Aggregate linear regression functions added in Firebird 3.0

Linear regression functions added in Firebird 3.0 :

REGR_AVGX(Y, X) = SUM(X) / N
REGR_AVGY(Y, X) = SUM(Y) / N
REGR_COUNT(Y, X) = N
REGR_INTERCEPT(Y, X) = REGR_AVGY(Y, X) – REGR_SLOPE(Y, X) * REGR_AVG_X(Y, X)
REGR_R2(Y, X) = POWER(CORR(Y, X),2)
REGR_SLOPE(Y, X) = COVAR_POP(Y, X)
REGR_SXX(Y, X) = N * VAR_POP(X)
REGR_SXY(Y, X) = N * COVAR_POP(Y, X)
REGR_SYY(Y, X) = N * VAR_POP(Y)

Contributed by Hajime Nakagami.

Github pull request: https://github.com/FirebirdSQL/core/pull/4