presto-memory/src/main/java/io/prestosql/plugin/memory/MemoryMetadata.java/MemoryMetadata/dropSchema
class MemoryMetadata:    @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);
        }

        verify(schemas.remove(schemaName));
    }