Q55
Consider the join_buffer_size parameter in MySQL Server.
Which two statements are true about the join buffer?
A. The value should be increased if the client performs several SELECT operations.
B. The join buffer is set per connection.
C. The join buffer is used to process sorts when complex joins are being performed.
D. The value should be increased from the default if the query joins large rows without using an
index.
E. The join buffer is global and can be changed only by restarting the server.
解析:
The minimum size of the buffer that is used for plain index scans, range index scans, and joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.
A错误,并不是select就会用到join_buffer,而是多表连接时才会用到
B正确,每个join buffer 是连接独立使用的
C错误,排序是sort_buffer_size控制的
D正确
E错误 ,join_buffer_size 是线程缓存 不是全局缓存
答案应该是 BD
Answer: C,D