IDENTITY type field in SQL Server database
WARNING: This tale of woe contains examples of code smells and poor design decisions, and technical debt.
If you are conversant with SOLID principles, practice TDD and unit test your work, DO NOT READ ON . Unless you want a good giggle at someone's misfortune and gloat in your own awesomeness knowing that you would never leave behind such a monumental pile of crap for your successors.
So, if you're sitting comfortably then I'll begin.
In this app that I have inherited and been supporting and bug fixing for the last 7 months I have been left with a DOOZY of a balls up by a developer that left 6 and a half months ago. Yes, 2 weeks after I started.
Anyway. In this app we have clients , employees and visits tables.
There is also a table called AppNewRef (or something similar) that ... wait for it ... contains the next record ID to use for each of the other tables. So, may contain data such as :-
TypeID Description NextRef
1 Employees 804
2 Clients 1708
3 Visits 56783
When the application creates new rows for Employees , it looks in the AppNewRef table, gets the value, uses that value for the ID, and then updates the NextRef column. Same thing for Clients , and Visits and all the other tables whose NextID to use is stored in here.
Yes, I know, no auto-numbering IDENTITY columns on this database. All under the excuse of "when it was an Access app". These ID's are held in the (VB6) code as longs. So, up to 2 billion 147 million records possible. OK, that seems to work fairly well. (apart from the fact that the app is updating and taking care of locking / updating, etc., and not the database)
So, our users are quite happily creating Employees , Clients , Visits etc. The Visits ID is steady increasing a few dozen at a time. Then the problems happen. Our clients are causing database corruptions while creating batches of visits because the server is beavering away nicely, and the app becomes unresponsive. So they kill the app using task manager instead of being patient and waiting. Granted the app does seem to lock up.
Roll on to earlier this year and developer Tim (real name. No protecting the guilty here) starts to modify the code to do the batch updates in stages, so that the UI remains 'responsive'. Then April comes along, and he's working his notice (you can picture the scene now, can't you ?) and he's beavering away to finish the updates.
End of April, and beginning of May we update some of our clients. Over the next few months we update more and more of them.
Unseen by Tim (real name, remember) and me (who started two weeks before Tim left) and the other new developer that started a week after, the ID's in the visit table start to take huge leaps upwards. By huge, I mean 10000, 20000, 30000 at a time. Sometimes a few hundred thousand.
Here's a graph that illustrates the rapid increase in IDs used.
Roll on November. Customer phones Tech Support and reports that he's getting an error. I look at the error message and ask for the database so I can debug the code. I find that the value is too large for a long. I do some queries, pull the information, drop it into Excel and graph it.
I don't think making the code handle anything longer than a long for the ID's is the right approach, as this app passes that ID into other DLL's and OCX's and breaking the interface on those just seems like a whole world of hurt that I don't want to encounter right now.
One potential idea that I'm investigating is try to modify the ID's so that I can get them down to a lower level. Essentially filling the gaps. Using the ROW_NUMBER function
What I'm thinking of doing is adding a new column to each of the tables that have a Foreign Key reference to these Visit ID's (not a proper foreign key mind, those constraints don't exist in this database). This new column could store the old (current) value of the Visit ID (oh, just to confuse things; on some tables it's called EventID , and on some it's called VisitID ).
Then, for each of the other tables that refer to that VisitID , update to the new value.
Ideas ? Suggestions ? Snippets of T-SQL to help all gratefully received.
Option one:
Explicitly constrain all of your foreign key relationships, and set them to be ON UPDATE CASCADE .
This will mean that whenever you change the ID, the foreign keys will automatically be updated.
Then you just run something like this...
WITH
resequenced AS
(
SELECT
ROW_NUMBER() OVER (ORDER BY id) AS newID,
*
FROM
yourTable
)
UPDATE
resequenced
SET
id = newID
I haven't done this in ages , so I forget if it causes problems mid-update by having two records with the same id value. If it does, you could do somethign like this first...
UPDATE yourTable SET id = -id
Option two:
Ensure that none of your foreign key relationships are explicitly defined. If they are, note them donw and remove them.
Then do something like...
CREATE TABLE temp AS
newID INT IDENTITY (1,1),
oldID INT
)
INSERT INTO temp (oldID) SELECT id FROM yourTable
/* Do this once for the table you are re-identifiering */
/* Repeat this for all fact tables holding that ID as a foreign key */
UPDATE
factTable
SET
foreignID = temp.newID
FROM
temp
WHERE
foreignID = temp.oldID
Then re-apply any existing foreign key relationships.
This is a pretty scary option. If you forget to update a table, you just borked your data. But, you can give that temp table a much nicer name and KEEP it.
Good luck. And may the lord have mercy on your soul. And Tim's if you ever meet him in a dark alley.
I would create a numbers table that has just a sequence from 1 to whatever max with an increment of 1 is for long and then change the logic of getting the maxid for visitid and maybe the others doing a right join between the numbers and the visits table. and then you can just look for te min of that number
select min(number) from visits right join numbers on visits.id = numbers.number
That way you get all the gaps filled in without having to change any of the other tables.
but I would just redo the whole database.
链接地址: http://www.djcxy.com/p/11430.html