Why not represent "never vacuumed" accurately wrt pg_class.relpages?

Hi,

estimate_rel_size() explains:

			/*
			 * HACK: if the relation has never yet been vacuumed, use a
			 * minimum size estimate of 10 pages.  The idea here is to avoid
			 * assuming a newly-created table is really small, even if it
			 * currently is, because that may not be true once some data gets
			 * loaded into it.  Once a vacuum or analyze cycle has been done
			 * on it, it's more reasonable to believe the size is somewhat
			 * stable.
			 *
			 * (Note that this is only an issue if the plan gets cached and
			 * used again after the table has been filled.  What we're trying
			 * to avoid is using a nestloop-type plan on a table that has
			 * grown substantially since the plan was made.  Normally,
			 * autovacuum/autoanalyze will occur once enough inserts have
			 * happened and cause cached-plan invalidation; but that doesn't
			 * happen instantaneously, and it won't happen at all for cases
			 * such as temporary tables.)
			 *
			 * We approximate "never vacuumed" by "has relpages = 0", which
			 * means this will also fire on genuinely empty relations.  Not
			 * great, but fortunately that's a seldom-seen case in the real
			 * world, and it shouldn't degrade the quality of the plan too
			 * much anyway to err in this direction.
			 *
			 * There are two exceptions wherein we don't apply this heuristic.
			 * One is if the table has inheritance children.  Totally empty
			 * parent tables are quite common, so we should be willing to
			 * believe that they are empty.  Also, we don't apply the 10-page
			 * minimum to indexes.
			 */

I don't quite get why we don't instead just represent "never vacuumed"
by storing a more meaningful value in relpages?  We could go for
InvalidBlockNumber, or even NULL (although the latter would be a bit
annoying due to not being mappable to a struct anymore).

I've seen numerous cases where relpages = 0 -> never vacuumed has caused
worse plans, and it just doesn't seem necessary?

Greetings,

Andres Freund