pgsql: Avoid extrapolating reltuples from tiny VACUUM samples

Avoid extrapolating reltuples from tiny VACUUM samples

Since b46e1e54d, on-access pruning can mark most pages all-visible
before VACUUM or ANALYZE establishes tuple density. VACUUM may then scan
only a tiny tail of mostly empty pages while reltuples is still unknown
or reflects a previously empty table.

When no prior density is available, treat a sample of at most one page
or less than two percent of the relation as insufficient and leave
reltuples unknown instead of extrapolating.

Reported-by: Vaibhav Jain <jainva(at)google(dot)com>
Author: Vaibhav Jain <jainva(at)google(dot)com>
Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://postgr.es/m/flat/CABa-Az2YXuX%2B_JH8VL89i7oA%2BDC74GvfkSsM9%2B92fGCeWYVCqg%40mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ab79429c26d3980535457cc618c79328a9ebf495

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 5 ++---
src/backend/commands/vacuum.c | 21 ++++++++++++++-------
2 files changed, 16 insertions(+), 10 deletions(-)