Oracle SQL theory

最新推荐文章于 2025-11-25 13:06:55 发布

原创最新推荐文章于 2025-11-25 13:06:55 发布 · 446 阅读

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文章标签：

#sql #oracle #library #cache #loops #filter

本文介绍SQL语句在数据库中的解析流程，包括软解析和硬解析的区别，以及数据的逻辑读取与物理读取过程。此外还讨论了不同类型的查询执行策略。

v$sql contains SQL statements cached in 'Library Cache' in 'System Global Area(SGA)'.
Soft parse - the SQL statement is already found in the library cache by hashing.
Hard parse - the SQL statement is not in the library cache, need parse the SQL, genereate the execution plan, update the library cache (need acquire the 'Latches (like spin lock)' to exculsivly update the lib cache area).
Logic read - if the data is already in the SGA.
Physical read - if the data is not in the SGA, need access physical disk to get the data and then update SGA if necessary (again need hold the Latches when updating the SGA).

Nested Loops (Usually Good) -
Filter (Usually Bad) -