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Archive-name: databases/sybase-faq/part5
URL: http://www.isug.com/Sybase_FAQ
Version: 1.7
Maintainer: David Owen
Last-modified: 2003/01/17
Posting-Frequency: posted every 3rd month
A how-to-find-the-FAQ article is posted on the intervening months.
User Database Administration
1.2.1 Changing varchar(m) to varchar(n)
1.2.2 Frequently asked questions on Table partitioning
1.2.3 How do I manually drop a table?
1.2.4 Why not create all my columns varchar(255)?
1.2.5 What's a good example of a transaction?
1.2.6 What's a natural key?
1.2.7 Making a Stored Procedure invisible
1.2.8 Saving space when inserting rows monotonically
1.2.9 How to compute database fragmentation
1.2.10 Tasks a DBA should do...
1.2.11 How to implement database security
1.2.12 How to shrink a database
1.2.13 How do I turn on auditing of all SQL text sent to the server
1.2.14 sp_helpdb/sp_helpsegment is returning negative numbers
Advanced Administration Basic Administration ASE FAQ
-------------------------------------------------------------------------------
1.2.1: Changing varchar(m) to varchar(n)
-------------------------------------------------------------------------------
Before you start:
select max(datalength(column_name))
from affected_table
In other words, please be sure you're going into this with your head on
straight.
How To Change System Catalogs
This information is Critical To The Defense Of The Free World, and you would be
Well Advised To Do It Exactly As Specified:
use master
go
sp_configure "allow updates", 1
go
reconfigure with override /* System 10 and below */
go
use victim_database
go
select name, colid
from syscolumns
where id = object_id("affected_table")
go
begin tran
go
update syscolumns
set length = new_value
where id = object_id("affected_table")
and colid = value_from_above
go
update sysindexes
set maxlen = maxlen + increase/decrease?
where id=object_id("affected_table")
and indid = 0
go
/* check results... cool? Continue... else rollback tran */
commit tran
go
use master
go
sp_configure "allow updates", 0
go
reconfigure /* System 10 and below */
go
Return to top
-------------------------------------------------------------------------------
1.2.2: FAQ on partitioning
-------------------------------------------------------------------------------
Index of Sections
* What Is Table Partitioning?
+ Page Contention for Inserts
+ I/O Contention
+ Caveats Regarding I/O Contention
* Can I Partition Any Table?
+ How Do I Choose Which Tables To Partition?
* Does Table Partitioning Require User-Defined Segments?
* Can I Run Any Transact-SQL Command on a Partitioned Table?
* How Does Partition Assignment Relate to Transactions?
* Can Two Tasks Be Assigned to the Same Partition?
* Must I Use Multiple Devices to Take Advantage of Partitions?
* How Do I Create A Partitioned Table That Spans Multiple Devices?
* How Do I Take Advantage of Table Partitioning with bcp in?
* Getting More Information on Table Partitioning
What Is Table Partitioning?
Table partitioning is a procedure that creates multiple page chains for a
single table.
The primary purpose of table partitioning is to improve the performance of
concurrent inserts to a table by reducing contention for the last page of a
page chain.
Partitioning can also potentially improve performance by making it possible to
distribute a table's I/O over multiple database devices.
Page Contention for Inserts
By default, ASE stores a table's data in one double-linked set of pages called
a page chain. If the table does not have a clustered index, ASE makes all
inserts to the table in the last page of the page chain.
When a transaction inserts a row into a table, ASE holds an exclusive page lock
on the last page while it inserts the row. If the current last page becomes
full, ASE allocates and links a new last page.
As multiple transactions attempt to insert data into the table at the same
time, performance problems can occur. Only one transaction at a time can obtain
an exclusive lock on the last page, so other concurrent insert transactions
block each other.
Partitioning a table creates multiple page chains (partitions) for the table
and, therefore, multiple last pages for insert operations. A partitioned table
has as many page chains and last pages as it has partitions.
