oracle中pga内存分配原则
pga_aggregate_target 通常缩写为P_A_T，该参数同时限制全局pga分配和私有工作区内存分配
在oracle9i以及10gr1中，单个sql操作内存使用存在如下限制：
对于串行操作，单个sql操作能够使用的pga内存按照一下原则分配：
MIN(5%*PGA_AGGREGATE_TARGET,100MB)
此处的5%*PGA_AGGREGATE_TARGET实际上是由_smm_nax_size决定的，该参数限制自动工作区模式下最大的工作区使用
对于并行操作：
30%PGA_AGGREGATE_TARGET/DOP(DOP=DEGREE IF PARALLELISM,并行度)
在oracle10gr2以及oracle11g中，内存使用存在如下限制：
对于串行操作，能够使用的pga内存按照以下原则分配：
如果P_A_T<500MB,则_smm_max_size=20%P_A_T
如果P_A_T在500MB和1000MB之间，_smm_max_size=100MB
如果P_A_T介于1001MB和2560MB之间，_smm_max_size=10%*P_A_T
如果P_A_T>2560MB则_smm_max_size=262060MB
对于并行操作，能够使用的pga内存按照以下原则分配：
50%PGA_AGGREGATE_TARGET/DOP (DOP=Degree of parallelism,并行度)
但是注意，当dop<5时，_smm_max_size限制生效，并行度超过5时另外一个限制并行的参数_smm_px_max_size才会生效
从oralce10g开始 pga算法受一个新增的隐含参数_newsort_abled影响，如果将该参数设置为false，则数据库会使用之前oracle9i中的算法规则：
sys@ORCL>@gethidpar.sql
输入 name 的值: newsort_enabled
原值 4: and x.ksppinm like'%&name%'
新值 4: and x.ksppinm like'%newsort_enabled%'
KSPPINM KSPPSTVL
_newsort_enabled TRUE
上述@gethidpar.sql内容：
select x.ksppinm,y.ksppstvl,x.ksppdesc from x$ksppi x,x$ksppcv y wherex.indx=y.indx
and y.inst_id=userenv('Instance')
and x.inst_id=userenv('Instance')
and x.ksppinm like '%&name%';
要理解pga的自动调整，还需要区分可调整内存(TUNABLE MEMORY SIZE) 与不可调整内存（UNTUNABLE MEMORY SIZE），可调整内存是由sql工作区使用的，其余部分是不可调整内存
Oracle在评估执行计划时会更具PGA_AGGREGATE_TARGET参数评估在sort，HASH_JOIN或Bitmap操作时能够使用的最大或最小内存，从而选择最优的执行计划
对于一个单纯的数据库服务器，通常我们需要保存20%的物理内存给操作系统使用，剩余80%可以分配给oracle使用，而oracle的内存由pga和sga构成，pga可以占用oracle消耗内存的20%（OLTP系统）至50%（DSS系统）
通过v$process 可以查询pga的相关使用情况：V$PROCESS displays information about the currently active processes.
| Column |
Datatype |
Description |
| ADDR |
RAW(4 | 8) |
Address of the process state object |
| PID |
NUMBER |
Oracle process identifier |
| SPID |
VARCHAR2(24) |
Operating system process identifier |
| PNAME |
VARCHAR2(5) |
Name of this process |
| USERNAME |
VARCHAR2(15) |
Operating system process username Note: Any two-task user coming across the network has "-T" appended to the username. |
| SERIAL# |
NUMBER |
Process serial number |
| TERMINAL |
VARCHAR2(30) |
Operating system terminal identifier |
| PROGRAM |
VARCHAR2(48) |
Program in progress |
| TRACEID |
VARCHAR2(255) |
Trace file identifier |
| TRACEFILE |
VARCHAR2(513) |
Trace file name of the process |
| BACKGROUND |
VARCHAR2(1) |
1 for a background process; NULL for a normal process |
| LATCHWAIT |
VARCHAR2(8) |
Address of the latch the process is waiting for; NULL if none |
| LATCHSPIN |
VARCHAR2(8) |
This column is obsolete |
| PGA_USED_MEM |
NUMBER |
PGA memory currently used by the process |
| PGA_ALLOC_MEM |
NUMBER |
PGA memory currently allocated by the process (including free PGA memory not yet released to the operating system by the server process) |
| PGA_FREEABLE_MEM |
NUMBER |
Allocated PGA memory which can be freed |
| PGA_MAX_MEM |
NUMBER |
Maximum PGA memory ever allocated by the process |
sys@ORCL>select pid,spid,pname,usernamefrom v$process;
PID SPID PNAMEUSERNAME
---------- ------------------------ --------------------
