Mysqlunionall efficiency problems

Mysqlunionall efficiency I have several tables with the same structure and need to perform join table queries. I use the following statement select & nbsp; sum (cnt) & nbsp; from & nbsp; (select & nbsp; count (*) & nbsp; as & nbsp; cnt & nbsp; from & nbsp; t1 & nbsp; w mysql union all efficiency issues

I have several tables with the same structure and need to perform join table query. I use the following statement:

Select sum (cnt) from (select count (*) as cnt from t1 where a> 100 union all select...) as t;
The sum of this sentence is fast.

Select * from t1 where a> 100 union all select * from t2 where a> 100 union all select * from t3 where a> 100 order by B desc limit;
This sentence seems to be very inefficient. it takes a long time to come out.

How to optimize it? thank you.


------ Solution --------------------
In the second sentence, order by B desc limit is estimated to affect the vertex. remove the vertex and try again.
------ Solution --------------------
In the union structure, only one order is allowed (except in subqueries)
Your writing method is obviously unreasonable, or it is outdated (because you have done manual table sharding and the segmentation is unreasonable)
Currently, mysql provides the partition function, as long as the setting is reasonable and the speed of a single small table is equal (although it is also cut into multiple data files)