v$sql v$sqlarea v$sqltext

這里做個筆記，純屬是加深印象：

V$SQL lists statistics on shared SQL areas without the GROUP BY clause and contains one row for each child of the original SQL text entered. Statistics displayed in V$SQL are normally updated at the end of query execution. However, for long running queries, they are updated every 5 seconds. This makes it easy to see the impact of long running SQL statements while they are still in progress.

明顯v$sql的信息來源于共享內存區shared SQL areas，也就是說相關SQL信息在數據庫重啟，數據庫緩存被刷，或相關SQL信息被踢出緩存區，此SQL的信息就不能在V$SQL里查詢。而V$SQL記錄每一個SQL的每一個子游標的信息。

V$SQLAREA displays statistics on shared SQL areas and contains one row per SQL string. It provides statistics on SQL statements that are in memory, parsed, and ready for execution.

同樣v$sqlarea也存儲于library cache，然而這個視圖卻做了一個統計分析，統計的方式是以sql string分組,也就是以sql_text文本做了group by，在此前提下對其他信息統計sum()。通過version_count可以得出SQL子游標的總數。所以簡單的說：v$sqlarea存放的是相同SQL語句不同version一個匯總。

version_count：Number of child cursors that are present in the cache under this parent

excutions : Total number of executions, totalled over all the child cursors

V$SQLTEXT displays the text of SQL statements belonging to shared SQL cursors in the SGA.

v$sql及v$sqlarea存放的更多的是性能數據，但其sql_text是不完整的的，如果想獲得完整的sql就要用v$sqltext。