Hmmm. On second thought, maybe one or the other is enough, either restrict the parameter to values where the approximation is good, or put out a clear documentation about when the approximation is not very good, but it may be still useful even if not precise.
I have removed restriction and expanded documentation in attaching patch v5.
Also I have recorded patch to CF 2017-09 — https://commitfest.postgresql.org/14/1206/.
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The Russian Postgres Company