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Any table with name starting with '#' is automatically put 
into tempdb database.
Temp tables allow developers to create and scan tables 
within a stored procedure - and
have the tables totally isolated from all other database 
connections. This is very
valuable when results need to be processed several times 
within a loop, or when
a complex result set is expected (like a crosstab). Note 
that temp table transactions
are logged within tempdb (exception: select into create 
statements).

Yes, it’s possible that we can login to one database with 
corresponding user id. Because we can have multiple users 
associated to same login, each corresponds to different 
database, so 
We can login to one database with corresponding user id by 
issuing command from command prompt
is : isql –U_user id –P_password –D database_name

When you install Adaptive Server, it contains these system 
databases:

The master database controls the user databases and the 
operation of Adaptive Server as a whole. It stores 
information about all user databases and their associated 
database devices.

The model database is used by Adaptive Server as a template 
for creating new user databases.
	A new database cannot be smaller than the model 
database.

? The sybsystemprocs database contains the system stored 
procedures.

The sybsystemdb database contains information about 
distributed transactions.

? The temporary database, tempdb, stores temporary objects, 
including temporary tables created with the name 
prefix ?tempdb..?.

? The sybsecurity database, contains the audit system for 
Adaptive Server. It consists of:

The system tables, sysaudits_01, sysaudits_02, ... 
sysaudits_08, which contain the audit trail
The sysauditoptions table, which contains rows describing 
the global audit options.
All other default system tables that are derived from model

Databases per Adaptive Server Enterprise -	32,767 
Server Size 	- 8 terabytes
Database size 	- 4 terabytes 
Page Size 	- 2K, 4K, 8K, 16K
Databases spanned by one update 	16 
Databases opened by one transaction 	16
Tables in a query 			50 
Logins per server 		2,147,516,416
Users per database 		2,146,484,223 
Groups per database 		1,032,193
Columns per table 			1024 
Arguments to stored procedure 		2048

The Sybase Server 
A Sybase server consists of: 
       a) Two processes, data server and backup server.
       b) Devices which house the databases; one database 
(master) contains system and configuration data.
       c) A configuration file which contains the server 
attributes. 
Memory Model 
The Sybase memory model consists of: 
	a) The program area, which is where the dataserver 
executable is stored.
	b) The data cache, stores recently fetched pages 
from the database device. 
	c) The stored procedure cache, which contains 
optimized sql calls. 
A database device stores the objects that make up 
databases. 
	The term device does not necessarily refer to a 
distinct physical device: it can refer to any piece of a 
disk (such as a disk partition) or a file in the file 
system that is used to store databases and their objects.

Each database device or file must be prepared and made 
known to Adaptive Server before it can be used for database 
storage. This process is called initialization.
A segment is a label that points to one or more database 
devices. When you first create a database, Adaptive Server 
creates three segments in the database,

System: Stores the database?s system tables.
Logsegment: Stores the database?s transaction log.
Default: Stores all other database objects?unless you 
create additional segments and store tables or indexes on 
the new segments by using create table...on segment_name or 
create index...on segment_name

use tempdb
go
sp_helpsegment "default"
go

Do any of the following 
1. restart the sybase database server
2. alter the size of the tempdb

If your tempdb gets full you can do one of following:
you can expand your tempdb or you can use

select lct_admin(0,2)
go
It will abort all open transactions.But be sure the task by 
confirming with the concern users.
Restarting the server is not recommanded.

lct_admin (0,2) would abort all open transactions, or you 
can go for altering the tempdb space. Multiple tempdb's is 
a feature which can be implemented to minimize such issues 
of tempdb getting full.