic happens. Connect to your 14.12 database using psql and execute the following SQL command to create an unlogged table with an identity column (which automatically creates a sequence):

    CREATE UNLOGGED TABLE foo (n INTEGER NOT NULL GENERATED BY DEFAULT AS IDENTITY);

    This single command sets the stage for the bug to manifest during the upgrade process. The key is the combination of UNLOGGED TABLE and the implicitly created logged sequence for the identity column.

  5. Stop the old database server: With the problematic schema in place, shut down the 14.12 server. This prepares the database for the upgrade process.

    /tmp/postgresql/14.12/bin/pg_ctl -D /tmp/postgresql/14.12/data stop

  6. Initialize the new database: Now, create a data directory for your new PostgreSQL version (15.7 or 16.3) and initialize a fresh database cluster.

    /tmp/postgresql/15.7/bin/initdb -D /tmp/postgresql/15.7/data -U postgres

  7. Run pg_upgrade: This is the moment of truth. Execute the pg_upgrade command, pointing it to the old and new database directories and binaries. Make sure to specify the correct paths for old-bindir, old-datadir, new-bindir, new-datadir, socketdir, and username.

    /tmp/postgresql/15.7/bin/pg_upgrade \
--old-bindir="/tmp/postgresql/14.12/bin" \
--old-datadir="/tmp/postgresql/14.12/data" \
--new-bindir="/tmp/postgresql/15.7/bin" \
--new-datadir="/tmp/postgresql/15.7/data" \
--socketdir="/tmp/postgresql/socket" \
--username="postgres" \
--verbose

    If you've followed these steps correctly, the pg_upgrade process should fail, producing the dreaded "unexpected request for new relfilenode" error.

  8. Inspect the logs: The output from pg_upgrade will point you to a log file. Examine this file (specifically the pg_upgrade_dump_*.log file) to confirm the error and the context in which it occurred. You'll see the pg_restore command failing while processing the sequence associated with the unlogged table.

    pg_restore: error: could not execute query: ERROR: unexpected request for new relfilenode in binary upgrade mode
Command was:
-- For binary upgrade, must preserve pg_class oids and relfilenodes
SELECT pg_catalog.binary_upgrade_set_next_heap_pg_class_oid('16384'::pg_catalog.oid);
SELECT pg_catalog.binary_upgrade_set_next_heap_relfilenode('16384'::pg_catalog.oid);

ALTER TABLE "public"."foo" ALTER COLUMN "n" ADD GENERATED BY DEFAULT AS IDENTITY (
SEQUENCE NAME "public"."foo_n_seq"
START WITH 1
INCREMENT BY 1
NO MINVALUE
NO MAXVALUE
CACHE 1
);
ALTER SEQUENCE "public"."foo_n_seq" SET LOGGED;

Why This Happens: A Deeper Dive

So, why does this combination of unlogged tables and logged sequences cause problems? It boils down to how PostgreSQL's binary upgrade process, managed by pg_upgrade, handles object identifiers (OIDs) and relfilenodes. The binary upgrade method aims to minimize data rewriting by preserving the physical layout of data files as much as possible. This means the OIDs and relfilenodes of existing database objects are ideally maintained during the upgrade.

However, the introduction of unlogged sequences in PostgreSQL 15 changed the landscape. Prior to version 15, sequences, even those associated with unlogged tables, were always logged. This meant they had certain behaviors and expectations tied to their logged nature. When pg_upgrade encounters this pre-15 setup, it attempts to reconcile the logged sequence with the unlogged table in the new version. This reconciliation process, specifically in binary upgrade mode, hits a snag when it tries to set the relfilenode for the sequence, leading to the “unexpected request” error. The system is essentially trying to modify a file identifier in a way that's incompatible with the binary upgrade's preservation goals.

Impact and Mitigation Strategies

This bug primarily affects users who:

  • Are upgrading from a PostgreSQL version prior to 15 (e.g., 14.x).
  • Are using the binary upgrade method with pg_upgrade.
  • Have schemas containing unlogged tables with identity columns (or other logged sequences associated with unlogged tables).

If you fall into this category, you'll likely encounter the upgrade failure described above. So, what can you do about it? Here are a few mitigation strategies:

  1. Upgrade to a version before 15, then to the desired version: As the bug seems to appear between 14 and later versions, upgrading to version 15 first and then to the target version (16, for instance) might bypass the issue. This approach essentially breaks the upgrade into smaller, potentially less problematic steps.

  2. Use the dump and restore upgrade method: Instead of pg_upgrade's binary upgrade, you can opt for the traditional dump and restore approach. This involves using pg_dump to create a logical backup of your old database and then using pg_restore to load it into the new database. While this method is generally slower than a binary upgrade, it's more resilient to schema differences and can often sidestep issues like this one. However, it requires downtime proportional to the database size.

  3. Manually adjust sequences before upgrading: A more hands-on approach involves modifying the sequences associated with unlogged tables before running pg_upgrade. You could potentially alter the sequences to be unlogged or adjust their ownership in a way that aligns with the expectations of the newer PostgreSQL version. This method requires careful planning and a deep understanding of PostgreSQL internals, so it's best suited for experienced DBAs.

  4. Address the issue in the upgrade scripts: For those comfortable with scripting, you can modify the upgrade scripts generated by pg_upgrade to handle the problematic sequences. This might involve removing or altering the commands that cause the "unexpected request" error. Again, this is an advanced technique that should be approached with caution.

Real-World Implications and Examples

To illustrate the impact, consider a real-world scenario: Imagine you're running a large e-commerce platform on PostgreSQL 14. You have several unlogged tables used for session data or temporary caching, each with an identity column for automatic ID generation. You're planning an upgrade to PostgreSQL 16 to take advantage of the latest performance improvements and features. You diligently follow the pg_upgrade instructions, but the upgrade fails with the dreaded relfilenode error. This unexpected roadblock can cause significant delays and disrupt your upgrade timeline. Understanding this bug and having a mitigation strategy in place can be crucial for a smooth upgrade process.

The Fix and Future Outlook

As of my last update, this bug was still an open issue. The PostgreSQL community is actively working on resolving it, and a fix is expected in future releases. Keep an eye on the PostgreSQL bug tracker and release notes for updates. In the meantime, the mitigation strategies outlined above should help you navigate this issue.

Conclusion: Stay Informed and Plan Ahead

Upgrading a database is always a critical operation, and encountering bugs along the way can be frustrating. By understanding the nuances of issues like bug #18562, you can better prepare for potential roadblocks and ensure a smoother upgrade experience. Remember to thoroughly test your upgrade process in a staging environment before applying it to your production database. Stay informed about the latest PostgreSQL releases and bug fixes, and always have a backup plan in place. Happy upgrading, folks!