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Archive-name: databases/sybase-faq/part14
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.
6.2.7: Hierarchy traversal - BOMs
-------------------------------------------------------------------------------
Alright, so you wanna know more about representing hierarchies in a relational
database? Before I get in to the nitty gritty I should at least give all of the
credit for this algorithm to: "_Hierarical_Structures:_The_Relational_Taboo!_,
_(Can_ Transitive_Closure_Queries_be_Efficient?)_", by Michael J. Kamfonas as
published in 1992 "Relational Journal" (I don't know which volume or issue).
The basic algorithm goes like this, given a tree (hierarchy) that looks roughly
like this (forgive the ASCII art--I hope you are using a fixed font to view
this):
a
/ \
/ \
/ \
b c
/ \ /|\
/ \ / | \
/ \ / | \
d e f | g
Note, that the tree need not be balanced for this algorithm to work.
The next step assigned two numbers to each node in the tree, called left and
right numbers, such that the left and right numbers of each node contain the
left and right numbers of the ancestors of that node (I'll get into the
algorithm for assigning these left and right numbers later, but, hint: use a
depth-first search):
1a16
/ \
/ \
/ \
2b7 8c15
/ \ /|\
/ \ / | \
/ \ / | \
3d4 5e6 9f10 11g12 13h14
Side Note: The careful observer will notice that these left and right
numbers look an awful lot like a B-Tree index.
So, you will notice that all of the children of node 'a' have left and right
numbers between 1 and 16, and likewise all of the children of 'c' have left and
right numbers between 8 and 15. In a slightly more relational format this table
would look like:
Table: hier
node parent left_nbr right_nbr
----- ------ -------- ---------
a NULL 1 16
b a 2 7
c a 8 15
d b 3 4
e b 5 6
f c 9 10
g c 11 12
h c 13 14
So, given a node name, say @node (in Sybase variable format), and you want to
know all of the children of the node you can do:
SELECT h2.node
FROM hier h1,
hier h2
WHERE h1.node = @node
AND h2.left_nbr > h1.left_nbr
AND h2.left_nbr < h1.right_nbr
If you had a table that contained, say, the salary for each node in your
hierarchy (assuming a node is actually a individual in a company) you could
then figure out the total salary for all of the people working underneath of
@node by doing:
SELECT sum(s.salary)
FROM hier h1,
hier h2,
salary s
WHERE h1.node = @node
AND h2.left_nbr > h1.left_nbr
AND h2.right_nbr > h1.right_nbr
AND s.node = h2.node
Pretty cool, eh? And, conversely, if you wanted to know how much it cost to
manage @node (i.e. the combined salary of all of the boss's of @node), you can
do:
SELECT sum(s.salary)
FROM hier h1,
hier h2,
salary s
WHERE h1.node = @node
AND h2.left_nbr < h1.left_nbr
AND h2.left_nbr > h1.right_nbr
AND s.node = h2.node
Now that you can see the algorithm in action everything looks peachy, however
the sticky point is the method in which left and right numbers get assigned.
And, unfortunately, there is no easy method to do this relationally (it can be
done, it just ain't that easy). For an real- world application that I have
worked on, we had an external program used to build and maintain the
hierarchies, and it was this program's responsibility to assign the left and
right numbers.
But, in brief, here is the algorithm to assign left and right numbers to every
node in a hierarchy. Note while reading this that this algorithm uses an array
as a stack, however since arrays are not available in Sybase, they are
(questionably) emulated using a temp table.
DECLARE @skip int,
@counter int,
@idx int,
@left_nbr int,
@node varchar(10)
/*-- Initialize variables --*/
SELECT @skip = 1000, /* Leave gaps in left & right numbers */
@counter = 0, /* Counter of next available left number */
@idx = 0 /* Index into array */
/*
* The following table is used to emulate an array for Sybase,
* for Oracle this wouldn't be a problem. :(
*/
CREATE TABLE #a (
idx int NOT NULL,
node varchar(10) NOT NULL,
left_nbr int NOT NULL
)
/*
* I know that I always preach about not using cursors, and there
* are ways to get around it, but in this case I am more worried
* about readability over performance.
*/
DECLARE root_cur CURSOR FOR
SELECT h.node
FROM hier h
WHERE h.parent IS NULL
FOR READ ONLY
/*
* Here we are populating our "stack" with all of the root
* nodes of the hierarchy. We are using the cursor in order
* to assign an increasing index into the "stack"...this could
* be done using an identity column and a little trickery.
*/
OPEN root_cur
FETCH root_cur INTO @node
WHILE (@@sqlstatus = 0)
BEGIN
SELECT @idx = @idx + 1
INSERT INTO #a VALUES (@idx, @node, 0)
FETCH root_cur INTO @node
END
CLOSE root_cur
DEALLOCATE CURSOR root_cur
/*
* The following cursor will be employed to retrieve all of
* the children of a given parent.
*/
DECLARE child_cur CURSOR FOR
SELECT h.node
FROM hier h
WHERE h.parent = @node
FOR READ ONLY
/*
* While our stack is not empty.
*/
WHILE (@idx > 0)
BEGIN
/*
* Look at the element on the top of the stack.
*/
SELECT @node = node,
@left_nbr = left_nbr
FROM #a
WHERE idx = @idx
/*
* If the element at the top of the stack has not been assigned
* a left number yet, then we assign it one and copy its children
* on the stack as "nodes to be looked at".
*/
IF (@left_nbr = 0)
BEGIN
/*
* Set the left number of the current node to be @counter + @skip.
* Note, we are doing a depth-first traversal, assigning left
* numbers as we go.
*/
SELECT @counter = @counter + @skip
UPDATE #a
SET left_nbr = @counter
WHERE idx = @idx
/*
* Append the children of the current node to the "stack".
*/
OPEN child_cur
FETCH child_cur INTO @node
WHILE (@@sqlstatus = 0)
BEGIN
SELECT @idx = @idx + 1
INSERT INTO #a VALUES (@idx, @node, 0)
FETCH child_cur INTO @node
END
CLOSE child_cur
END
ELSE
BEGIN
/*
* It turns out that the current node already has a left
* number assigned to it, so we just need to assign the
* right number and update the node in the actual
* hierarchy.
