发出一个sql语句或sql命令，让数据库执行，那么sql语句在数据库内部就要被解析，建立它的execution plan(执行规划(计划))，然后按照执行计划去执行。
每一个sql语句都要被解析，解析成原子操作去执行。解析好的sql语句都会存在共享池里面。
Shared Pool used to store:
--Most recently executed SQL statements
--Most recently used data definitions
It consists of two key performance-related memory structures:
--Library Cache
--Data Dictionary Cache
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How to resize the shared pool:
alter system set shared_pool_size = 64M;
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Library Cache:
It stores information about the most recently used SQL and PL/SQL statements
(存储的是经过编译解析后的SQL语句和PL/SQL语句)
Enables the sharing of commonly used statements.
Is managed by a Last Recently Used(LRU) algorithm.
Consists of two structures:
--Shared SQL area
--Shared PL/SQL area
Size determined by the Shared Pool Sizing
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(如果现在执行的sql语句已经在LibraryCache中了，就不需要再解析，就可以提高执行速度)