问题
I am writing a code which supports different versions of Sybase ASE. I am using union queries and the problem is that different version of Sybase ASE supports different number of tables in union query. The union query is dynamic and will be formed depending on the number of database present in the server.
Is there any way in which I can find the max number of tables supported by a particular Sybase ASE? The only solution that I know right now is to fetch the version using query and pick out the version number from the result and set the number accordingly in the code. But this is not a very good solution. I tried checking if there are any tables which have stores this value but nothing came up. Can anyone suggest any solution for this?
回答1:
Since that's my SAP response you've re-posted here, I'll add some more notes ...
that was a proof of concept that answered the basic question of how to get the info via T-SQL; it was assumed anyone actually looking to implement the solution would (eventually) get around to addressing the various issues re: overhead/maintenance, eg ...
setting a tracefile is going to require permissions to do it; which permissions depends on whether or not you've got granular permissions enabled (see the notes for the 'set tracefile' command in the Reference manual); you'll need to decide if/how you want to grant the permissions to other users
while it's true you cannot re-use the tracefile, you can create a proxy table for the directory where the tracefile exists, then 'delete' the tracefile from the directory, eg:
create proxy_table tracedir external directory at '/tmp'
go
delete tracedir where filename = 'my_serverlmiits'
go
if you could have multiple copies of the proxy table solution running at the same time then you'll obviously (?) need to make sure you generate a unique tracefile name for each session; while you could do this by appending @@spid to the file name, you could also add the login name (suser_name()), the kpid (select KPID from master..monProcess where SPID = @@spid), etc; you'll also want to make sure such a file doesn't exist before trying to create it (eg, delete tracedir where filename = '.....'; set tracefile ...)
your error (when selecting from the proxy table) appears to be related to your client application running in transaction isolation level 0 (which, by default, requires a unique index on the table ... not something you're going to accomplish against a proxy table pointing to an OS file); try setting your isolation level to 1, or use a client application that doesn't default to isolation level 0 (eg, that example runs fine with the basic isql command line tool)
if this solution were to be productionalized then you'll probably want to get a separate filesystem allocated so that any 'run away' tracing sessions don't fill up an important filesystem (eg, /var, /tmp, $SYBASE, etc)
also from a production/security perspective, I'd probably want to investigate the possibility of encapsulating a lot of the details in a DBA/system proc (created to execute under the permissions of the creator) so as to ensure developers can't create tracefiles in the 'wrong' directories ... and on and on and on re: control/security ...
Then again ...
If you're going to be doing this a LOT ... and you're only interested in the max number of tables in a (union) query, then it'd probably be much easier to just build a static if/then/else (or case) expression that matches your ASE version with the few possible numbers (see RobV's post).
Let's face it, how often are really, Really, REALLY going to be building a query with more than, say, 100 tables, let alone 500, 1000, more? [You really don't want to deal with trying to tune such a monster!! YIKES] Realistically speaking, I can't see any reason why you'd want to productionalize the proxy table solution just to access a single row from dbcc serverlimits when you could just implement a hard limit (eg, max of 100 tables).
And the more I think about it, as a DBA I'm going to do whatever I can to make sure your application can't create some monster, multi-hundred table query that ends up bogging down my dataserver simply because the developer couldn't come up with a more efficient solution. [And heaven forbid this type of application gets rolled out to the general user community, ie, I'd have to deal with dozens/hundreds of copies of this monster running in my dataserver?!?!?!]
回答2:
You can get such limits by running 'dbcc serverlimits' (enable traceflag 3604 first).
Up until version 15.7, the maximum was 256. In 16.0, this was raised to 512. In 16.0 SP01, this was raised again to 1023.
回答3:
I suggest you open a case/ticket with SAP support to know if there is any system tables that store this information. If there is none, I would implement the tedious solution you mentionned and will monitor the following error in the ASE15.7 logs:
CR 805525 -- If you exceed the number of tables in a UNION query you can get a signal 11 in ord_getrowbounds instead of an error message.
回答4:
This is the answer that I got from the SAP community
-- enable trace file for your spid
set tracefile '/tmp/my_serverlimits' for @@spid
go
-- dump dbcc serverlimits output to your tracefile
dbcc serverlimits
go
-- turn off tracing
set tracefile off for @@spid
go
-- enable external file access:
sp_configure 'enable file access',1
go
-- create proxy table pointing at the trace file
create proxy_table dbcc_serverlimits external file at '/tmp/my_serverlimits'
go
-- find our column name ('record' of type varchar(255) in this case)
sp_help dbcc_serverlimits
go
-- extract the desired row; store the 'record' value in a @variable
-- and parse for the desired info ...
select * from dbcc_serverlimits where lower(record) like '%union%'
go
record
------------------------------------------------------------------------
Max number of user tables overall in a statement using UNIONs : 512
There are some problems with this approach though. First issue is setting trace file. I am going to use this code mostly daily and in Sybase, I think we can't delete or overwrite a trace file. Second is regarding the proxy table. Proxy table will have to be deleted, but this can be taken care with the following code
IF
exists (select 1 from
sysobjects where type = 'U' and name = 'dbcc_serverlimits')
begin
drop table
dbcc_serverlimits
end
go
Final problem comes when a select query is made from dbcc_serverlimits table. It throws the following error
Could not execute statement. The optimizer could not find a unique index which it could use to scan table 'dbo.dbcc_serverlimits' for cursor 'jconnect_implicit_26'. SQLCODE=311 Server=************, Severity Level=16, State=2, Transaction State=1, Line=1 Line 24
select * from dbcc_serverlimits
All this command will have to be written in procedure (that is what I am thinking). Any more elegant solution?
来源：https://stackoverflow.com/questions/43361888/finding-total-number-of-tables-in-union-query