I/O Contention
Partitioning a table can improve I/O contention when ASE writes information in
the cache to disk. If a table's segment spans several physical disks, ASE
distributes the table's partitions across fragments on those disks when you
create the partitions.
A fragment is a piece of disk on which a particular database is assigned space.
Multiple fragments can sit on one disk or be spread across multiple disks.
When ASE flushes pages to disk and your fragments are spread across different
disks, I/Os assigned to different physical disks can occur in parallel.
To improve I/O performance for partitioned tables, you must ensure that the
segment containing the partitioned table is composed of fragments spread across
multiple physical devices.
Caveats Regarding I/O Contention
Be aware that when you use partitioning to balance I/O you run the risk of
disrupting load balancing even as you are trying to achieve it. The following
scenarios can keep you from gaining the load balancing benefits you want:
* You are partitioning an existing table. The existing data could be sitting
on any fragment. Because partitions are randomly assigned, you run the risk
of filling up a fragment. The partition will then steal space from other
fragments, thereby disrupting load balancing.
* Your fragments differ in size.
* The segment maps are configured such that other objects are using the
fragments to which the partitions are assigned.
* A very large bcp job inserts many rows within a single transaction. Because
a partition is assigned for the lifetime of a transaction, a huge amount of
data could go to one particular partition, thus filling up the fragment to
which that partition is assigned.
Can I Partition Any Table?
No. You cannot partition the following kinds of tables:
1. Tables with clustered indexes (as of release 11.5 it is possible to have a
clustered index on a partitioned table)
2. ASE system tables
3. Work tables
4. Temporary tables
5. Tables that are already partitioned. However, you can unpartition and then
re-partition tables to change the number of partitions.
How Do I Choose Which Tables To Partition?
You should partition heap tables that have large amounts of concurrent insert
activity. (A heap table is a table with no clustered index.) Here are some
examples:
1. An "append-only" table to which every transaction must write
2. Tables that provide a history or audit list of activities
3. A new table into which you load data with bcp in. Once the data is loaded
in, you can unpartition the table. This enables you to create a clustered
index on the table, or issue other commands not permitted on a partition
table.
Does Table Partitioning Require User-Defined Segments?
No. By design, each table is intrinsically assigned to one segment, called the
default segment. When a table is partitioned, any partitions on that table are
distributed among the devices assigned to the default segment.
In the example under "How Do I Create A Partitioned Table That Spans Multiple
Devices?", the table sits on a user-defined segment that spans three devices.
Can I Run Any Transact-SQL Command on a Partitioned Table?
No. Once you have partitioned a table, you cannot use any of the following
Transact-SQL commands on the table until you unpartition it:
1. drop table
2. sp_placeobject
3. truncate table
4. alter table table_name partition n
On releases of ASE prior to 11.5 it was not possible to create a clustered
index on a partitioned table either.
How Does Partition Assignment Relate to Transactions?
A user is assigned to a partition for the duration of a transaction. Assignment
of partitions resumes with the first insert in a new transaction. The user
holds the lock, and therefore partition, until the transaction ends.
For this reason, if you are inserting a great deal of data, you should batch it
into separate jobs, each within its own transaction. See "How Do I Take
Advantage of Table Partitioning with bcp in?", for details.
Can Two Tasks Be Assigned to the Same Partition?
Yes. ASE randomly assigns partitions. This means there is always a chance that
two users will vie for the same partition when attempting to insert and one
would lock the other out.
The more partitions a table has, the lower the probability of users trying to
write to the same partition at the same time.
Must I Use Multiple Devices to Take Advantage of Partitions?
It depends on which type of performance improvement you want.
Table partitioning improves performance in two ways: primarily, by decreasing
page contention for inserts and, secondarily, by decreasing i/o contention.
"What Is Table Partitioning?" explains each in detail.
If you want to decrease page contention you do not need multiple devices. If
you want to decrease i/o contention, you must use multiple devices.
How Do I Create A Partitioned Table That Spans Multiple Devices?
