fix(postgres): explain a dropped table instead of raw driver text

[Gilbert09]

Problem

When a table or column a Postgres sync reads is dropped or renamed on the source, the sync stopped, which is correct, but stored the raw psycopg error as latest_error: relation "..." does not exist followed by a SQL fragment. The customer got no next step, and the message echoed their own relation name and query back into the error panel.

Changes

• A dropped or renamed table or column now shows "A table or column this sync reads no longer exists in your source database. Remove it...", instead of the raw driver text. The failure stays non-retryable.
• Give the existing does not exist non-retryable bucket an actionable message. It was None, which fell back to the raw text.
• Mechanical: a Postgres test asserts the message is surfaced. The Supabase realtime-partition test is updated, because a regular missing relation now gets this generic message while realtime partitions keep their specific one through first-match selection.

How did you test this code?

Ran TestPostgresSourceNonRetryableErrors and the Supabase source tests. Added test_missing_relation_surfaces_actionable_message, which catches a revert of the bucket back to None or raw text. Reworked the Supabase parametrized test to assert realtime partitions keep their specific message while other dropped relations get the generic one. Did not run the full warehouse suite, because this sandbox has no dev stack.

Automatic notifications

• Publish to changelog?

Docs update

None.

🤖 Agent context

Autonomy: Fully autonomous

Found while triaging a batch of production warehouse sync failure classes. Skills invoked: /writing-user-facing-copy, /writing-tests, /writing-code-comments, /writing-pr-descriptions.

The does not exist key is deliberately broad; it already caught both relation and column drops, and the new message covers both. Supabase inherits this bucket and keeps its realtime-partition message ahead of it. Relation names in the tests are invented.

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[github-actions]

Hey @Gilbert09! 👋

It looks like your git author email on this PR isn't your @posthog.com address (owerstom@gmail.com). Since you're on the PostHog team, it's worth pointing your local git author email at your @posthog.com address. Why it matters:

• Consistent work identity in git history — internal tooling that attributes commits to team members keys off your @posthog.com address.
• Keeps team contributions easy to tell apart from external community ones when scanning history.

You can fix it for this repo with:

git config user.email "you@posthog.com"

Or set it globally with git config --global user.email "you@posthog.com". No need to redo this PR — just a nudge for next time. 🙂

[github-actions]

🤖 CI report

⚠️ Trunk lane — backend Python lane

This PR is assigned to the backend Python lane. It runs backend Python tests and may merge in parallel with PRs in other lanes.

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