*/
SELECT @counter = @counter + @skip
UPDATE h
SET left_nbr = @left_nbr,
right_nbr = @counter
WHERE h.node = @node
/*
* "Pop" the current node off our "stack".
*/
DELETE #a WHERE idx = @idx
SELECT @idx = @idx - 1
END
END /* WHILE (@idx > 0) */
DEALLOCATE CURSOR child_cur
While reading through this, you should notice that assigning the left and right
numbers to the entire hierarchy is very costly, especially as the size of the
hierarchy grows. If you put the above code in an insert trigger on the hier
table, the overhead for inserting each node would be phenomenal. However, it is
possible to reduce the overall cost of an insertion into the hierarchy.
1. By leaving huge gaps in the left & right numbers (using the @skip
variable), you can reduce the circumstances in which the numbers need to be
reassigned for a given insert. Thus, as long as you can squeeze a new node
between an existing pair of left and right numbers you don't need to do the
re-assignment (which could affect all of the node in the hierarchy).
2. By keeping an extra flag around in the hier table to indicate which nodes
are leaf nodes (this could be maintained with a trigger as well), you avoid
placing leaf nodes in the array and thus reduce the number of updates.
Deletes on this table should never cause the left and right numbers to be
re-assigned (you could even have a trigger automagically re-parent orphaned
hierarchy nodes).
All-in-all, this algorithm is very effective as long as the structure of the
hierarchy does not change very often, and even then, as you can see, there are
ways of getting around a lot of its inefficiencies.
Back to top
-------------------------------------------------------------------------------
6.2.8: Calling OS commands from a trigger or a stored procedure
-------------------------------------------------------------------------------
11.5 and above
The Adaptive Server (11.5) will allow O/S calls from within stored procedures
and triggers. These stored procedures are known as extended stored procedures.
Pre-11.5
Periodically folks ask if it's possible to make a system command or call a UNIX
process from a Trigger or a Stored Procedure.
Guaranteed Message Processing
The typical ways people have implemented this capability is:
1. Buy Open Server and bind in your own custom stuff (calls to system() or
custom C code) and make Sybase RPC calls to it.
2. Have a dedicated client application running on the server box which
regularly scans a table and executes the commands written into it (and
tucks the results into another table which can have a trigger on it to
gather results...). It is somewhat tricky but cheaper than option 1.
Sybase ASE 10.0.2.5 and Above - syb_sendmsg()
This release includes a new built-in function called syb_sendmsg(). Using this
function you can send a message up to 255 bytes in size to another application
from the ASE. The arguments that need to be passed to syb_sendmsg() are the IP
address and port number on the destination host, and the message to be sent.
The port number specified can be any UDP port, excluding ports 1-1024, not
already in use by another process. An example is:
1> select syb_sendmsg("120.10.20.5", 3456, "Hello")
2> go
This will send the message "Hello" to port 3456 at IP address '120.10.20.5'.
Because this built-in uses the UDP protocol to send the message, the ASE does
not guarantee the receipt of the message by the receiving application.
Also, please note that there are no security checks with this new function.
It is possible to send sensitive information with this command and Sybase
strongly recommends caution when utilizing syb_sendmsg to send sensitive
information across the network. By enabling this functionality, the user
accepts any security problems which result from its use (or abuse).
To enable this feature you should run the following commands as the System
Security Officer.
1. Login to the ASE using 'isql'.
2. Enable the syb_sendmsg() feature using sp_configure.
1> sp_configure "allow sendmsg", 1
2> go
1> sp_configure "syb_sendmsg port number", <port number>
2> go
1> reconfigure with override -- Not necessary with 11.0 and above
2> go
The server must be restarted to set the port number.
Using syb_sendmsg() with Existing Scripts
Since syb_sendmsg() installs configuration parameter "allow sybsendmsg",
existing scripts that contain the syntax
1> sp_configure allow, 1
2> go
to enable updates to system tables should be altered to be fully qualified as
in the following:
1> sp_configure "allow updates", 1
2> go
If existing scripts are not altered they will fail with the following message:
1> sp_configure allow, 1
2> go
Configuration option is not unique.
duplicate_options
----------------------------
allow updates
allow sendmsg
(return status = 1)
(The above error is a little out of date for the latest releases of ASE, there
are now 8 rows that contain "allow", but the result is the same.)
Backing Out syb_sendmsg()
The syb_sendmsg() function requires the addition on two config values. If it
becomes necessary to roll back to a previous ASE version which does not include
syb_sendmsg(), please follow the instructions below.
1. Edit the RUNSERVER file to point to the SWR ASE binary you wish to use.
2. isql -Usa -P<sa password> -Sserver_name -n -iunconfig.sendmsg -ooutput_file
Sample C program
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <fcntl.h>
main(argc, argv)
int argc; char *argv[];
{
struct sockaddr_in sadr;
int portnum,sck,dummy,msglen;
char msg[256];
if (argc <2) {
printf("Usage: udpmon <udp portnum>\n");
exit(1);
}
if ((portnum=atoi(argv[1])) <1) {
printf("Invalid udp portnum\n");
exit(1);
}
if ((sck="socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP))" < 0) {
printf("Couldn't create socket\n");
exit(1);
}
sadr.sin_family = AF_INET;
sadr.sin_addr.s_addr = inet_addr("0.0.0.0");
sadr.sin_port = portnum;
if (bind(sck,&sadr,sizeof(sadr)) < 0) {
printf("Couldn't bind requested udp port\n");
exit(1);
}
for (;;)
{
if((msglen="recvfrom(sck, msg, sizeof(msg), 0, NULL, &dummy))" < 0)
printf("Couldn't recvfrom() from udp port\n");
printf("%.*s\n", msglen, msg);
}
}
Back to top
-------------------------------------------------------------------------------
6.2.9: Identities and Sequential Keys
-------------------------------------------------------------------------------
This has several sections, culled from various sources. It is better described
as "Everything you've ever wanted to know about identities." It will serve to
answer the following frequently asked questions:
What are the Features and Advantages of using Identities?
