presto-main/src/main/java/io/prestosql/metadata/MetadataManager.java/MetadataManager/listSchemaNames
class MetadataManager:    @Override
    public List<String> listSchemaNames(Session session, String catalogName)
    {
        Optional<CatalogMetadata> catalog = getOptionalCatalogMetadata(session, catalogName);

        ImmutableSet.Builder<String> schemaNames = ImmutableSet.builder();
        if (catalog.isPresent()) {
            CatalogMetadata catalogMetadata = catalog.get();
            ConnectorSession connectorSession = session.toConnectorSession(catalogMetadata.getCatalogName());
            for (CatalogName connectorId : catalogMetadata.listConnectorIds()) {
                ConnectorMetadata metadata = catalogMetadata.getMetadataFor(connectorId);
                metadata.listSchemaNames(connectorSession).stream()
                        .map(schema -> schema.toLowerCase(Locale.ENGLISH))
                        .forEach(schemaNames::add);
            }
        }
        return ImmutableList.copyOf(schemaNames.build());
    }