Creating a partitioned table that spans multiple devices is a multi-step
procedure. In this example, we assume the following:
* We want to create a new segment rather than using the default segment.
* We want to spread the partitioned table across three devices, data_dev1,
data_dev2, and data_dev3.
Here are the steps:
1. Define a segment:
sp_addsegment newsegment, my_database,data_dev1
2. Extend the segment across all three devices:
sp_extendsegment newsegment, my_database, data_dev2
sp_extendsegment newsegment, my_database, data_dev3
3. Create the table on the segment:
create table my_table
(names, varchar(80) not null)
on newsegment
4. Partition the table:
alter table my_table partition 30
How Do I Take Advantage of Table Partitioning with bcp in?
You can take advantage of table partitioning with bcp in by following these
guidelines:
1. Break up the data file into multiple files and simultaneously run each of
these files as a separate bcp job against one table.
Running simultaneous jobs increases throughput.
2. Choose a number of partitions greater than the number of bcp jobs.
Having more partitions than processes (jobs) decreases the probability of
page lock contention.
3. Use the batch option of bcp in. For example, after every 100 rows, force a
commit. Here is the syntax of this command:
bcp table_name in filename -b100
Each time a transaction commits, ASE randomly assigns a new partition for
the next insert. This, in turn, reduces the probability of page lock
contention.
Getting More Information on Table Partitioning
For more information on table partitioning, see the chapter on controlling
physical data placement in the ASE Performance and Tuning Guide.
Return to top
-------------------------------------------------------------------------------
1.2.3: How to manually drop a table
-------------------------------------------------------------------------------
Occasionally you may find that after issuing a drop table command that the ASE
crashed and consequently the table didn't drop entirely. Sure you can't see it
but that sucker is still floating around somewhere.
Here's a list of instructions to follow when trying to drop a corrupt table:
1. sp_configure allow, 1
go
reconfigure with override
go
2. Write db_id down.
use db_name
go
select db_id()
go
3. Write down the id of the bad_table:
select id
from sysobjects
where name = bad_table_name
go
4. You will need these index IDs to run dbcc extentzap. Also, remember that if
the table has a clustered index you will need to run extentzap on index
"0", even though there is no sysindexes entry for that indid.
select indid
from sysindexes
where id = table_id
go
5. This is not required but a good idea:
begin transaction
go
6. Type in this short script, this gets rid of all system catalog information
for the object, including any object and procedure dependencies that may be
present.
Some of the entries are unnecessary but better safe than sorry.
declare @obj int
select @obj = id from sysobjects where name =
delete syscolumns where id = @obj
delete sysindexes where id = @obj
delete sysobjects where id = @obj
delete sysprocedures where id in
(select id from sysdepends where depid = @obj)
delete sysdepends where depid = @obj
delete syskeys where id = @obj
delete syskeys where depid = @obj
delete sysprotects where id = @obj
delete sysconstraints where tableid = @obj
delete sysreferences where tableid = @obj
delete sysdepends where id = @obj
go
7. Just do it!
commit transaction
go
8. Gather information to run dbcc extentzap:
use master
go
sp_dboption db_name, read, true
go
use db_name
go
checkpoint
go
9. Run dbcc extentzap once for each index (including index 0, the data level)
that you got from above:
use master
go
dbcc traceon (3604)
go
dbcc extentzap (db_id, obj_id, indx_id, 0)
go
dbcc extentzap (db_id, obj_id, indx_id, 1)
go
Notice that extentzap runs twice for each index. This is because the
last parameter (the sort bit) might be 0 or 1 for each index, and you
want to be absolutely sure you clean them all out.
10. Clean up after yourself.
sp_dboption db_name, read, false
go
use db_name
go
checkpoint
go
sp_configure allow, 0
go
reconfigure with override
go
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-------------------------------------------------------------------------------
1.2.4: Why not max out all my columns?
-------------------------------------------------------------------------------
People occasionally ask the following valid question:
Suppose I have varying lengths of character strings none of which should
exceed 50 characters.