What are the Problems with and Disadvantages of Identities?
Common Questions about Identities
* Is Identity the equivalent of Oracle's Auto-sequencing?
* How do I configure a table to use the Identity field?
* How do I configure the burn factor?
* How do I find out if my tables have Identities defined?
* What is my current identity burn factor vulnerability?
How do I optimize the performance of a table that uses Identities?
How do I recover from a huge gap in my identity column?
How do I fix a table that has filled up its identity values?
OK, I hate identities. How do I generate sequential keys without using the
Identity feature?
How do I optimize a hand-made sequential key system for best performance?
- Question 8.1 of the comp.database.sybase FAQ has a quick blurb about
identities and sequential numbers. Search down in the page for the section
titled, "Generating Sequential Numbers." Question 8.1 is a general document
describing Performance and Tuning topics to be considered and thus doesn't go
into as much detail as this page.
- There's a white paper by Malcolm Colton available from the sybase web site.
Goto the Sybase web site http://www.sybase.com and type Surrogate in the search
form. Select the Surrogate Primary Keys, Concurrency, and the Cache Hit Ratio
document.
-------------------------------------------------------------------------------
Advantages/Features of Using Identities
There's an entire section devoted to Identity columns in the ASE Reference
manual, Chapter 5
Sybase System 10 introduced many changes over the 4.9.x architecture. One of
these changes was the Identity feature. The identity column is a special column
type that gets automatically updated by the server upon a new row insert. Its
purpose is to guarantee a unique row identifier not based on the other data in
the row. It was integrated with the server and made memory based for fast value
retrieval and no locking (as was/is the case with homegrown sequential key
generation schemes).
The Advantages and Features of Identities include:
* A non-SQL based solution to the problem of having an default unique value
assigned to a row. ASE prefetches identity values into cache and adds them
automatically to rows as they're inserted into tables that have a type
Identity column. There's no concurrency issues, no deadlocking in
high-insert situations, and no possibility of duplicate values.
* A high performance Unique identifier; ASE's optimizer is tuned to work well
with Unique indexes based on the identity value.
* The flexibility to insert into the identity field a specific value in the
case of a mistaken row deletion. (You can never update however). You
accomplish this by:
1> set identity_insert [datababase]..[table] on
2> go
Note however that the System will not verify the uniqueness of the value
you specifically insert (unless of course you have a unique index existing
on the identity column).
* The flexibility during bcp to either retain existing identity values or to
reset them upon bcping back in. To retain the specific identity values
during a bcp out/in process, bcp your data out normally (no special
options). Then create your bcp in target table with ddl specifying the
identity column in the correct location. Upon bcp'ing back in, add the "-E"
option at the end of the bcp line, like this (from O/S prompt):
% bcp [database]..[new_table] in [bcp datafile] -Usa -S[server] -f [fmt file] -E
For procedures on resetting identity values during a bcp, see the section
regarding Identity gaps.
* Databasewide Identity options: 1) The ability to set Sybase to
automatically create an Identity column on any table that isn't created
with a primary key or a unique constraint specified. 2) Sybase can
automatically include an Identity field in all indexes created,
guaranteeing all will be unique. These two options guarantee increased
index performance optimization and guarantees the use of updateable cursors
and isolation level 0 reads.
These features are set via sp_dboption, like this:
1> sp_dboption [dbname], "auto identity", true
2> go
or
1> sp_dboption [dbname], "identity in nonunique index", true
2> go
To tune the size of the auto identity (it defaults to precision 10):
1> sp_configure "size of auto identity", [desired_precision]
2> go
(the identity in nonunique index db_option and the size of auto identity
sp_configure value are new with System 11: the auto identity existed with
the original Identity feature introduction in System 10)
Like other dboptions, you can set these features on the model database
before creating new databases and all your future databases will be
configured. Be warned of the pitfalls of large identity gaps however; see
the question regarding Burn Factor Vulnerability in the Common Questions
about Identities section.
* The existence of the @@identity global variable, which keeps track of the
identity value assigned during the last insert executed by the server. This
variable can be used programming SQL around tables that have identity
values (in case you need to know what the last value inserted was). If the
last value inserted in the server was to a non-identity table, this value
will be "0."
Back to start of 6.2.9
-------------------------------------------------------------------------------
Disadvantages/Drawbacks of Using Identities
Despite its efficacy of use, the Identity has some drawbacks:
* The mechanism that Sybase uses to allocate Identities involves a memory
based prefetch scheme for performance. The downside of this is, during
non-normal shutdowns of ASE (shutdown with nowait or flat out crashes) ASE
will simply discard or "burn" all the unused identity values it has
pre-allocated in memory. This sometimes leaves large "gaps" in your
monotonically increasing identity columns and can be unsettling for some
application developers and/or end users.
NOTE: Sybase 11.02.1 (EBF 6717) and below had a bug (bugid 96089) which
would cause "large gaps to occur in identity fields after polite
shutdowns." The Sybase 11.02.2 rollup (EBF 6886) fixed this problem. If
you're at or below 11.02.1 and you use identities, you should definitely
upgrade.
* (paraphrased from Sybooks P&T guide, Chapter 6): If you do a large number
of inserts and you have built your clustered index on an Identity column,
you will have major contention and deadlocking problems. This will
instantly create a hot spot in your database at the point of the last
inserted row, and it will cause bad contention if multiple insert requests
are received at once. Instead, create your clustered index on a field that
will somewhat randomize the inserts across the physical disk (such as last
name, account number, social security number, etc) and then create a
non-clustered index based on the identity field that will "cover" any
eligible queries.
The drawback here, as pointed out in the Identity Optimization section in
more detail, is that clustering on another field doesn't truly resolve the
concurrency issues. The hot spot simply moves from the last data page to
the last non-clustered index page of the index created on the Identity
column.
