public void setFirstPhaseRanking(String expression) {
        try {
            firstPhaseRanking = new RankingExpressionFunction(parseRankingExpression(FIRST_PHASE, List.of(), expression), false);
        } catch (ParseException e) {
            throw new IllegalArgumentException("Invalid first-phase function", e);
        }
    }
Returns the ranking expression to use by this. This expression must not be edited.
Returns null if no expression is set.
config-model/src/main/java/com/yahoo/schema/RankProfile.java