ask Tom解釋 connect processes sesssions 的區別
ask Tom解釋的很清楚,如下[@more@]
You Asked
Hi Tom What's the difference between connections, sessions and processes? I read a note from Metalink about the difference but I simply dont get it! May you give a brief explanation? Thank you
and we said...
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: ops$tkyte@ORA920.US.ORACLE.COM> select username from v$session where username is not null; USERNAME ------------------------------ OPS$TKYTE one session, ME ops$tkyte@ORA920.US.ORACLE.COM> select username, program from v$process; USERNAME PROGRAM --------------- ------------------------------------------------ PSEUDO ora920 oracle@tkyte-pc-isdn.us.oracle.com (PMON) ora920 oracle@tkyte-pc-isdn.us.oracle.com (DBW0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (LGWR) ora920 oracle@tkyte-pc-isdn.us.oracle.com (CKPT) ora920 oracle@tkyte-pc-isdn.us.oracle.com (SMON) ora920 oracle@tkyte-pc-isdn.us.oracle.com (RECO) ora920 oracle@tkyte-pc-isdn.us.oracle.com (CJQ0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (QMN0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (S000) ora920 oracle@tkyte-pc-isdn.us.oracle.com (D000) ora920 oracle@tkyte-pc-isdn.us.oracle.com (ARC0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (ARC1) tkyte oracle@tkyte-pc-isdn.us.oracle.com (TNS V1-V3) 14 rows selected. you can see all of the backgrounds and my dedicated server... ops$tkyte@ORA920.US.ORACLE.COM> 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! ops$tkyte@ORA920.US.ORACLE.COM> select username from v$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 ops$tkyte@ORA920.US.ORACLE.COM> select username, program from v$process; USERNAME PROGRAM --------------- ------------------------------------------------ PSEUDO ora920 oracle@tkyte-pc-isdn.us.oracle.com (PMON) ora920 oracle@tkyte-pc-isdn.us.oracle.com (DBW0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (LGWR) ora920 oracle@tkyte-pc-isdn.us.oracle.com (CKPT) ora920 oracle@tkyte-pc-isdn.us.oracle.com (SMON) ora920 oracle@tkyte-pc-isdn.us.oracle.com (RECO) ora920 oracle@tkyte-pc-isdn.us.oracle.com (CJQ0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (QMN0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (S000) ora920 oracle@tkyte-pc-isdn.us.oracle.com (D000) ora920 oracle@tkyte-pc-isdn.us.oracle.com (ARC0) ora920 oracle@tkyte-pc-isdn.us.oracle.com (ARC1) tkyte oracle@tkyte-pc-isdn.us.oracle.com (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 ops$tkyte@ORA920.US.ORACLE.COM> 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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