From e54a758d24dab056bb7f50d26c57a3c8761cc44a Mon Sep 17 00:00:00 2001 From: Dean Rasheed Date: Wed, 6 Oct 2021 13:16:51 +0100 Subject: Fix corner-case loss of precision in numeric_power(). This fixes a loss of precision that occurs when the first input is very close to 1, so that its logarithm is very small. Formerly, during the initial low-precision calculation to estimate the result weight, the logarithm was computed to a local rscale that was capped to NUMERIC_MAX_DISPLAY_SCALE (1000). However, the base may be as close as 1e-16383 to 1, hence its logarithm may be as small as 1e-16383, and so the local rscale needs to be allowed to exceed 16383, otherwise all precision is lost, leading to a poor choice of rscale for the full-precision calculation. Fix this by removing the cap on the local rscale during the initial low-precision calculation, as we already do in the full-precision calculation. This doesn't change the fact that the initial calculation is a low-precision approximation, computing the logarithm to around 8 significant digits, which is very fast, especially when the base is very close to 1. Patch by me, reviewed by Alvaro Herrera. Discussion: https://postgr.es/m/CAEZATCV-Ceu%2BHpRMf416yUe4KKFv%3DtdgXQAe5-7S9tD%3D5E-T1g%40mail.gmail.com --- src/backend/utils/adt/numeric.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) (limited to 'src/backend/utils/adt/numeric.c') diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c index 796f5173f1c..1de744855f3 100644 --- a/src/backend/utils/adt/numeric.c +++ b/src/backend/utils/adt/numeric.c @@ -10266,9 +10266,13 @@ power_var(const NumericVar *base, const NumericVar *exp, NumericVar *result) */ ln_dweight = estimate_ln_dweight(base); + /* + * Set the scale for the low-precision calculation, computing ln(base) to + * around 8 significant digits. Note that ln_dweight may be as small as + * -SHRT_MAX, so the scale may exceed NUMERIC_MAX_DISPLAY_SCALE here. + */ local_rscale = 8 - ln_dweight; local_rscale = Max(local_rscale, NUMERIC_MIN_DISPLAY_SCALE); - local_rscale = Min(local_rscale, NUMERIC_MAX_DISPLAY_SCALE); ln_var(base, &ln_base, local_rscale); -- cgit v1.2.3