Pushdown the limit if possible(group by partition column); Fix the queri...

...es that should not push down limit
On partition 1, I have A3 values:
  1
  1
  3

On partition 2 I have A3 values:
   2
   2

If I have an index on A3, I can have serial aggs on the partitions. Even if I have hash aggs, maybe 1 hashes before 3.

    select A3, count(*) as tag from T3 group by A3 order by tag limit 1;

Explain to me how it's OK to push down limit 1 to partition 1 just because A3 is the partition key.
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On partition 1 I have: