mysql各個內存參數的介紹，分線程獨享和全局共享兩大類

mysql的內存參數分別有兩大類，一類是線程獨享的內存，一類是全局共享的內存 線程獨享內存：join_buffer_size、sort_buffer_size、read_buffer_size順序讀取數據緩沖區、read_rnd_buffer_size隨機讀取數據緩沖區、bulk_insert_buffer_size批量插入暫存使用內存、tmp_table_size內部臨時表使用內存、max_heap_table_size內存表使用內存

join_buffer_size: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.When Batched Key Access is used, the value of join_buffer_size defines how large the batch of keys is in each request to the storage engine用于普通索引掃描、范圍索引掃描和不使用索引因而執行全表掃描的聯接的緩沖區的最小大小。當使用批處理密鑰訪問時，join_buffer_size的值定義了向存儲引擎發出的每個請求中的批處理密鑰的大小

sort_buffer_size:Each session that must perform a sort allocates a buffer of this size每個必須執行排序的會話都會分配一個這種大小的緩沖區

read_buffer_size:Each thread that does a sequential scan for a MyISAM table allocates a buffer of this size (in bytes) for each table it scans對MyISAM表進行順序掃描的每個線程為其掃描的每個表分配一個這種大小(以字節為單位)的緩沖區

tmp_table_size:The maximum size of internal in-memory temporary tables. 內存中內部臨時表的最大大小。mysql臨時表分為兩種，一種是使用create temporary table創建的，稱為為外部臨時表，一種是因union、order by、group by、distinct等語句產生的，稱為內部臨時表

max_heap_table_size:This variable sets the maximum size to which user-created MEMORY tables are permitted to grow此變量設置允許用戶創建的內存表增長的最大大小

全局共享內存：key_buffer_size(MyISAM索引緩存)、query_cache_size為緩存查詢結果而分配的內存量、thread_cache_size服務器應該緩存多少個線程以供重用、table_open_cache所有線程打開的表的數量、binlog_cache_size二進制日志緩沖區、innodb_log_buffer_size事務日志緩沖區、innodb_buffer_pool_size數據和索引緩沖區

key_buffer_size：Index blocks for MyISAM tables are buffered and are shared by all threads.MyISAM表的索引塊被緩沖并由所有線程共享。

query_cache_size：The amount of memory allocated for caching query results為緩存查詢結果而分配的內存量

thread_cache_size：How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there.The default value= 8+(max_connections/100)
MySQL里面為了提高客戶端請求創建連接過程的性能,提供了一個連接池也就是 Thread_Cache池,將空閑的連接線程放在連接池中,而不是立即銷毀.這樣的好處就是,當又有一個新的請求的時候,mysql不會立即去創建連接 線程,而是先去Thread_Cache中去查找空閑的連接線程,如果存在則直接使用,不存在才創建新的連接線程.

table_open_cache：The number of open tables for all threads. 為所有線程打開的表的數量。

binlog_cache_size：The size of the cache to hold changes to the binary log during a transaction.在事務處理期間，緩存保存對二進制日志的更改的大小。

innodb_log_buffer_size：The size in bytes of the buffer that InnoDB uses to write to the log files on disk.InnoDB用來寫入磁盤上的日志文件的緩沖區的字節大小。

innodb_buffer_pool_size：The size in bytes of the buffer pool, the memory area where InnoDB caches table and index data. The default value is 134217728 bytes (128MB).緩沖池的字節大小，InnoDB緩存表和索引數據的內存區域。默認值是134217728字節(128MB)