Ranges increase fast after restore database

I have one 3 nodes cluster.
After restore database serval times,total ranges number will be very high. After every restore database, the total ranges will increase fast.
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Also there is another intestring thing, many ranges belong to none of database:
select count(*) as ranges,database_name,sum(range_size) from crdb_internal.ranges group by database_name order by ranges ;
ranges | database_name | sum
---------±--------------±--------------
2 | bank | 63
16 | modi | 4677685910
19 | tpcc | 489164761
22 | testdb | 189053
23 | system | 251898897
34 | pzh | 3916
72 | defaultdb | 71917020
1463 | v7dev | 13239324008
4185 | v7dev_prd | 311030754263
22268 | | 4219631005

So I have 2 questions:
1.Why so many ranges and belong to none of database?
2.Are these ranges useful and how to delete these ranges?

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