* If you fill up your identity values, no more inserts can occur. This can be
a big problem, especially if you have a large number of inserts and you
have continually crashed your server. However this problem most often
occurs when you try to alter a table and add an Identity column that's too
small, or if you try to bcp into a table with an identity column thetas too
small. If this occurs, follow the procedures for recovering from identity
gaps.
* I've heard (but not been able to reproduce) that identities jump
significantly when dumping and loading databases. Not confirmed.
NOTE: there are several other System 11 bugs related to Identities. EBF
7312 fixes BugId 97748, which caused duplicate identity values to be
inserted at times. EBF 6886 fixed (in addition to the above described bug)
an odd bug (#82460) which caused a server crash when bcping into a table w/
an identity added via alter table. As always, try to stay current on EBFs.
Back to start of 6.2.9
-------------------------------------------------------------------------------
Common questions about Identities
Is the Identity the equivalent of Oracle's auto-sequencing?:
Answer: More or less yes. Oracle's auto-sequencing feature is somewhat
transparent to the end user and automatically increments if created as a
primary key upon a row insert. The Sybase Identity column is normally specified
at table creation and thus is a functional column of the table. If however you
set the "auto identity" feature for a database, the tables created will have a
"hidden" identity column that doesn't even appear when you execute a select *
from [table]. See the Advantages of Identities for more details.
* How do I configure Identities?: You can either create your table initially
with the identity column:
1> create table ident_test
2> (text_field varchar(10),
3> ident_field numeric(5,0) identity)
4> go
Or alter an existing table and add an identity column:
1> alter table existing_table
2> add new_identity_field numeric(7,0) identity
3> go
When you alter a table and add an identity column, the System locks the
table while systematically incrementing and adding unique values to each
row. IF YOU DON'T SPECIFY a precision, Sybase defaults the size to 18!
Thats 1,000,000,000,000,000,000-1 possible values and some major major
problems if you ever crash your ASE and burn a default number of values...
(10^18 with the default burn factor will burn 5^14 or 500,000,000,000,000
values...yikes).
* How do I Configure the burn factor?: The number of identity values that
gets "burned" upon a crash or a shutdown can by found by logging into the
server and typing:
1> sp_configure "identity burning set factor"
2> go
the Default value set upon install is 5000. The number "5000" in this case
is read as ".05% of all the potential identity values you can have in this
particular case will be burned upon an unexpected shutdown." The actual
number depends on the size of the identity field as you specified it when
you created your table.
To set the burn factor, type:
1> sp_configure "identity burning set factor", [new value]
2> go
This is a static change; the server must be rebooted before it takes
effect.
* How do I tell which tables have identities?: You can tell if a table has
identities one of two ways:
1. sp_help [tablename]: there is a field included in the sp_help output
describing a table called "Identity." It is set to 1 for identity
fields, 0 otherwise.
2. Within a database, execute this query:
1> select object_name(id) "table",name "column", prec "precision"
2> from syscolumns
3> where convert(bit, (status & 0x80)) = 1
4> go
this will list all the tables and the field within the table that serves as
an identity, and the size of the identity field.
* What is my identity burn factor vulnerability right now?:
In other words, what would happen to my tables if I crashed my server right
now?
Identities are created type numeric, scale 0, and precision X. A precision
of 9 means the largest identity value the server will be able to process is
10^9-1, or 1,000,000,000-1, or 999,999,999. However, when it comes to
Burning identities, the server will burn (based on the default value of
5000) .05% of 1,000,000,000 or 500,000 values in the case of a crash. (You
may think an identity precision allowing for 1 Billion rows is optimistic,
but I once saw a precision set at 14...then the database crashed and their
identity values jumped 5 TRILLION. Needless to say they abandoned their
original design. Even worse, SQL server defaults precision to 18 if you
don't specify it upon table creation...that's a MINIMUM 10,000,000,000 jump
in identity values upon a crash with the absolute minimum burn factor)
Lets say you have inserted 5 rows into a table, and then you crash your
server and then insert 3 more rows. If you select all the values of your
identity field, it will look like this:
1> select identity_field from id_test
2> go
identity_field
--------------
1
2
3
4
5
500006
500007
500008
(8 rows affected)
Here's your Identity burning options (based on a precision of 10^9 as
above):
Burn value % of values # values burned during crash
5000 .05% 500,000
1000 .01% 100,000
100 .001% 10,000
10 .0001% 1,000
1 .00001% 100
So, the absolute lowest amount of numbers you'll burn, assuming you
configure the burn factor down to 1 (sp_configure "identity burning set
factor", 1) and a precision of 9, is 100 values.
Back to start of 6.2.9
---------------------------------------------------------------------------
Optimizing your Identity setup for performance and maintenance
If you've chosen to use Identities in your database, here are some
configuration tips to avoid typical Identity pitfalls:
+ Tune the burn factor!: see the vulnerability section for a discussion
on what happens to identity values upon ASE crashes. Large jumps in
values can crash front ends that aren't equipped to handle and process
numbers upwards of 10 Trillion. I've seen Powerbuilder applications
crash and/or not function properly when trying to display these large
identity values.
+ Run update statistics often on tables w/ identities: Any index with an
identity value as the first column in the search condition will have
its performance severely hampered if Update statistics is not run
frequently. Running a nightly update statistics/sp_recompile job is a
standard DBA task, and should be run often regardless of the existence
of identities in your tables.
+ Tune the "Identity Grab Size": ASE defaults the number of Identity
values it pre-fetches to one (1). This means that in high insert
environments the Server must constantly update its internal identity
placeholder structure before adding the row. By tuning this parameter
up:
1> sp_configure "identity grab size", [number]
2> go
You can prefetch larger numbers of values for each user as they log
into the server an insert rows. The downside of this is, if the user
doesn't use all of the prefetched block of identity values, the unused
values are lost (seeing as, if another user logs in the next block gets
assigned to him/her). This can quickly accelerate the depletion of
identity values and can cause gaps in Identity values.