Is there any advantage of last_name varchar(50) over this last_name varchar
(255)?
That is, for simplicity, can I just define all my varying strings to be
varchar(255) without even thinking about how long they may actually be? Is
there any storage or performance penalty for this.
There is no performance penalty by doing this but as another netter pointed
out:
If you want to define indexes on these fields, then you should specify the
smallest size because the sum of the maximal lengths of the fields in the
index can't be greater than 256 bytes.
and someone else wrote in saying:
Your data structures should match the business requirements. This way the
data structure themselves becomes a data dictionary for others to model
their applications (report generation and the like).
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-------------------------------------------------------------------------------
1.2.5: What's a good example of a transaction?
-------------------------------------------------------------------------------
This answer is geared for Online Transaction Processing (OTLP)
applications.
To gain maximum throughput all your transactions should be in stored procedures
- see Q1.5.8. The transactions within each stored procedure should be short and
simple. All validation should be done outside of the transaction and only the
modification to the database should be done within the transaction. Also, don't
forget to name the transaction for sp_whodo - see Q9.2.
The following is an example of a good transaction:
/* perform validation */
select ...
if ... /* error */
/* give error message */
else /* proceed */
begin
begin transaction acct_addition
update ...
insert ...
commit transaction acct_addition
end
The following is an example of a bad transaction:
begin transaction poor_us
update X ...
select ...
if ... /* error */
/* give error message */
else /* proceed */
begin
update ...
insert ...
end
commit transaction poor_us
This is bad because:
* the first update on table X is held throughout the transaction. The idea
with OLTP is to get in and out fast.
* If an error message is presented to the end user and we await their
response, we'll maintain the lock on table X until the user presses return.
If the user is out in the can we can wait for hours.
Return to top
-------------------------------------------------------------------------------
1.2.6: What's a natural key?
-------------------------------------------------------------------------------
Let me think back to my database class... okay, I can't think that far so I'll
paraphrase... essentially, a natural key is a key for a given table that
uniquely identifies the row. It's natural in the sense that it follows the
business or real world need.
For example, assume that social security numbers are unique (I believe it is
strived to be unique but it's not always the case), then if you had the
following employee table:
employee:
ssn char(09)
f_name char(20)
l_name char(20)
title char(03)
Then a natural key would be ssn. If the combination of _name and l_name were
unique at this company, then another natural key would be f_name, l_name. As a
matter of fact, you can have many natural keys in a given table but in practice
what one does is build a surrogate (or artificial) key.
The surrogate key is guaranteed to be unique because (wait, get back, here it
goes again) it's typically a monotonically increasing value. Okay, my
mathematician wife would be proud of me... really all it means is that the key
is increasing linearly: i+1
The reason one uses a surrogate key is because your joins will be faster.
If we extended our employee table to have a surrogate key:
employee:
id identity
ssn char(09)
f_name char(20)
l_name char(20)
title char(03)
Then instead of doing the following:
where a.f_name = b.f_name
and a.l_name = a.l_name
we'd do this:
where a.id = b.id
We can build indexes on these keys and since Sybase's atomic storage unit is
2K, we can stash more values per 2K page with smaller indexes thus giving us
better performance (imagine the key being 40 bytes versus being say 4 bytes...
how many 40 byte values can you stash in a 2K page versus a 4 byte value? --
and how much wood could a wood chuck chuck, if a wood chuck could chuck wood?)
Does it have anything to do with natural joins?
Um, not really... from "A Guide to Sybase..", McGovern and Date, p. 112:
The equi-join by definition must produce a result containing two identical
columns. If one of those two columns is eliminated, what is left is called
the natural join.
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-------------------------------------------------------------------------------
1.2.7: Making a Stored Procedure invisible
-------------------------------------------------------------------------------
System 11.5 and above
It is now possible to encrypt your stored procedure code that is stored in the
syscomments table. This is preferred than the old method of deleting the data
as deleting will impact future upgrades. You can encrypt the text with the
sp_hidetext system procedure.
