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Library Cache is mainly used to store executable objects (such as SQL statements, PL/SQL blocks and related objects, etc.) and their related information (parsing information, executable code, related objects and information, related control structures, etc.) in the system, so as to realize fast access and sharing of these executable objects and their related information.

Hash Bucket is mainly used for fast location and access of specific executable objects. Each hash bucket corresponds to a linked list connecting multiple executable objects, and the expression form of these executable objects in the linked list is the handle of the executable object. When it is necessary to locate or access a specific object, the system can quickly know the hash bucket where the specific object should be located by doing specific hash calculation on the specific SQL or PL/SQL object. The linked list of each hash bucket is relatively short. Especially in older versions of Oracle, hash bucket lists are shorter, so locating and accessing specific objects in hash bucket lists is very fast. As for the number of hash buckets in the Library Cache, the system will automatically calculate it according to the size of the Shared Pool, so as to ensure that the hash buckets are sufficient, and at the same time, the number of objects in the hash bucket will not be too large, which will affect the efficiency of location and access. The hash bucket diagram is shown below.

4. Hash Latch

As mentioned above, each hash bucket connects multiple executable objects together through a linked list, and these executable objects exist in the form of object handles, which contain relevant properties of these objects, such as names, tags, pointers to object memory addresses, etc. The Library Cache Handle is shown below.

6. Library Cache Object

The Handle on the linked list in the hash bucket corresponds to executable objects (Library Cache Objects), which are composed of some independent heaps. Specifically, when the Handle on the linked list in the hash bucket points to these executable objects, it actually points to its first heap (heap0), which records pointer information pointing to other heaps of the object. These heaps specifically include:

1) Dependency Table: information about other objects on which the current object (Library Cache Object, LCO) depends, such as tables, views, indexes, etc. on which the object depends.

2) Child Table: Child LCO of the current object and its related information.

3) Data Block: SQL statement, execution plan, execution text and other information of the current object.

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When an Oracle user submits an executable object such as SQL or PL/SQL, this generates an object called a cursor in the Library Cache that can be shared by multiple sessions, saving resources and increasing efficiency. At the same time, each cursor is expressed and qualified by two parts, namely, the parent cursor and the child cursor, which can be queried and understood through the system-related dynamic view. Parent cursors and child cursors are shown below.

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