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
Branch
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REL9_6_STABLE
Details
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https://git.postgresql.org/pg/commitdiff/0de8f9bc866a6a54ef891f07890d35455e5e6a00
Modified Files
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src/backend/utils/adt/numeric.c | 6 +++++-
src/test/regress/expected/numeric.out | 6 ++++++
src/test/regress/sql/numeric.sql | 1 +
3 files changed, 12 insertions(+), 1 deletion(-)
