Well, could you write a trigger that would do what you need? AFAIR analyze data is stored no matter transaction boundaries. You could store some counters in session vars and issue an explicit analyze when enough rows were added.
I'm thinking that a variant of (2) might be simpler to inplement:
(I think Matt C essentially beat me to this suggestion - he originally discovered this issue). It is probably good enough for only *new* plans to react to the increased/increasing number of in progress rows. So this would require backends doing significant numbers of row changes to either directly update pg_statistic or report their in progress numbers to the stats collector. The key change here is the partial execution numbers would need to be sent. Clearly one would need to avoid doing this too often (!) - possibly only when number of changed rows > autovacuum_analyze_scale_factor proportion of the relation concerned or similar.
Are you loading using COPY? Why not break down the load into chunks?
INSERT - but we could maybe workaround by chunking the INSERT. However that *really* breaks the idea that in SQL you just say what you want, not how the database engine should do it! And more practically means that the most obvious and clear way to add your new data has nasty side effects, and you have to tip toe around muttering secret incantations to make things work well :-)
I'm still thinking that making postgres smarter about having current stats for getting the actual optimal plan is the best solution.
