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Question and Answer

Tom Kyte

Thanks for the question.

Asked: September 28, 2002 - 10:19 am UTC

Answered by: Tom Kyte - Last updated: April 08, 2016 - 3:25 am UTC

Category: Database - Version: 8.1.7.4

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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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