public static Collector<Tuple2<Double, Double>, ?, Optional<Double>> covarianceDouble() {
        return filter(t -> t.v1 != null && t.v2 != null, covarianceDouble(t -> t.v1, t -> t.v2));
    }
Get a {@link Collector} that calculates the <code>COVAR_POP()</code> function.
jOOL-java-8/src/main/java/org/jooq/lambda/Agg.java