pgsql: Scale VACUUM reltuples estimates after relation growth

Scale VACUUM reltuples estimates after relation growth

VACUUM kept the old reltuples value when it scanned at most one page,
even if the relation size changed. This left stale estimates after table
growth or shrinkage. Commit b46e1e54d made scanning one page more likely
by allowing newly populated pages to become all-visible before VACUUM.

Preserve reltuples only when relation size is unchanged; otherwise, scale
the old tuple density to the current size.

Only backpatch as far as 19, even though it was present before, to avoid
changing stats logic in stable branches.

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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/eacabd7d031cdc449ae7938dd8b75e6edb3d03b8

Modified Files
--------------
src/backend/commands/vacuum.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)