prestodb/presto · error · PrestoException

ALREADY_EXISTS

ALREADY_EXISTS

Error message

Schema [%s] already exists

What it means

The Memory connector's MemoryMetadata.createSchema is guarded against re-creating an existing schema. Because all schemas are held in-memory and checked with schemas.contains(schemaName), creating a schema name that is already registered throws PrestoException ALREADY_EXISTS with the schema name in brackets. This is a standard idempotency guard for DDL.

Source

Thrown at presto-memory/src/main/java/com/facebook/presto/plugin/memory/MemoryMetadata.java:113

    @Inject
    public MemoryMetadata(NodeManager nodeManager, MemoryConnectorId connectorId)
    {
        this.nodeManager = requireNonNull(nodeManager, "nodeManager is null");
        this.connectorId = requireNonNull(connectorId, "connectorId is null").toString();
        this.schemas.add(SCHEMA_NAME);
    }

    @Override
    public synchronized List<String> listSchemaNames(ConnectorSession session)
    {
        return ImmutableList.copyOf(schemas);
    }

    @Override
    public synchronized void createSchema(ConnectorSession session, String schemaName, Map<String, Object> properties)
    {
        if (schemas.contains(schemaName)) {
            throw new PrestoException(ALREADY_EXISTS, format("Schema [%s] already exists", schemaName));
        }
        schemas.add(schemaName);
    }

    @Override
    public synchronized void dropSchema(ConnectorSession session, String schemaName)
    {
        if (!schemas.contains(schemaName)) {
            throw new PrestoException(NOT_FOUND, format("Schema [%s] does not exist", schemaName));
        }

        boolean tablesExist = tables.values().stream()
                .anyMatch(table -> table.getSchemaName().equals(schemaName));

        if (tablesExist) {
            throw new PrestoException(SCHEMA_NOT_EMPTY, "Schema not empty: " + schemaName);
        }

View on GitHub (pinned to 55bb57d202)

Solutions

  1. Use CREATE SCHEMA IF NOT EXISTS memory.<name>
  2. Check existence first via SHOW SCHEMAS FROM memory before creating
  3. Use a unique schema name per test/deploy run
  4. Catch ALREADY_EXISTS and treat it as success in setup scripts

Example fix

// before
CREATE SCHEMA memory.test_schema;
// after
CREATE SCHEMA IF NOT EXISTS memory.test_schema;
Defensive patterns

Strategy: try-catch

Validate before calling

// Skip creation when schema already exists
if (metadata.listSchemaNames(session, "memory").contains(schemaName)) {
    return; // already exists, nothing to do
}
metadata.createSchema(session, schemaName, ImmutableMap.of());

Try / catch

try {
    metadata.createSchema(session, schemaName, properties);
} catch (PrestoException e) {
    if (ALREADY_EXISTS.equals(e.getErrorCode())) {
        log.info("Schema {} already exists, ignoring", schemaName);
    } else throw e;
}

Prevention

When it happens

Trigger: Running CREATE SCHEMA memory.<name> twice without IF NOT EXISTS; concurrent session re-running bootstrap DDL against a memory catalog that already created the schema.

Common situations: Idempotent deployment scripts that re-run CREATE SCHEMA on every deploy; test harnesses sharing a memory catalog across test runs; race between two setup steps in the same session.

Related errors

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/cb06b5858d04e37f. Report an issue: GitHub.