1. max_connections open_files_limit table_open_cache 如何配置
table_open_cache = max_connections * N (N is the maximun number of tables open in each connection)
open_files_limit = 2*table_open_cache (open one table needs at least one file handler and another one for index, index file handler can share for the same table)
2. wait_timeout interactive_timeout 如何配置？
3. view algorithm: merge temptable undefined
build view from another view will have bad performance
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