Oracle體系結構由例項和一組資料檔案組成，例項由SGA記憶體區，SGA意思是共享記憶體區，由share pool(共享池)、data buffer(資料緩衝區)、log buffer(日誌緩衝區)組成
SGA記憶體區的share pool是解析SQL並儲存執行計劃的，然後SQL根據執行計劃獲取資料時先看data buffer裡是否有資料，沒資料才從磁碟讀，然後還是讀到data buffer裡，下次就直接讀data buffer的，當SQL更新時，data buffer的資料就必須寫入磁碟備份，為了保護這些資料，才有log buffer，這就是大概的原理簡介
系統結構關係圖如圖，圖來自《收穫，不止SQL優化》一書：
下面介紹共享池、資料緩衝、日誌緩衝方面調優的例子
共享池相關例子
未使用使用繫結變數的情況，進行一下批量寫資料，在登入系統，經常用的sql是select * from sys_users where username='admin'或者什麼什麼的，假如有很多使用者登入，就需要執行很多次這樣類似的sql，能不能用一條SQL代表？意思是不需要Oracle優化器每次都解析sql獲取執行計劃，對於這種類似的sql是沒必要的，Oracle提供了繫結變數的方法，可以用於調優sql，然後一堆sql就可以用
select * from sys_users where username=:x
這裡用一個變數x表示，具體例子如下，
新建一張表來測試
create table t (x int);不使用繫結遍歷，批量寫資料
begin
for i in 1 .. 1000
loop
execute immediate
'insert into t values('|| i ||')';
commit;
end loop;
end;
/輸出
已用時間: 00: 00: 00.80
加上繫結遍歷，繫結變數是用:x的形式
begin
for i in 1 .. 100
loop
execute immediate
'insert into t values( :x )' using i;
commit;
end loop;
end;
/已用時間: 00: 00: 00.05
資料緩衝相關例子
這裡介紹和資料快取相關例子
(1) 清解析快取
//建立一個表來測試
SQL> create table t as select * from dba_objects;
表已建立。
//設定列印行數
SQL> set linesize 1000
//設定執行計劃開啟
SQL> set autotrace on
//列印出時間
SQL> set timing on
//查詢一下資料
SQL> select count(1) from t;
COUNT(1)
----------
72043
已用時間: 00: 00: 00.10
//清一下緩衝區快取(ps:這個sql不能隨便在生產環境執行)
SQL> alter system flush buffer_cache;
系統已更改。
已用時間: 00: 00: 00.08
//清一下共享池快取(ps:這個sql不能隨便在生產環境執行)
SQL> alter system flush shared_pool;
//再次查詢，發現查詢快了
SQL> select count(1) from t;
COUNT(1)
----------
72043
已用時間: 00: 00: 00.12
SQL>日誌緩衝相關例子
這裡說明一下，日誌關閉是可以提供效能的，不過在生生產環境還是不能隨便用，只能說是一些特定建立，SQL如：
alter table [表名] nologging;調優擴充知識
這些是看《收穫，不止SQL優化》一書的小記
(1) 批量寫資料事務問題
對於迴圈批量事務提交的問題，commit放在迴圈內和放在迴圈外的區別，
放在迴圈內，每次執行就提交一次事務，這種時間相對比較少的
begin
for i in 1 .. 1000
loop
execute immediate
'insert into t values('|| i ||')';
commit;
end loop;
end;放在迴圈外，sql迴圈成功，再提交一次事務，這種時間相對比較多一點
begin
for i in 1 .. 1000
loop
execute immediate
'insert into t values('|| i ||')';
end loop;
commit;
end;《收穫，不止SQL優化》一書提供的指令碼，用於檢視邏輯讀、解析、事務數等等情況：
select s.snap_date,
decode(s.redosize, null, '--shutdown or end--', s.currtime) "TIME",
to_char(round(s.seconds / 60, 2)) "elapse(min)",
round(t.db_time / 1000000 / 60, 2) "DB time(min)",
s.redosize redo,
round(s.redosize / s.seconds, 2) "redo/s",
s.logicalreads logical,
round(s.logicalreads / s.seconds, 2) "logical/s",
physicalreads physical,
round(s.physicalreads / s.seconds, 2) "phy/s",
s.executes execs,
round(s.executes / s.seconds, 2) "execs/s",
s.parse,
round(s.parse / s.seconds, 2) "parse/s",
s.hardparse,
round(s.hardparse / s.seconds, 2) "hardparse/s",
s.transactions trans,
round(s.transactions / s.seconds, 2) "trans/s"
from (select curr_redo - last_redo redosize,
curr_logicalreads - last_logicalreads logicalreads,
curr_physicalreads - last_physicalreads physicalreads,
curr_executes - last_executes executes,
curr_parse - last_parse parse,
curr_hardparse - last_hardparse hardparse,
curr_transactions - last_transactions transactions,
round(((currtime + 0) - (lasttime + 0)) * 3600 * 24, 0) seconds,
to_char(currtime, 'yy/mm/dd') snap_date,
to_char(currtime, 'hh24:mi') currtime,
currsnap_id endsnap_id,
to_char(startup_time, 'yyyy-mm-dd hh24:mi:ss') startup_time
from (select a.redo last_redo,
a.logicalreads last_logicalreads,
a.physicalreads last_physicalreads,
a.executes last_executes,
a.parse last_parse,
a.hardparse last_hardparse,
a.transactions last_transactions,
lead(a.redo, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_redo,
lead(a.logicalreads, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_logicalreads,
lead(a.physicalreads, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_physicalreads,
lead(a.executes, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_executes,
lead(a.parse, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_parse,
lead(a.hardparse, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_hardparse,
lead(a.transactions, 1, null) over(partition by b.startup_time order by b.end_interval_time) curr_transactions,
b.end_interval_time lasttime,
lead(b.end_interval_time, 1, null) over(partition by b.startup_time order by b.end_interval_time) currtime,
lead(b.snap_id, 1, null) over(partition by b.startup_time order by b.end_interval_time) currsnap_id,
b.startup_time
from (select snap_id,
dbid,
instance_number,
sum(decode(stat_name, 'redo size', value, 0)) redo,
sum(decode(stat_name,
'session logical reads',
value,
0)) logicalreads,
sum(decode(stat_name,
'physical reads',
value,
0)) physicalreads,
sum(decode(stat_name, 'execute count', value, 0)) executes,
sum(decode(stat_name,
'parse count (total)',
value,
0)) parse,
sum(decode(stat_name,
'parse count (hard)',
value,
0)) hardparse,
sum(decode(stat_name,
'user rollbacks',
value,
'user commits',
value,
0)) transactions
from dba_hist_sysstat
where stat_name in
('redo size',
'session logical reads',
'physical reads',
'execute count',
'user rollbacks',
'user commits',
'parse count (hard)',
'parse count (total)')
group by snap_id, dbid, instance_number) a,
dba_hist_snapshot b
where a.snap_id = b.snap_id
and a.dbid = b.dbid
and a.instance_number = b.instance_number
order by end_interval_time)) s,
(select lead(a.value, 1, null) over(partition by b.startup_time order by b.end_interval_time) - a.value db_time,
lead(b.snap_id, 1, null) over(partition by b.startup_time order by b.end_interval_time) endsnap_id
from dba_hist_sys_time_model a, dba_hist_snapshot b
where a.snap_id = b.snap_id
and a.dbid = b.dbid
and a.instance_number = b.instance_number
and a.stat_name = 'DB time') t
where s.endsnap_id = t.endsnap_id
order by s.snap_date, time desc;