Aggregate before the join
With enable_eager_aggregate the planner can aggregate t2 by the join key first (1000 rows become 10), join, then finalize.
task
Show the plan of SELECT t1.a, avg(t2.c) FROM eager_agg_t1 t1 JOIN eager_agg_t2 t2 ON t1.b = t2.b GROUP BY t1.a ORDER BY t1.a.