(this feature is new with System 11)
+ Do NOT build business rules around Identity values. More generally
speaking the recommendation made by DBAs is, if your end users are EVER
going to see the identity field during the course of doing their job,
then DON'T use it. If your only use of the Identity field is for its
advertised purpose (that being solely to have a uniquely identifying
row for a table to index on) then you should be fine.
+ Do NOT build your clustered index on your Identity field, especially if
you're doing lots of inserts. This will create a hot spot of contention
at the point of insertion, and in heavier OLTP environments can be
debilitating.
- There is an excellent discussion in document http://www.sybase.com/
detail?id=860 on the performance and tuning aspects of Identities. It
supplements some of the information located here (Note: this will open in a
new browser window).
Back to start of 6.2.9
---------------------------------------------------------------------------
Recovery from Large Identity value gaps or
Recovery from Identity insert errors/Full Identity tables
This section will discuss how to re-order the identity values for a table
following a crash/abnormal shutdown that has resulted in huge gaps in the
values. The same procedure is used in cases where the identity field has
"filled up" and does not allow inserts anymore. Some applications that use
Identities are not truly candidates for this process (i.e., applications
that depend on the identity field for business purposes as opposed to
simple unique row identifiers). Applications like this that wish to rid
their dependence on identities will have to re-evaluate their database
design.
+ Method 1:bcp out and in:
- First, (from O/S command line):
% bcp database..table out [data_file] -Usa -S[server] -N
This will create a binary bcp datafile and will force the user to
create a .fmt file. The -N option tells the server to skip the identity
field while bcp'ing out.
- drop and recreate the table in question from ddl (make sure your
table ddl specifies the identity field).
- Now bcp back in:
% bcp database.table in [data_file -Usa -S[server] -f[fmt file] -N
The -N option during bcp in tells the server to ignore the data file's
placeholder column for the defined identity column.
Coincidentally, if you bcp out w/o the -N option, drop the table,
recreate from ddl specifying the identity field, and bcp back in w/o
the -N option, the same effect as above occurs.
(note: if you bcp out a table w/ identity values and then want to
preserve the identity values during the bcp back in, use the "-E"
option.)
+ Method 2: select into a new table, adding the identity column as you go
: Follow this process:
1> select [all columns except identity column]
2> [identity column name ] = identity(desired_precision)
3> into [new_table]
4> from [old table]
5> go
+ There are alternate methods that perform the above in multi steps, and
might be more appropriate in some situations.
o You can bcp out all the fields of a table except the identity
column (create the bcp format file from the original table, edit
out the identity column, and re-bcp). At this point you can create
a new table with or without the identity column; if you create it
with, as you bcp back in the Server will assign new identity
values. If you create it without, you can bcp back in normally and
then alter the table and add the identity later.
o You can select all columns but the identity into a new table, then
alter that table and add an identity later on.
Back to start of 6.2.9
---------------------------------------------------------------------------
How do I generate Sequential Keys w/o the Identity feature?
There are many reasons not to use the Identity feature of Sybase. This
section will present several alternative methods, along with their
advantages and drawbacks. The methods are presented in increasing order of
complexity. The most often implemented is Method 3, which is a more robust
version of Method 2 and which uses a surrogate-key storage table.
Throughout this section the test table I'm adding lines to and generating
sequential numbers for is table inserttest, created like this:
1> create table inserttest
2> (testtext varchar(25), counter int)
3> go
+ Method 1: Create your table with a column called counter of type int.
Then, each time you insert a row, do something like this:
1> begin tran
2> declare @nextkey int
3> select @nextkey=max(counter)+1 from inserttest holdlock
4> insert inserttest (testtext,counter) values ("test_text,@nextkey")
5> go
1> commit tran
2> go
This method is rather inefficient, as large tables will take minutes to
return a max(column) value, plus the entire table must be locked for
each insert (since the max() will perform a table scan). Further, the
select statement does not guarantee an exclusive lock when it executes
unless you have the "holdlock" option; so either duplicate values might
be inserted to your target table or you have massive deadlocking.
+ Method 2: See Question 10.1.1 of the comp.database.sybase FAQ is the
May 1994 (Volume 3, Number 2) Sybase Technical Note (these links will
open in a new browser window). Search down in the tech note for the
article titled, "How to Generate Sequential Keys for Table Key
Columns." This has a simplistic solution that is expanded upon in
Method 3.
+ Method 3: Create a holding table for keys in a common database: Here's
our central holding table.
1> create table keystorage
2> (tablename varchar(25),
4> lastkey int)
5> go
And initially populate it with the tablenames and last values inserted
(enter in a 0 for tables that are brand new).
1> insert into keystorage (tablename,lastkey)
2> select "inserttest", max(counter) from inserttest
3> go
Now, whenever you go to insert into your table, go through a process
like this:
1> begin tran
2> update keystorage set lastkey=lastkey+1 where tablename="inserttest"
3> go
1> declare @lastkey int
2> select @lastkey = lastkey from keystorage where tablename="inserttest"
3> insert inserttest (testtext,counter) values ("nextline",@lastkey)
4> go
1> commit tran
2> go
There is plenty of room for error checking with this process: for
example (code adapted from Colm O'Reilly ([email protected])
post to Sybase-L 6/20/97):
1> begin tran
2> update keystorage set lastkey=lastkey+1 where tablename="inserttest"
3> if @@rowcount=1
4> begin
5> declare @lastkey int
6> select @lastkey=lastkey from keystorage where tablename="inserttest"
7> end
8> commit tran
9> begin tran
10> if @lastkey is not null
11> begin
12> insert inserttest (testtext,counter) values ("third line",@lastkey)
13> end
14> commit tran
15> go
This provides a pretty failsafe method of guaranteeing the success of
the select statements involved in the process. You still have a couple
of implementation decisions though:
o One transaction or Two? The above example uses two transactions to
complete the task; one to update the keystorage and one to insert
the new data. Using two transactions reduces the amount of time the
lock is held on keystorage and thus is better for high insertion
applications. However, the two transaction method opens up the
possibility that the first transaction will commit and the second
will roll back, leaving a gap in the sequential numbers. (of
course, this gap is small potatoes compared to the gaps that occur
in Identity values). Using one transaction (deleting lines 8 and 9
in the SQL above) will guarantee absolutely no gaps in the values,
but will lock the keystorage table longer, reducing concurrency in
high insert applications.
o Update first or select first? The examples given generally update
the keystorage table first, THEN select the new value. Performing
the select first (you will have to rework the creation scheme
slightly; by selecting first you're actually getting the NEXT key
to add, where as by updating first, the keystorage table actually
holds the LAST key added) you allow the application to continue
processing while it waits for the update lock on the table.
