Currently, the tri-compare operator for big_decimal (operator <=>), uses
a precise but potentially very expensive algorithm for comparing the
numbers: it first brings them to the same scale, then compares the
normalized unscaled values. big_decimal has abritrary precisions,
therefore the stored numbers can be arbitrarily large.
In extreme cases, comparing two numbers can result in huge amount of
memory allocated and stalls. If this type is used int he primary key of
a table, these comparisons can make the node completely unresponsive.
This patch adds the following fast-paths to operator <=>:
* An early return for the case of equal scales.
* An early return for different signs.
* An early return for the case where one or both of the numbers are 0.
* A fast algorithm for detecting the case where the there is a big
difference between the two numbers. This algorithm works only with the
scales and is able to compare the two numbers by using only one division
and some additions and substractions. This algorithm is imprecise and
when the numbers are closer than its confidence window, it will
fall-back to the current slow but precise tri-compare.
All but the last case should have been fast before as well, but the
scale-compare algorithm makes a huge difference. Numbers, which would
previously make the node unresponsive, now compare in constant-time.
Fixes: scylladb/scylladb#21716Closesscylladb/scylladb#21715
b6a9c79af3