被面试官问到，但我答不上来的的问题(1)

What Are the DB2 Catalog and Directory?

The DB2 Catalog is the central repository for DB2 object and user meta data. DB2 is constantly referring to that meta data as it processes applications and queries. The physical condition of the table spaces and indexes that comprise the DB2 Catalog is therefore a major component in overall DB2 subsystem performance.

Likewise, the DB2 Directory contains internal control structures such as DBDs. skeleton cursor tables, and skeleton package tables that can be accessed only by DB2 itself. The information in the DB2 Directory is critical for database access, utility processing, plan and package execution, and logging. It is quite obvious that efficient access to this critical information should be of paramount importance.

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简单地说，DB2 Catalog是存放DB2对象和用户元数据的表空间和索引的集合。它的物理状况是所有DB2子系统性能中的主要部分。

DB2 Directory包括内部控制结构的信息，比如DBD(DataBase Description)，skeleton cursor tables和skeleton package tables这些只能被DB2自身access的东西。对数据库访问，Utility运行，plan和package的执行以及日志记录来说都是至关重要的信息。