Pre-System 11.5
Perhaps you are trying to prevent the buyer of your software from defncopy'ing
all your stored procedures. It is perfectly safe to delete the syscomments
entries of any stored procedures you'd like to protect:
sp_configure "allow updates", 1
go
reconfigure with override /* System 10 and below */
go
use affected_database
go
delete syscomments where id = object_id("procedure_name")
go
use master
go
sp_configure "allow updates", 0
go
I believe in future releases of Sybase we'll be able to see the SQL that is
being executed. I don't know if that would be simply the stored procedure name
or the SQL itself.
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-------------------------------------------------------------------------------
1.2.8: Saving space when inserting rows monotonically
-------------------------------------------------------------------------------
If the columns that comprise the clustered index are monotonically increasing
(that is, new row key values are greater than those previously inserted) the
following System 11 dbcc tune will not split the page when it's half way full.
Rather it'll let the page fill and then allocate another page:
dbcc tune(ascinserts, 1, "my_table")
By the way, SyBooks is wrong when it states that the above needs to be reset
when ASE is rebooted. This is a permanent setting.
To undo it:
dbcc tune(ascinserts, 0, "my_table")
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-------------------------------------------------------------------------------
1.2.9: How to compute database fragmentation
-------------------------------------------------------------------------------
Command
dbcc traceon(3604)
go
dbcc tab(production, my_table, 0)
go
Interpretation
A delta of one means the next page is on the same track, two is a short seek,
three is a long seek. You can play with these constants but they aren't that
important.
A table I thought was unfragmented had L1 = 1.2 L2 = 1.8
A table I thought was fragmented had L1 = 2.4 L2 = 6.6
How to Fix
You fix a fragmented table with clustered index by dropping and creating the
index. This measurement isn't the correct one for tables without clustered
indexes. If your table doesn't have a clustered index, create a dummy one and
drop it.
Return to top
-------------------------------------------------------------------------------
1.2.10: Tasks a DBA should do...
-------------------------------------------------------------------------------
A good presentation of a DBA's duties has been made available by Jeff Garbus (
jeffg@soaringeagleltd.com) of Soaring Eagle Consulting Ltd (http://
www.soaringeagleltd.com) and numerous books can be found here. These are
Powerpoint slides converted to web pages and so may be difficult to view with a
text browser!
An alternative view is catalogued below. (OK, so this list is crying out for a
bit of a revamp since checkstorage came along Ed!)
DBA Tasks
+-------------------------------------------------------------------------+
| Task | Reason | Period |
|------------------------+---------------+--------------------------------|
| | I consider | If your ASE permits, daily |
| | these the | before your database dumps. If |
| dbcc checkdb, | minimal | this is not possible due to |
| checkcatalog, | dbcc's to | the size of your databases, |
| checkalloc | ensure the | then try the different options |
| | integrity of | so that the end of, say, a |
| | your database | week, you've run them all. |
|------------------------+---------------+--------------------------------|
| Disaster recovery | Always be | |
| scripts - scripts to | prepared for | |
| rebuild your ASE in | the worst. | |
| case of hardware | Make sure to | |
| failure | test them. | |
|------------------------+---------------+--------------------------------|
| scripts to logically | | |
| dump your master | You can | |
| database, that is bcp | selectively | |
| the critical system | rebuild your | |
| tables: sysdatabases, | database in | Daily |
| sysdevices, syslogins, | case of | |
| sysservers, sysusers, | hardware | |
| syssegments, | failure | |
| sysremotelogins | | |
|------------------------+---------------+--------------------------------|
| | A system | |
| | upgrade is | After any change as well as |
| %ls -la <disk_devices> | known to | daily |
| | change the | |
| | permissions. | |
|------------------------+---------------+--------------------------------|
| dump the user | CYA* | Daily |
| databases | | |
|------------------------+---------------+--------------------------------|
| dump the transaction | CYA | Daily |
| logs | | |
|------------------------+---------------+--------------------------------|
