What causes "Function '%s' already exists" in prestodb/presto?

Use CREATE FUNCTION ... OR REPLACE (or pass replace=true to createFunction).

prestodb/presto · error · PrestoException

GENERIC_USER_ERROR

GENERIC_USER_ERROR

Error message

Function '%s' already exists

What it means

InMemoryFunctionNamespaceManager.createFunction throws GENERIC_USER_ERROR when a function with the same SqlFunctionId already exists and replace=false. The in-memory manager keys functions by id; creating a duplicate without replace would silently shadow the existing definition, so it is rejected.

Source

Thrown at presto-function-namespace-managers-common/src/main/java/com/facebook/presto/functionNamespace/testing/InMemoryFunctionNamespaceManager.java:72

    public InMemoryFunctionNamespaceManager(String catalogName, SqlFunctionExecutors sqlFunctionExecutors, SqlInvokedFunctionNamespaceManagerConfig config)
    {
        super(catalogName, sqlFunctionExecutors, config);
    }

    @Override
    public void setBlockEncodingSerde(BlockEncodingSerde blockEncodingSerde)
    {
        // Do not need to do anything here since InMemoryFunctionNamespaceManager cannot execute functions
    }

    @Override
    public synchronized void createFunction(SqlInvokedFunction function, boolean replace)
    {
        checkFunctionLanguageSupported(function);
        SqlFunctionId functionId = function.getFunctionId();
        if (!replace && latestFunctions.containsKey(function.getFunctionId())) {
            throw new PrestoException(GENERIC_USER_ERROR, format("Function '%s' already exists", functionId.getId()));
        }

        SqlInvokedFunction replacedFunction = latestFunctions.get(functionId);
        long version = 1;
        if (replacedFunction != null) {
            version = parseLong(replacedFunction.getRequiredVersion()) + 1;
        }
        latestFunctions.put(functionId, function.withVersion(String.valueOf(version)));
    }

    @Override
    public void alterFunction(QualifiedObjectName functionName, Optional<List<TypeSignature>> parameterTypes, AlterRoutineCharacteristics alterRoutineCharacteristics)
    {
        throw new PrestoException(NOT_SUPPORTED, "Alter Function is not supported in InMemoryFunctionNamespaceManager");
    }

    @Override
    public synchronized void dropFunction(QualifiedObjectName functionName, Optional<List<TypeSignature>> parameterTypes, boolean exists)

View on GitHub (pinned to 55bb57d202)

Solutions

Use CREATE FUNCTION ... OR REPLACE (or pass replace=true to createFunction).
Drop or clear the existing function before re-creating it (dropFunction is unsupported here, so use a fresh manager instance in tests).
Guard creation with a existence check via listFunctions/getFunction before calling createFunction.

Example fix

// before
CREATE FUNCTION test_ns.double_it(x BIGINT) RETURNS BIGINT RETURN x * 2;
-- second run fails
// after
CREATE OR REPLACE FUNCTION test_ns.double_it(x BIGINT) RETURNS BIGINT RETURN x * 2;

Defensive patterns

Strategy: validation

Validate before calling

boolean exists = manager.listFunctions(functionName).stream()
        .anyMatch(f -> f.getFunctionId().equals(function.getFunctionId()));
// only call createFunction(..., false) when !exists, otherwise use replace=true

Try / catch

try {
    manager.createFunction(function, false);
} catch (PrestoException e) {
    if (e.getMessage().contains("already exists")) {
        manager.createFunction(function, true); // OR REPLACE
    }
}

Prevention

Use CREATE OR REPLACE for idempotent function setup scripts.
Check existing functions before creating in shared test fixtures.
Isolate each test with its own InMemoryFunctionNamespaceManager instance.
Make setup methods idempotent rather than additive.

When it happens

Trigger: Calling createFunction(function, replace=false) when latestFunctions already contains function.getFunctionId(); running the same CREATE FUNCTION twice without OR REPLACE; test setup re-registering a helper function.

Common situations: Repeated test setup against the shared in-memory manager; forgetting the 'OR REPLACE' clause; idempotency issues where a script reruns CREATE FUNCTION statements.

Related errors

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/3f0d0e6d5fa0b8ae. Report an issue: GitHub.