void setFirstPhaseRanking(RankingExpression rankingExpression) {
        this.firstPhaseRanking = new RankingExpressionFunction(new ExpressionFunction(FIRST_PHASE, List.of(), rankingExpression), false);
    }
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