| dump the master | CYA | After any change as well as |
| database | | daily |
|------------------------+---------------+--------------------------------|
| | This is the | |
| System 11 and beyond - | configuration | |
| save the $DSQUERY.cfg | that you've | After any change as well as |
| to tape | dialed in, | daily |
| | why redo the | |
| | work? | |
|------------------------+---------------+--------------------------------|
| | | Depending on how often your |
| | | major tables change. Some |
| | | tables are pretty much static |
| | | (e.g. lookup tables) so they |
| update statistics on | To ensure the | don't need an update |
| frequently changed | performance | statistics, other tables |
| tables and | of your ASE | suffer severe trauma (e.g. |
| sp_recompile | | massive updates/deletes/ |
| | | inserts) so an update stats |
| | | needs to be run either nightly |
| | | /weekly/monthly. This should |
| | | be done using cronjobs. |
|------------------------+---------------+--------------------------------|
| create a dummy ASE and | | |
| do bad things to it: | See disaster | When time permits |
| delete devices, | recovery! | |
| destroy permissions... | | |
|------------------------+---------------+--------------------------------|
| Talk to the | It's better | |
| application | to work with | As time permits. |
| developers. | them than | |
| | against them. | |
|------------------------+---------------+--------------------------------|
| Learn new tools | So you can | As time permits. |
| | sleep! | |
|------------------------+---------------+--------------------------------|
| Read | Passes the | Priority One! |
| comp.databases.sybase | time. | |
+-------------------------------------------------------------------------+
* Cover Your Ass
Return to top
-------------------------------------------------------------------------------
1.2.11: How to implement database security
-------------------------------------------------------------------------------
This is a brief run-down of the features and ideas you can use to implement
database security:
Logins, Roles, Users, Aliases and Groups
* sp_addlogin - Creating a login adds a basic authorisation for an account -
a username and password - to connect to the server. By default, no access
is granted to any individual databases.
* sp_adduser - A user is the addition of an account to a specific database.
* sp_addalias - An alias is a method of allowing an account to use a specific
database by impersonating an existing database user or owner.
* sp_addgroup - Groups are collections of users at the database level. Users
can be added to groups via the sp_adduser command.
A user can belong to only one group - a serious limitation that Sybase
might be addressing soon according to the ISUG enhancements requests.
Permissions on objects can be granted or revoked to or from users or
groups.
* sp_role - A role is a high-level Sybase authorisation to act in a specific
capacity for administration purposes. Refer to the Sybase documentation for
details.
Recommendations
Make sure there is a unique login account for each physical person and/or
process that uses the server. Creating generic logins used by many people or
processes is a bad idea - there is a loss of accountability and it makes it
difficult to track which particular person is causing server problems when
looking at the output of sp_who. Note that the output of sp_who gives a
hostname - properly coded applications will set this value to something
meaningful (ie. the machine name the client application is running from) so you
can see where users are running their programs. Note also that if you look at
master..sysprocesses rather than just sp_who, there is also a program_name.
Again, properly coded applications will set this (eg. to 'isql') so you can see
which application is running. If you're coding your own client applications,
make sure you set hostname and program_name via the appropriate Open Client
calls. One imaginative use I've seen of the program_name setting is to
incorporate the connection time into the name, eg APPNAME-DDHHMM (you have 16
characters to play with), as there's no method of determining this otherwise.
Set up groups, and add your users to them. It is much easier to manage an
object permissions system in this way. If all your permissions are set to
groups, then adding a user to the group ensures that users automatically
inherit the correct permissions - administration is *much* simpler.
Objects and Permissions
Access to database objects is defined by granting and/or revoking various
access rights to and from users or groups. Refer to the Sybase documentation
for details.
