这里做个笔记，纯属是加深印象：
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.
明显v$sql的信息来源于共享内存区shared SQL areas，也就是说相关SQL信息在数据库重启，数据库缓存被刷，或相关SQL信息被踢出缓存区，此SQL的信息就不能在V$SQL里查询。而V$SQL记录每一个SQL的每一个子游标的信息。
V$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.
同样v$sqlarea也存储于library cache，然而这个视图却做了一个统计分析，统计的方式是以sql string,也就是以sql_text文本做了group by，在此前提下对其他信息统计sum()。通过version_count可以得出SQL指针的总数。所以简单的说：v$sqlarea存放的是相同SQL语句不同version一个汇总。
V$SQLTEXT displays the text of SQL statements belonging to shared SQL cursors in the SGA.
v$sql及v$sqlarea存放的更多的是性能数据，但其sql_text是不完整的的，如果想获得完整的sql就要用v$sqltext。