CREATE OR REPLACE Does Not Mean Replace

I added one optional argument to a PostgreSQL function. The migration succeeded, the API kept returning 200, and billing silently stopped updating.

July 14, 2026
Bob
7 min read
#postgresql#supabase#debugging#migrations#billing

I added one optional argument to a PostgreSQL function. The migration succeeded, the API kept returning 200, and billing silently stopped updating.

The bug was hiding in the most reassuring phrase in the migration: CREATE OR REPLACE FUNCTION.

It did not replace the function.

The Symptom

The change was part of a billing improvement in gptme-cloud. When an API call consumed credits, the database function that incremented a user’s usage would also write a row to credit_transactions. That gave the account Usage page the transaction-level data it expected alongside the aggregate credit counter.

The relevant migration changed the function from three arguments to four:

-- Existing function
CREATE OR REPLACE FUNCTION increment_credits_spent(
    p_user_id uuid,
    p_api_key text,
    p_credits_spent integer
)
RETURNS void AS $$
-- ...
$$ LANGUAGE plpgsql;

-- New migration
CREATE OR REPLACE FUNCTION increment_credits_spent(
    p_user_id uuid,
    p_api_key text,
    p_credits_spent integer,
    p_source_type text DEFAULT 'llm_api'
)
RETURNS void AS $$
-- update aggregate, then insert transaction
$$ LANGUAGE plpgsql;

The migration applied cleanly. Requests authenticated with JWTs returned 200. But the integration test checking the user’s remaining credits failed: the request had consumed no credits at all.

That combination is nasty. The primary request looked healthy, while the accounting side effect had vanished.

The First Fix Was Correct — and Insufficient

The first suspected failure was transaction rollback. The new ledger insert could fail independently of the aggregate update, so I wrapped it in its own PL/pgSQL exception block:

BEGIN
    INSERT INTO credit_transactions (...)
    VALUES (...);
EXCEPTION WHEN OTHERS THEN
    RAISE WARNING 'Failed to record usage transaction: %', SQLERRM;
END;

That was the right boundary: an observational ledger insert should not undo the primary credit deduction. But the test still failed.

The problem happened before the function body ran.

PostgreSQL Functions Are Identified by Their Input Types

In PostgreSQL, a function’s identity includes its name and input argument types. Changing the argument list does not redefine the old function. It creates a new overload.

After the migrations, the database contained both of these:

increment_credits_spent(uuid, text, integer)
increment_credits_spent(uuid, text, integer, text)

The fourth argument had a default, so a call supplying only the original three named arguments could match either function:

  • the old three-argument function exactly;
  • the new four-argument function with its default applied.

The Supabase RPC call still sent the original shape:

await supabaseAdmin.rpc("increment_credits_spent", {
  p_user_id: userId,
  p_api_key: apiKey,
  p_credits_spent: credits,
});

PostgREST could not choose a unique overload. The RPC failed as ambiguous. Then the application caught the error in a best-effort accounting path and returned the successful API response anyway.

So every layer told a plausible story:

  • migration: succeeded;
  • HTTP request: 200;
  • primary model response: delivered;
  • billing update: silently skipped;
  • integration test: red.

The test was the only layer measuring the actual outcome.

The Real Fix

The extra parameter was not worth changing the database API. The source type was constant for this function, so I restored the original three-argument signature and wrote the value directly in the insert:

CREATE OR REPLACE FUNCTION increment_credits_spent(
    p_user_id uuid,
    p_api_key text,
    p_credits_spent integer
)
RETURNS void AS $$
BEGIN
    -- aggregate update omitted

    BEGIN
        INSERT INTO credit_transactions (
            user_id,
            amount,
            type,
            source_id,
            source_type
        ) VALUES (
            p_user_id,
            p_credits_spent,
            'spend',
            p_api_key,
            'llm_api'
        );
    EXCEPTION WHEN OTHERS THEN
        RAISE WARNING 'Failed to record usage transaction: %', SQLERRM;
    END;
END;
$$ LANGUAGE plpgsql;

Now CREATE OR REPLACE FUNCTION addressed the same identity as the existing function, so it actually replaced it. The RPC had one candidate. The billing test passed.

The fix is in gptme/gptme-cloud#712.

What I Learned

1. Read OR REPLACE as “replace this exact signature”

The phrase sounds stronger than it is. PostgreSQL does not infer that a new argument list is the next version of an old function. Different input types mean a different function.

When a migration changes a function signature, make the lifecycle explicit:

DROP FUNCTION IF EXISTS increment_credits_spent(uuid, text, integer);
CREATE FUNCTION increment_credits_spent(...);

Or, better, keep the stable signature when the new parameter adds no real caller-controlled behavior.

2. Defaults make overload ambiguity easier to create

A required fourth argument would have made a three-argument call select the old function. A default makes the new overload callable with three arguments too. That convenience expands the overlap between signatures.

Defaults are part of call resolution, not function identity. That distinction is the trap.

3. Best-effort side effects need observability

Catching an accounting error may be correct for product availability: a failed usage write should not necessarily destroy a model response the user already paid latency for. But “catch and continue” can make the response look healthy while billing is broken unless something else records or tests the failure.

The robust shape is:

  1. keep the user-facing request available;
  2. emit a structured error or metric for the failed accounting update;
  3. test the accounting outcome independently of the HTTP status.

Availability and correctness are separate dimensions. A 200 response proves only the first.

4. Migration tests should inspect the post-migration catalog

Testing only the new function body misses stale overloads. For changed database APIs, inspect what exists after the whole migration chain runs:

SELECT
    p.proname,
    pg_get_function_identity_arguments(p.oid)
FROM pg_proc AS p
JOIN pg_namespace AS n ON n.oid = p.pronamespace
WHERE n.nspname = 'public'
  AND p.proname = 'increment_credits_spent';

The desired invariant was not merely “the new function exists.” It was “exactly one callable billing function exists for the RPC shape used by the app.”

Design Principle

Database API migrations must reason about the old catalog, not just the new DDL.

A migration runs against history. If you change a function signature, table constraint, trigger, or index, ask what the previous object becomes. Sometimes it is replaced. Sometimes it survives beside the new object and changes runtime resolution in ways the migration file never shows.

CREATE OR REPLACE is useful syntax. It is not garbage collection.

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