Recommendations
The ideal setup has all database objects being owned by the dbo, meaning no
ordinary users have any default access at all. Specific permissions users
require to access the database are granted explicitly. As mentioned above - set
permissions for objects to a group and add users to that group. Any new user
added to the database via the group then automatically obtains the correct set
of permissions.
Preferably, no access is granted at all to data tables, and all read and write
activity is accomplished through stored procedures that users have execute
permission on. The benefit of this from a security point of view is that access
can be rigidly controlled with reference to the data being manipulated, user
clearance levels, time of day, and anything else that can be programmed via
T-SQL. The other benefits of using stored procedures are well known (see Q1.5.8
). Obviously whether you can implement this depends on the nature of your
application, but the vast majority of in-house-developed applications can rely
solely on stored procedures to carry out all the work necessary. The only
server-side restriction on this method is the current inability of stored
procedures to adequately handle text and image datatypes (see Q1.5.12). To get
around this views can be created that expose only the necessary columns to
direct read or write access.
Views
Views can be a useful general security feature. Where stored procedures are
inappropriate views can be used to control access to tables to a lesser extent.
They also have a role in defining row-level security - eg. the underlying table
can have a security status column joined to a user authorisation level table in
the view so that users can only see data they are cleared for. Obviously they
can also be used to implement column-level security by screening out sensitive
columns from a table.
Triggers
Triggers can be used to implement further levels of security - they could be
viewed as a last line of defence in being able to rollback unauthorised write
activity (they cannot be used to implement any read security). However, there
is a strong argument that triggers should be restricted to doing what they were
designed for - implementing referential integrity - rather being loaded up with
application logic.
Administrative Roles
With Sybase version 10 came the ability to grant certain administrative roles
to user accounts. Accounts can have sa-level privilege, or be restricted to
security or operator roles - see sp_role.
Recommendations
The use of any generic account is not a good idea. If more than one person
requires access as sa to a server, then it is more accountable and traceable if
they each have an individual account with sa_role granted.
Return to top
-------------------------------------------------------------------------------
1.2.12: How to Shrink a Database
-------------------------------------------------------------------------------
Warning: This document has not been reviewed. Treat it as alpha-test
quality information and report any problems and suggestions to
bret@sybase.com
It has historically been difficult to shrink any database except tempdb
(because it is created fresh every boot time). The two methods commonly used
have been:
1. Ensure that you have scripts for all your objects (some tools like SA
Companion, DB Artisan or dbschema.pl from Sybperl can create scripts from
an existing database), then bcp out your data, drop the database, recreate
it smaller, run your scripts, and bcp in your data.
2. Use a third-party tool such as DataTool's SQL Backtrack, which in essence
automates the first process.
This technote outlines a third possibility that can work in most cases.
An Unsupported Method to Shrink a Database
This process is fairly trivial in some cases, such as removing a recently added
fragment or trimming a database that has a log fragment as its final
allocation, but can also be much more complicated or time consuming than the
script and bcp method.
General Outline
The general outline of how to do it is:
1. Make a backup of the current database
2. Migrate data from sysusages fragments with high lstart values to fragments
with low lstart values.
3. Edit sysusages to remove high lstart fragments that no longer have data
allocations.
4. Reboot ASE.
Details
1. Dump your database. If anything goes wrong, you will need to recover from
this backup!
2. Decide how many megabytes of space you wish to remove from your database.
3. Examine sysusages for the database. You will be shrinking the database by
removing the fragments with the highest lstart values. If the current
fragments are not of appropriate sizes, you may need to drop the database,
recreate it so there are more fragments, and reload the dump.
A trivial case: An example of a time when you can easily shrink a
database is if you have just altered it and are sure there has been no
activity on the new fragment. In this case, you can directly delete the
last row in sysusages for the db (this row was just added by alter db)
and reboot the server and it should come up cleanly.
4. Change the segmaps of the fragments you plan to remove to 0. This will
prevent future data allocations to these fragments.
