presto-main/src/main/java/io/prestosql/event/QueryMonitor.java/QueryMonitor/mergeSessionAndCatalogProperties
class QueryMonitor:    private static Map<String, String> mergeSessionAndCatalogProperties(SessionRepresentation session)
    {
        Map<String, String> mergedProperties = new LinkedHashMap<>(session.getSystemProperties());

        // Either processed or unprocessed catalog properties, but not both.  Instead of trying to enforces this while
        // firing events, allow both to be set and if there is a duplicate favor the processed properties.
        for (Map.Entry<String, Map<String, String>> catalogEntry : session.getUnprocessedCatalogProperties().entrySet()) {
            for (Map.Entry<String, String> entry : catalogEntry.getValue().entrySet()) {
                mergedProperties.put(catalogEntry.getKey() + "." + entry.getKey(), entry.getValue());
            }
        }
        for (Map.Entry<CatalogName, Map<String, String>> catalogEntry : session.getCatalogProperties().entrySet()) {
            for (Map.Entry<String, String> entry : catalogEntry.getValue().entrySet()) {
                mergedProperties.put(catalogEntry.getKey().getCatalogName() + "." + entry.getKey(), entry.getValue());
            }
        }
        return ImmutableMap.copyOf(mergedProperties);
    }