However, performing the update first guarantees uniqueness (selects
are not exclusive).
Some DBAs experienced with this keystorage table method warn of large
amounts of blocking in high insert activity situations, a potential
drawback.
+ Method 4: Enhance the above method by creating an insert trigger on
your inserttest table that performs the next-key obtainment logic. Or
you could create an insert trigger on keystorage which updates the
table and obtains your value for you. Integrating the trigger logic to
your application might make this approach more complex. Also, because
of the nature of the trigger you'll have to define the sequence number
columns as allowing NULL values (a bad thing if you're depending on the
sequential number as your primary key). Plus, triggers will slow the
operation down because after obtaining the new value via trigger,
you'll have to issue an extra update command to insert the rest of your
table values.
+ Method 5: (Thanks to John Drevicky ([email protected]))
The following procedure is offered as another example of updating and
returning the Next Sequential Key, with an option that allows automatic
reuse of numbers......
-----------------------------------------------------------------
----
--
DECLARE @sql_err int, @sql_count int
--
begin tran
--
select @out_seq = 0
--
UPDATE NEXT_SEQUENCE
SET next_seq_id
= ( next_seq_id
* ( sign(1 + sign(max_seq_id - next_seq_id) ) -- evaluates: 0 [when
-- next > max]; else 1
* sign(max_seq_id - next_seq_id) -- evaluates: 0 [when next = max];
-- 1 [next < max];
-- -1 [next > max]
) -- both evaluate to 1 when next < max
) + 1 -- increment by [or restart at] 1
WHERE seq_type = @in_seq_type
--
select @sql_err = @@error, @sql_count = @@rowcount
--
IF @sql_err = 0 and @sql_count = 1
BEGIN
select @out_seq = next_seq_id
from NEXT_SEQUENCE
where seq_type = @in_seq_type
--
commit tran
return 0
END
ELSE
BEGIN
RAISERROR 44999 'Error %1! returned from proc derive_next_sequence...no update occurred', @sql_err
rollback tran
END
+ Other Methods: there are several other implementation alternatives
available that involve more complex logic but which might be good
solutions. One example has a central table that stores pre-inserted
sequential numbers that are deleted as they're inserted into the
production rows. This method allows the sequence numbers to be recycled
if their associated row is deleted from the production table. An
interesting solution was posted to Sybase-L 6/20/97 by Matt Townsend (
[email protected]) and is based on the millisecond field of the
date/time stamp. His solution guarantees uniqueness without any
surrogate tables or extra inserts/updates, and is a superior performing
solution to other methods described here (including Identities), but
cannot support exact sequential numbers. Some other solutions are
covered in a white paper available at Sybase's Technical library
discussing Sequential Keys (this will open in a new browser window).
Back to start of 6.2.9
---------------------------------------------------------------------------
Optimizing your home grown Sequential key generating process for any
version of Sybase
+ max_rows_per_page/fillfactor/table padding to simulate row level
locking: This is the most important tuning mechanism when creating a
hand -made sequence key generation scheme. Because of Sybase's page
level locking mechanism, your concurrency performance in higher-insert
activity situations could be destroyed unless the server only grabs one
row at a time. However since Sybase doesn't currently have row-level
locking, we simulate row-level locking by creating our tables in such a
way as to guarantee one row per 2048 byte page.
o For pre-System 11 servers; Calculate the size of your rows, then
create dummy fields in the table that get populated with junk but
which guarantee the size of the row will fill an entire page. For
example (code borrowed from Gary Meyer's 5/8/94 ISUG presentation (
[email protected])):
1> create table keystorage
2> (tablename varchar(25),
3> lastkey int,
4> filler1 char(255) not null,
5> filler2 char(255) not null,
6> filler3 char(255) not null,
7> filler4 char(255) not null,
8> filler5 char(255) not null,
9> filler6 char(255) not null,
9> filler7 char(255) not null)
10> with fillfactor = 100
11> go
We use 7 char(255) fields to pad our small table. We also specify
the fillfactor create table option to be 100. A fillfactor of 100
tells the server to completely fill every data page. Now, during
your initial insertion of a line of data, do this:
1> insert into keystorage
2> (tablename,lastkey,
3> filler1,filler2,filler3,filler4,filler5,filler6,filler7)
4> values
5> ("yourtable",0,
6> replicate("x",250),replicate("x",250),
7> replicate("x",250),replicate("x",250),
8> replicate("x",250),replicate("x",250),
9> replicate("x",250))
10> go
This pads the row with 1750 bytes of junk, almost guaranteeing
that, given a row's byte size limit of 1962 bytes (a row cannot
span more than one page, thus the 2048 page size minus server
overhead == 1962), we will be able to simulate row level locking.
o In Sybase 11, a new create table option was introduced:
max_rows_per_page. It automates the manual procedures above and
guarantees at a system level what we need to achieve; one row per
page.
1> create table keystorage
2> (tablename varchar(25),
3> lastkey int)
4> with max_rows_per_page = 1
5> go
+ Create unique clustered indexes on the tablename/entity name within
your keystorage table. This can only improve its performance. Remember
to set max_rows_per_page or the fillfactor on your clustered index, as
clustered indexes physically reorder the data.
+ Break up the process into multiple transactions wherever possible; this
will reduce the amount of time any table lock is held and will increase
concurrency in high insertion environments.
