有什么方法可以使UNION ALL运行更快？

Question

I have a lot of exactly same tables. TableA,TableB,TableC,TableD etc. which I want to create views from. Doing select * from TableA takes 20ms, doing select * from tableB takes 20ms, but doing (select * from TableA) union all (select * from TableB) takes over 20 minutes. Those tables have exactly same columns. Is there any settings in my.cnf that I need to change, or a way to create a view that would run faster? All tables have 1.5m to about 10m rows.

Results of explain

PRIMARY TableA  ALL                 28808685    
UNION   TableB  ALL                 15316215    
UNION RESULT    <union1,2>  ALL     Using temporary

Table structure: 10 varchar(20)'s, 5 unsigned INTs.

Answer 1

My guess is that select * from TableA does not take 20 ms. It takes 20 ms to start returning results.

Although I am going to answer your question, you should revisit your data structure. Having multiple tables with the same layout is usually a really bad idea. Instead, you should have a single table with all the rows.

But, you don't seem to have that.

Try running the union all without parentheses:

select * from TableA union all
select * from TableB;

MySQL has a habit of materializing subqueries. I'm not sure if it does this with union all subqueries, but given your description of the problem, that sees likely.