php的pconnect
PHP在連結MYSQL時有兩個FUNCTIONCONNECT與PCONNECT這兩個函式的差別在哪裡？一般的標準答案都是：PCONNECT是持續性的連接，PHP會重複使用已經存在的資料庫連線
但是再追問下去，能說得出來的人就少了許多常見的疑問有：什麼時候要用CONNECT什麼時候用PCONNECT?mysql_close何時使用？為什麼DATABASE上有一堆SLEEP的PROCESS？明明用PCONNECT為何每次RESOURCE ID都不同？
先回歸到最簡單的解釋，pconnect會去檢查是否有已經存在的資料庫連線。若有，就使用舊的連線；若無，就新開啟一個連線但是這裡所說得檢查連線，是指這一個APACHE行程所開啟的資料庫連線而不是這一台WEB SERVER所開啟的資料庫連線。一台WEB SERVER上可能有上百個APAHE行程（2.0預設最大150個）也就是說 在最大情況下可能會有上百個PCONNECT造成的資料庫連線（實際上會遠少於此）所以同樣使用PCONNECT是會出現不同RESOURCE ID的因為他可能是在不同行程上面執行
而PCONNECT是不會在查詢完成後就關閉，而是會等待一定時間這個時間可以由MYSQL上的wait_timeout設定而且mysql_close並無法關閉pconnect所開啟的連線，只能關閉connect所開啟的資料庫連線。當然如果沒有CLOSE在經過TIMEOUT時間之後也是會被DB DROP掉。
PCONNECT必須要小心使用，在寫的不好的CODE中，會殘留一大批的DATABASE CONNECTION。最大值可以這樣計算每台SERVER的APACHE行程數 * 所有WEB SERVEWR數量當然不需要我來提醒，每個連線都會消耗DB的CPU TIME與MEMORY
寫WEB CODE時，不論使用何種語言，有兩點一定要再動手寫之前先有答案一是，要從DATABASE抓取什麼資料二是，要傳送什麼資料到USER端(其實這也不只用於WEB）DATABASE的連線是很慢的，能夠一次抓取完所需的資料，就不要分兩次
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