+ Use Stored Procedures: Put the SQL commands that update the keystorage
table and then insert the updated key value into a stored procedure.
Stored procedures are generally faster than individual SQL statements
in your code because procedures are pre-compiled and have optimization
plans for index usage stored in Sybase's system tables.
+ Enhance the keystorage table to contain a fully qualified table name as
opposed to just the tablename. This can be done by adding fields to the
table definition or by just expanding the entity name varchar field
definition. Then place the keystorage table in a central location/
common database that applications share. This will eliminate multiple
keystorage tables but might add length to queries (since you have to do
cross-database queries to obtain the next key).
- There is an excellent discussion located in the whitepapers section
of Sybase's home page discussing the performance and tuning aspects of
any type of Sequential key use. It supplements the information here
(note: this page will open in a new browser window).
Back to start of 6.2.9
Back to top
-------------------------------------------------------------------------------
6.2.10: How can I execute dynamic SQL with ASE?
-------------------------------------------------------------------------------
Adaptive Server Enterprise: System 12
ASE 12 supports dynamic SQL, allowing the following:
declare @sqlstring varchar(255)
select @sqlstring = "select count(*) from master..sysobjects"
exec (@sqlstring)
go
Adaptive Server Enterprise: 11.5 and 11.9
There is a neat trick that was reported first by Bret Halford ( [email protected]
). (If anyone knows better, point me to the proof and I will change this!) It
utilises the CIS features of Sybase ASE.
* Firstly define your local server to be a remote server using
sp_addserver LOCALSRV,sql_server[,INTERFACENAME]
go
* Enable CIS
sp_configure "enable cis",1
go
* Finally, use sp_remotesql, sending the sql to the server defined in point
1.
declare @sqlstring varchar(255)
select @sqlstring = "select count(*) from master..sysobjects"
sp_remotesql LOCALSRV,@sqlstring
go
Remember to ensure that all of the databases referred to in the SQL string are
fully qualified since the call to sp_remotesql places you back in your default
database.
Sybase ASE (4.9.x, 10.x and 11.x before 11.5)
Before System 11.5 there was no real way to execute dynamic SQL. Rob Verschoor
has some very neat ideas that fills some of the gaps (http://www.euronet.nl/
~syp_rob/dynsql.html).
Dynamic Stored Procedure Execution
With System 10, Sybase introduced the ability to execute a stored procedure
dynamically.
declare @sqlstring varchar(255)
select @sqlstring = "sp_who"
exec @sqlstring
go
For some reason Sybase chose never to document this feature.
Obviously all of this is talking about executing dynamic SQL within the server
itself ie stored procedures and triggers. Dynamic SQL within client apps is a
different matter altogether.
Back to top
-------------------------------------------------------------------------------
6.2.11: Is it possible to concatenate all the values from a column and return a
single row?
-------------------------------------------------------------------------------
Hey, this was quite cool I thought. It is now possible to concatenate a series
of strings to return a single column, in a sort of analogous manner to sum
summing all of the numbers in a column. Obviously, in versions before 12.5,
the longest string that you can have is 255 characters, but with very long
varchars, this may prove useful to someone.
Use a case statement, a la,
1> declare @string_var varchar(255)
2>
3> select @string_var = ""
4>
5> select @string_var = @string_var +
6> (case 1 when 1
7> then char_col
8> end)
9> from tbl_a
10>
11> print "%1!", @string_var
12> go
(1 row affected)
ABCDEFGH
(8 rows affected)
1> select * from tbl_a
2> go
char_col
--------
A
B
C
D
E
F
G
H
(8 rows affected)
1>
Back to top
-------------------------------------------------------------------------------
6.2.12: Selecting rows N to M without Oracle's rownum?
-------------------------------------------------------------------------------
Sybase does not have a direct equivalent to Oracle's rownum but its
functionality can be emulated in a lot of cases.
If you are simply trying to retrieve the first N rows of a table, then simple
use:
set rowcount
replacing <N> with your desired number of rows. (set rowcount 0 restores
normality.) If it is simply the last N rows, then use a descending order-by
clause in the select.
1> set rowcount
2> go
1> select foo
2> from bar
3> order by barID desc
4> go
If you are trying to retrieve rows 100 to 150, say, from a table in a given
order. You could use this to retrieve rows for a set of web pages, but there
are probably more efficient ways using cursors or well written queries or even
Sybperl! The general idea is select the rows into a temporary table adding an
identity column at the same time. Only select enough rows to do the job using
the rowcount trick. Finally, return the rows from the temporary table where
the identity column is between 100 and 150. Something like this:
set rowcount 150
select pseudo_key = identity(3),
col1,
col2
into #tempA
from masterTable
where clause...
order by 2,3
select col1,col2 from #tempA where pseudo_key between 100 and 150
Remember to reset rowcount back to 0 before issuing any more SQL or you will
only get back 150 rows!
A small optimisation would be to select only the key columns for the source
table together with the identity key. Once you have the set of rows you require
in the temporary table, join this back to the source using the key columns to
get any data that you require.
An alternative, which might be better if you needed to join back to this table
a lot, would be to insert enough rows to cover the range as before, but then
delete the set of unwanted rows. This would be a very efficient mechanism if
the majority of your queries involved the first few rows of a table. A typical
application for this might be a search engine displaying relevant items first.
The chances are that the user is going to be bored after the first couple of
pages and go back to playing 'Internet Doom'.
set rowcount 150
select col1,
col2
into #tempA
from masterTable
where clause...
set rowcount 100
delete #tempA
Sybase does not guarantee to return rows in any particular order, so the delete
may not delete the correct set of rows. In the above example, you should add an
order-by to the 'select' and build a clustered index on a suitable key in the
temporary table.
The following stored proc was posted to the Sybase-L mailing list and uses yet
another mechanism. You should check that it works as expected in your
environment since it relies on the fact a variable will be set using the last
row that is returned from a result set. This is not published behaviour and is
not guaranteed by Sybase.