Note: If any of the fragments you are using have user defined segments
on them, drop those segments before doing this.
sp_configure "allow updates", 1
go
reconfigure with override -- not necessary in System 11
go
update sysusages
set segmap = 0
where dbid = <dbid>
and lstart = <lstart>
go
dbcc dbrepair(<dbname>, remap)
go
Ensure that there is at least one data (segmap 3) and one log (segmap 4)
fragment, or one mixed (segmap 7) fragment.
If the server has been in use for some time, you can shrink it by deleting
rows from sysusages for the db, last rows first, after making sure that no
objects have any allocations on the usages.
5. Determine which objects are on the fragments you plan to remove.
traceon(3604)
go
dbcc usedextents( dbid,0,0,1)
go
Find the extent with the same value as the lstart of the first fragment you
plan to drop. You need to migrate every object appearing from this point on
in the output.
6. Migrate these objects onto earlier fragments in the database.
Objids other than 0 or 99 are objects that you must migrate or drop. You
can migrate a user table by building a new clustered index on the table
(since the segmap was changed, the new allocations will not go on this
fragment).
You can migrate some system tables (but not all) using the sp_fixindex
command to rebuild its clustered index. However, there are a few system
tables that cannot have their clustered indexes rebuilt, and if they have
any allocations on the usage, you are out of luck.
If the objid is 8, then it is the log. You can migrate the log by ensuring
that another usage has a log segment (segmap 4 or 7). Do enough activity on
the database to fill an extents worth of log pages, then checkpoint and
dump tran.
Once you have moved all the objects, delete the row from sysusages and
reboot the server.
Run dbcc checkdb and dbcc checkalloc on the database to be sure you are ok,
then dump the database again.
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1.2.13: How do I audit the SQL sent to the server?
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This does not seem to be well documented, so here is a quick means of auditing
the SQL text that is sent to the server. Note that this simply audits the SQL
sent to the server. So, if your user process executes a big stored procedure,
all you will see here is a call to the stored procedure. None of the SQL that
is executed as part of the stored procedure will be listed.
Firstly, you need to have installed Sybase security (which involves installing
the sybsecurity database and loading it using the script $SYBASE/scripts/
installsecurity). Read the Sybase Security Administration Manual, you may
want to enable a threshold procedure to toggle between a couple of audit
tables. Be warned, that the default configuration option "suspend auditing
when device full" is set to 1. This means that the server will suspend all
normal SQL operations if the audit database becomes full and the sso logs in
and gets rid of some data. You might want to consider changing this to 0
unless yours is a particularly sensitive installation.
Once that is done, you need to enable auditing. If you haven't already, you
will need to restart ASE in order to start the audit subsystem. Then comes the
bit that does not seem well documented, you need to select an appropriate audit
option, and the one for the SQL text is "cmdtext". From the sybsecurity
database, issue
sp_audit "cmdtext",<username>,"all","on"
for each user on the system that wish to collect the SQL for. sp_audit seems
to imply that you can replace "<username>" with all, but I get the error
message "'all' is not a valid user name". Finally, enable auditing for the
system as a whole using
sp_configure "auditing",1
go
If someone knows where in the manuals this is well documented, I will add a
link/reference.
Note: The stored procedure sp_audit had a different name under previous
releases. I think that it was called sp_auditoption. Also, to get a full list
of the options and their names, go into sybsecurity and simply run sp_audit
with no arguments.
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1.2.14: sp_helpdb/sp_helpsegment is returning negative numbers
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A number of releases of ASE return negative numbers for sp_helpdb. One solution
given by Sybase is to restart the server. Hmm... not always possible. An
alternative is to use the dbcc command 'usedextents'. Issue the following:
dbcc traceon(3604)
dbcc usedextents(, 0, 1, 1)
and the problem should disappear. This is actually a solved case, Sybase solved
case no: 10454336, go to http://info.sybase.com/resolution/detail.stm?id_number
=10454336 to see more information.
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Advanced Administration Basic Administration ASE FAQ
--
- David Alex Lamb, one of the *.answers moderators
news-answers-request@mit.edu
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