Delete table rows in loop - sql

Question

We are using SQL Server 2000. We have a heavy database with over 100000 images. Currently I'm deleting records with this query:
DELETE FROM T_JBSHEETDATA
WHERE (F_JBREF NOT IN (SELECT JOB_REF_NUMBER
FROM T_JBDTLS))
but unfortunately it only deletes 500 records at a time. If I take more records the server dies (server timeout). How do I create a loop of x rows until it's finished?

Answer 1

Below is a way to do "top N deletes".
A few ideas. You can configure the #TopSize until you find the good "goldie locks" value. Not too big, not too small.
You could also remove the while loop, and then track the #RowCount (after the delete statement)......and if you had client code....return the delete-count, and keep calling the (stored procedure?) over and over until the delete count was zero.
NOW, I would see if an index could improve performance before resorting to the below.
But I'm trying to answer your question..............as asked.
/* START TSQL */
if exists (SELECT * FROM information_schema.tables WHERE table_schema = 'dbo' and table_name = 'Television')
BEGIN
DROP TABLE [dbo].[Television]
END
GO
CREATE TABLE [dbo].[Television] (
TelevisionUUID [uniqueidentifier] not null default NEWSEQUENTIALID() ,
TelevisionName varchar(64) not null ,
TelevisionKey int not null ,
IsCheckedOut bit default 0
)
GO
ALTER TABLE dbo.Television ADD CONSTRAINT PK_Television_TelevisionUUID
PRIMARY KEY CLUSTERED (TelevisionUUID)
GO
ALTER TABLE dbo.Television ADD CONSTRAINT CK_Television_TelevisionName_UNIQUE
UNIQUE (TelevisionName)
GO
set nocount on
declare #counter int
select #counter = 11000
declare #currentTVName varchar(24)
declare #TopSize int
select #TopSize = 10
while #counter > 10000 /* this loop counter is ONLY here for fake data,….do not use this syntax for production code */
begin
select #currentTVName = 'TV: '+ convert(varchar(24) , #counter)
INSERT into dbo.Television ( TelevisionName , TelevisionKey ) values ( #currentTVName , #counter)
select #counter = #counter - 1
end
/* Everything above is just setup data, the crux of the code is below this line */
select count(*) as TV_Total_COUNT_Pre from dbo.Television
declare #DeleteLoopCounter int
select #DeleteLoopCounter = 0
while exists ( select top 1 * from dbo.Television )
BEGIN
select #DeleteLoopCounter = #DeleteLoopCounter + 1
;
WITH cte1 AS
( SELECT
TOP (#TopSize)
TelevisionUUID , /* <<Note, the columns here must be available to the output */
IsCheckedOut
FROM
dbo.Television tv
WITH ( UPDLOCK, READPAST , ROWLOCK ) /* <<Optional Hints, but helps with concurrency issues */
/* WHERE conditions can be put there as well */
ORDER BY /* order by is optional, and I would probably remove it for a delete operation */
tv.TelevisionKey DESC
)
/* UPDATE cte1 SET IsCheckedOut = 1 */ /* this code has nothing to do with the delete solution, but shows how you could use this same trick for an "update top N" */
Delete deleteAlias
from dbo.Television deleteAlias
where exists ( select null from cte1 innerAlias where innerAlias.TelevisionUUID = deleteAlias.TelevisionUUID )
;
print '/#DeleteLoopCounter/'
print #DeleteLoopCounter
print ''
select count(*) as TV_Total_COUNT_Post from dbo.Television
END
EDIT
Sql Server 2000 specific info:
NOTE. Since you have 2000, you will have to HARD CODE the #TopSize value. But I will leave the code "as is" for future readers. Again, you'll have to remove #TopSize and then use a value like "1000" or similar. Remove the () around the #TopSize near the select.

Answer 2

You might be able to improve the performance of your query instead of trying to figure out how to delete in a loop.
1. Make sure there is an index on table T_JBDTLS.JOB_REF_NUMBER. If it's missing that might be the cause of the slowdown.
2. Change your query to use a join instead of a sub-select. Something like:
DELETE FROM T_JBSHEETDATA
FROM T_JBSHEETDATA D
LEFT JOIN T_JBDTLS J ON D.F_JBREF = J.JOB_REF_NUMBER
WHERE J.JOB_REF_NUMBER IS NULL
3. Are there any triggers on the table that you're deleting from? What about cascade deletes? Triggers on the table that is being cascade-deleted?

Answer 3

You may also want to try a "not exists" clause instead of a "not in".
I believe the below is the correct translation.
Delete deleteAlias
/* select deleteAlias.* */
from dbo.T_JBSHEETDATA deleteAlias
where not exists ( select null from dbo.[T_JBDTLS] innerDets where innerDets.JOB_REF_NUMBER = deleteAlias.F_JBREF )
Here is a generic Northwind version that will delete any Order(s) that do(es) not have any (child) Order-Details.
Use Northwind
GO
Delete deleteAlias
/* select deleteAlias.* */
from dbo.Orders deleteAlias
where not exists ( select null from dbo.[Order Details] innerDets where innerDets.OrderId = deleteAlias.OrderId )