CREATE PROCEDURE dbo.sp_get_posts
@perpage INT,
@pagenumber INT
WITH RECOMPILE
AS
-- if we're on the first page no need to go through the @postid push
IF @pagenumber = 1
BEGIN
SET ROWCOUNT @perpage
SELECT ...
RETURN
END
-- otherwise
DECLARE @min_postid NUMERIC( 8, 0 ),
@position INT
SELECT @position = @perpage * ( @pagenumber - 1 ) + 1
SET ROWCOUNT @position
-- What happens here is it will select through the rows
-- and order the whole set.
-- It will stop push postid into @min_postid until it hits
-- ROWCOUNT and does this out of the ordered set (a work
-- table).
SELECT @min_postid = postid
FROM post
WHERE ...
ORDER BY postid ASC
SET ROWCOUNT @perpage
-- we know where we want to go (say the 28th post in a set of 50).
SELECT ...
FROM post
WHERE postid >= @min_postid
...
ORDER BY postid ASC
Yet another solution would be to use a loop and a counter. Probably the least
elegant, but again, it would depend on what you were trying to do as to what
would be most appropriate.
As you can see, none of these are particularly pretty. If you know of a better
method, please forward it to [email protected].
Back to top
-------------------------------------------------------------------------------
6.2.13: How can I return number of rows that are returned from a grouped query
without using a temporary table?
-------------------------------------------------------------------------------
This question is certainly not rocket science, but it is often nice to know how
many rows are returned as part of a group by. This might be for a report or a
web query, where you would want to tell the user how many rows were returned on
page one. It is easy using a temp table, but how to do it without a temp table
is a little harder. I liked this solution and thought that it might not be
obvious to everyone, it was certainly educational to me. Thanks go to Karl Jost
for a very nice answer.
So, give data like:
name item
---- ----
Brown 1
Smith 2
Brown 5
Jones 7
you wish to return a result set of the form:
name sum(item) rows
---- --------- ----
Brown 6 3
Jones 7 3
Smith 2 3
rather than
name sum(item) rows
---- --------- ----
Brown 6 2
Jones 7 1
Smith 2 1
Use the following, beguilingly simple query:
select name, sum(item), sum(sign(count(*)))
from data
group by name
Back to top
-------------------------------------------------------------------------------
Useful SQL Tricks SQL Fundamentals ASE FAQ
Useful SQL Tricks
6.3.1 How to feed the result set of one stored procedure into another.
6.3.2 Is it possible to do dynamic SQL before ASE 12?
Open Client SQL Advanced ASE FAQ
-------------------------------------------------------------------------------
Note: A number of the following tips require CIS to be enabled (at this precise
moment, all of them require CIS :-) The optimiser does take on a different
slant, however small, when CIS is enabled, so it is up to you to ensure that
things don't break when you do turn it on. Buyer beware. Test, test, test and
when you have done that, check some more.
-------------------------------------------------------------------------------
6.3.1: How to feed the result set of one stored procedure into another.
-------------------------------------------------------------------------------
I am sure that this is all documented, but it is worth adding here. It uses
CIS, as do a number of useful tricks. CIS is disabled by default before 12.0
and not available before 11.5. It is courtesy of BobW from
sybase.public.ase.general, full acceditation will be granted if I can find out
who he is. Excellent tip!
So, the scenario is that you have a stored procedure, AP_A, and you wish to use
the result set that it returns in a query.
Create a proxy table for SP_A.
create table proxy_SP_A (
a int,
b int,
c int,
_p1 int null,
_p2 int null
) external procedure
at "SELF.dbname.dbo.SP_A"
Columns a, b, c correspond to the result set of SP_A. Columns _p1, _p2
correspond to the @p1, @p2 parameters of SP_A. "SELF" is an alias put in
sysservers to refer back to the local server.
If you only have one row returned the proxy table can be used with the
following:
declare @a int, @b int, @c int
select @a = a, @b = b, @c = c from proxy_SP_B
where _p1 = 3 and _p2 = 5
More rows can be handled with a cursor.
Back to top
-------------------------------------------------------------------------------
6.3.2: Is it possible to do dynamic SQL before ASE 12?
-------------------------------------------------------------------------------
Again, using CIS, it is possible to fake dynamic SQL. Obviously for this to
work, CIS must be enabled. In addition, the local server must be added to
sysservers as a remote server. There is a stored procedure, sp_remotesql, that
takes as an arguments a remote server and a string, containing SQL.
As before, adding SELF as the 'dummy' server name pointing to the local server
as if it were a remote server, we can execute the following:
sp_remotesql "SELF","select * from sysdatabases"
Which will do just what you expect, running the query on the local machine. The
stored proc will take 251 (according to its own documentation) arguments of
char(255) or varchar(255) arguments, and concatenate them all together. So we
can do the following:
1> declare @p1 varchar(255),@p2 varchar(255),@p3 varchar(255), @p4 varchar(255)
2>
3> select @p1 = "select",
4> @p2 = " name ",
5> @p3 = "from ",
6> @p4 = "sysdatabases"
7>
8> exec sp_remotesql "SELF", @p1, @p2, @p3, @p4
9> go
(1 row affected)
name
------------------------------
bug_track
dbschema
master
model
sybsystemprocs
tempdb
(6 rows affected, return status = 0)
Obviously, when the parameters are concatenated, they must form a legal T-SQL
statement. If we remove one of the spaces from the above statement, then we
see:
1> declare @p1 varchar(255),@p2 varchar(255),@p3 varchar(255), @p4 varchar(255)
2>
3> select @p1 = "select",
4> @p2 = "name ",
5> @p3 = "from ",
6> @p4 = "sysdatabases"
7>
8> exec sp_remotesql "SELF", @p1, @p2, @p3, @p4
9> go
Msg 156, Level 15, State 1
, Line 1
Incorrect syntax near the keyword 'from'.
(1 row affected, return status = 156)
Back to top
-------------------------------------------------------------------------------
Open Client SQL Advanced ASE FAQ
--
- David Alex Lamb, one of the *.answers moderators
[email protected]
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