1
2 7780 PMON SYSTEM
3 6156 VKTM SYSTEM
4 7864 GEN0 SYSTEM
5 3604 DIAG SYSTEM
6 7916 DBRM SYSTEM
7 7940 PSP0 SYSTEM
8 7216 DIA0 SYSTEM
9 7264 MMAN SYSTEM
10 7324 DBW0 SYSTEM
11 8020 LGWR SYSTEM
12 7972 CKPT SYSTEM
13 7908 SMON SYSTEM
14 6864 RECO SYSTEM
15 1604 MMON SYSTEM
16 1440 MMNL SYSTEM
17 8176 D000 SYSTEM
18 6324 S000 SYSTEM
19 3976 SMCO SYSTEM
20 4560 W000 SYSTEM
23 6332 ARC0 SYSTEM
24 4732 ARC1 SYSTEM
25 1436 ARC2 SYSTEM
26 6732 ARC3 SYSTEM
27 7796 QMNC SYSTEM
28 8660 SYSTEM
31 6424 Q000 SYSTEM
32 892 Q001 SYSTEM
35 7976 CJQ0 SYSTEM
选择spid为8660为例：
sys@ORCL>select pid,spid,username,pga_used_mem,pga_alloc_mem,pga_freeable_mem,pga_max_mem fromv$process where spid=8660;
PID SPID USERNAME PGA_USED_MEMPGA_ALLOC_MEM PGA_FREEABLE_MEM PGA_MAX_MEM
---------- --------------------------------------- ------------ ------------- ---------------- -----------
28 8660 SYSTEM 1161714 1786898 524288 1786898
通过v$process_memory视图可以进一步知道pga内存消耗在什么地方：
V$PROCESS_MEMORY displaysdynamic PGA memory usage by named component categories for each process.
| Column |
Datatype |
Description |
| PID |
NUMBER |
Oracle process identifier |
| SERIAL# |
NUMBER |
Oracle process serial number |
| CATEGORY |
VARCHAR2(15) |
Category name. Categories include "SQL", "PL/SQL", "OLAP" and "JAVA". Special categories are "Freeable" and "Other". Freeable memory has been allocated to the process by the operating system, but has not been allocated to a category. "Other" memory has been allocated to a category, but not to one of the named categories. |
| ALLOCATED |
NUMBER |
Bytes of PGA memory allocated by the process for the category. For the "Freeable" category, it is the amount of free PGA memory eligible to be released to the operating system. |
| USED |
NUMBER |
Bytes of PGA memory used by the process for the category. For "Freeable", the value is zero. For "Other", the value is NULL for performance reasons. |
| MAX_ALLOCATED |
NUMBER |
Maximum bytes of PGA memory ever allocated by the process for the category. |
sys@ORCL>selectp.program,p.spid,pm.category,pm.allocated,pm.allocated,pm.used,pm.max_allocatedfrom v$process p,v$process_memory pm where p.pid=pm.pid and p.spid=8660;
PROGRAM SPID CATEGORY ALLOCATED ALLOCATED USED MAX_ALLOCATED
---------------------------------------------------------------------------------------- --------------- ---------- ---------- -----------------------
ORACLE.EXE (SHAD) 8660 SQL 15416 15416 11888 418096
ORACLE.EXE (SHAD) 8660 PL/SQL 28296 28296 20136 29440
ORACLE.EXE (SHAD) 8660 Freeable 327680 327680 0
ORACLE.EXE (SHAD) 8660 Other 1415506 1415506 1415506
还可以通过pid得到当前执行的sql语句：
SELECT /*+ ORDERED */
sql_text
FROM v$sqltext a
WHERE (a.hash_value, a.address) IN (
SELECT DECODE (sql_hash_value,
0, prev_hash_value,
sql_hash_value
),
DECODE (sql_hash_value, 0,prev_sql_addr, sql_address)
FROM v$session b
WHERE b.paddr = (SELECT addr
FROM v$processc
WHERE c.spid =TO_NUMBER ('&pid', 'xxxx')))
ORDER BY piece ASC