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