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WINDOWS下的程序员出身,偶尔也写一些linux平台下小程序, 后转行数据库行业,专注于ORACLE和DB2的运维和优化。 同时也是ios移动开发者。欢迎志同道合的朋友一起研究技术。 数据库技术交流群:58308065,23618606

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分类: Oracle

2012-01-19 10:34:46

A process is a physical process or thread.
 
On unix, you can see a process with "ps" for example.  It is there.
 
There are many types of processes in Oracle -- background processes like SMON, PMON,
RECO, ARCH, CKPT, EMNn, DBWR, etc.....  And user processes like dedicated servers or
shared server (multi-threaded server -- aka MTS -- configuration)

A connection is a "physical circuit", a pathway to a database.  You can be connected to a
database yet have 0 or 1 or MORE sessions going on that connection.  We can see that with
sqlplus, consider (single user system here, its all about me)

[tkyte@tkyte-pc-isdn tkyte]$ ps -auxww | grep oracleora920
[tkyte@tkyte-pc-isdn tkyte]$ sqlplus /nolog
SQL*Plus: Release 9.2.0.1.0 - Production on Sat Sep 28 10:36:03 2002
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.
 
idle> !ps -auxww | grep oracleora920
tkyte    19971  0.0  0.1  2196  916 pts/1    S    10:36   0:00 /bin/bash -c ps -auxww |
grep oracleora920
tkyte    19973  0.0  0.1  1736  564 pts/1    S    10:36   0:00 grep oracleora920
 
no process, no nothing
 
idle> connect /
Connected.
idle> !ps -auxww | grep oracleora920
ora920   19974  1.5  2.2 230976 11752 ?      S    10:36   0:00 oracleora920
(DESCRIPTION=(LOCAL=YES)(ADDRESS=(PROTOCOL=beq)))
tkyte    19975  0.0  0.1  2196  916 pts/1    S    10:36   0:00 /bin/bash -c ps -auxww |
grep oracleora920
tkyte    19977  0.0  0.1  1736  564 pts/1    S    10:36   0:00 grep oracleora920
 
got my process now...
 
idle> disconnect
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production
idle> !ps -auxww | grep oracleora920
ora920   19974  0.6  2.3 230976 11876 ?      S    10:36   0:00 oracleora920
(DESCRIPTION=(LOCAL=YES)(ADDRESS=(PROTOCOL=beq)))
tkyte    19978  0.0  0.1  2196  916 pts/1    S    10:36   0:00 /bin/bash -c ps -auxww |
grep oracleora920
tkyte    19980  0.0  0.1  1736  564 pts/1    S    10:36   0:00 grep oracleora920
 
idle> select * from dual;

SP2-0640: Not connected
 
still have my process, but no session, the message is a little "misleading". 
Technically -- I have a connection, I don't have a session

further, autotrace in sqlplus can be used to show that you can have
a) a connection
b) that uses a single process
c) to service two sessions:

> select username from v$session where username is not
null;
USERNAME
------------------------------
OPS$TKYTE
one session, ME
> select username, program from v$process;
USERNAME        PROGRAM
--------------- ------------------------------------------------
                PSEUDO
ora920          (PMON)
ora920          (DBW0)
ora920          (LGWR)
ora920          (CKPT)
ora920          (SMON)
ora920          (RECO)
ora920          (CJQ0)
ora920          (QMN0)
ora920          (S000)
ora920          (D000)
ora920          (ARC0)
ora920          (ARC1)
tkyte           (TNS V1-V3)
14 rows selected.
you can see all of the backgrounds and my dedicated server...
> set autotrace on statistics;
Autotrace for statistics uses ANOTHER session so it can query up the stats for your
CURRENT session without impacting the STATS for that session!

> select username from v$session where username is not
null;
USERNAME
------------------------------
OPS$TKYTE
OPS$TKYTE

see, two sessions but....
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        418  bytes sent via SQL*Net to client
        499  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed
> select username, program from v$process;
USERNAME        PROGRAM
--------------- ------------------------------------------------
                PSEUDO
ora920          (PMON)
ora920          (DBW0)
ora920          (LGWR)
ora920          (CKPT)
ora920          (SMON)
ora920          (RECO)
ora920          (CJQ0)
ora920          (QMN0)
ora920          (S000)
ora920          (D000)
ora920          (ARC0)
ora920          (ARC1)
tkyte           (TNS V1-V3)
14 rows selected.
same 14 processes...
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
       1095  bytes sent via SQL*Net to client
        499  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         14  rows processed
>

I'll try to put it into a single, simple paragraph:
A connection is a physical circuit between you and the database.  A connection might be
one of many types -- most popular begin DEDICATED server and SHARED server.  Zero, one or
more sessions may be established over a given connection to the database as show above
with sqlplus.  A process will be used by a session to execute statements.  Sometimes
there is a one to one relationship between CONNECTION->SESSION->PROCESS (eg: a normal
dedicated server connection).  Sometimes there is a one to many from connection to
sessions (eg: like autotrace, one connection, two sessions, one process).  A process does
not have to be dedicated to a specific connection or session however, for example when
using shared server (MTS), your SESSION will grab a process from a pool of processes in
order to execute a statement.  When the call is over, that process is released back to
the pool of processes.
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