基础知识之SQL执行过程

--此处优化器的模式为基于成本的ALL_ROWS
SQL> show parameter optimizer_mode

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode                       string      ALL_ROWS
--收集统计信息

SQL>exec dbms_stats.gather_table_stats('YWBZ','TEST',CACASDE=>TRUE);

硬分析语法解析
SQL>select count fromm dual
第 1 行出现错误:ORA-00923: 未找到要求的 FROM 关键字
语义解析
SQL>select count from dual;
第 1 行出现错误:ORA-00904: "COUNT": 标识符无效
SQL>alter system flush shared_pool;
系统已更改。
SQL>alter system flush shared_pool;
系统已更改。 
SQL>select sql_text,parse_calls,loads from v$sql where sql_text like 'select count(*)from%';
 未选定行
SQL>select count(*) from v$sql;
 COUNT(*)
----------
 74 
SQL>select sql_text,parse_calls,loads from v$sql where sql_text like 'select count(*)from%';
SQL_TEXT PARSE_CALLS LOADS
---------------------------------------------- ----------
select count(*) from v$sql 1 1
 软解析（确保未被置换出共享池）
SQL>select count(*) from v$sql;
COUNT(*)
---------- 
141 
SQL>select sql_text,parse_calls,loads from v$sql where sql_text like 'select count(*)from%';
SQL_TEXT PARSE_CALLS LOADS
---------------------------------------------- ----------
select count(*) from v$sql 2 1

--此处优化器的模式为基于成本的ALL_ROWS
SQL> show parameter optimizer_mode

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode                       string      ALL_ROWS
--收集统计信息

SQL>exec dbms_stats.gather_table_stats('YWBZ','TEST',CACASDE=>TRUE);

硬分析语法解析
SQL>select count fromm dual
第 1 行出现错误:ORA-00923: 未找到要求的 FROM 关键字
语义解析
SQL>select count from dual;
第 1 行出现错误:ORA-00904: "COUNT": 标识符无效
SQL>alter system flush shared_pool;
系统已更改。
SQL>alter system flush shared_pool;
系统已更改。 
SQL>select sql_text,parse_calls,loads from v$sql where sql_text like 'select count(*)from%';
 未选定行
SQL>select count(*) from v$sql;
 COUNT(*)
----------
 74 
SQL>select sql_text,parse_calls,loads from v$sql where sql_text like 'select count(*)from%';
SQL_TEXT PARSE_CALLS LOADS
---------------------------------------------- ----------
select count(*) from v$sql 1 1
 软解析（确保未被置换出共享池）
SQL>select count(*) from v$sql;
COUNT(*)
---------- 
141 
SQL>select sql_text,parse_calls,loads from v$sql where sql_text like 'select count(*)from%';
SQL_TEXT PARSE_CALLS LOADS
---------------------------------------------- ----------
select count(*) from v$sql 2 1