What is the best practice of deleting large amount of records?

Question

hi,
I need your suggestions on best practice of deleting large amount of records of SQL Azure regularly.
Scenario:
I have a SQL Azure database (P1) to which I insert data every day, to prevent the database size grow too fast, I need a way to  remove all the records which is older than 3 days every day.
For on-premise SQL server, I can use SQL Server Agent/job, but, since SQL Azure does not support SQL Job yet, I have to use a Web job which scheduled to run every day to delete all old records.
To prevent the table locking when deleting too large amount of records, in my automation or web job code, I limit the amount of deleted records to
5000 and batch delete count to 1000 each time when calling the deleting records stored procedure:
1. Get total amount of old records (older then 3 days)
2. Get the total iterations: iteration = (total count/5000)
3. Call SP in a loop:
for(int i=0;i<iterations;i++)
   Exec PurgeRecords @BatchCount=1000, @MaxCount=5000
And the stored procedure is something like this:
 BEGIN
  INSERT INTO @table
  SELECT TOP (@MaxCount) [RecordId] FROM [MyTable] WHERE [CreateTime] < DATEADD(DAY, -3, GETDATE())
 END
 DECLARE @RowsDeleted INTEGER
 SET @RowsDeleted = 1
 WHILE(@RowsDeleted > 0)
 BEGIN
  WAITFOR DELAY '00:00:01'
  DELETE TOP (@BatchCount) FROM [MyTable] WHERE [RecordId] IN (SELECT [RecordId] FROM @table)
  SET @RowsDeleted = @@ROWCOUNT
 END
It basically works, but the performance is not good. One example is, it took around 11 hours to delete around 1.7 million records, really too long time...
Following is the web job log for deleting around 1.7 million records:
[01/12/2015 16:06:19 > 2f578e: INFO] Start getting the total counts which is older than 3 days
[01/12/2015 16:06:25 > 2f578e: INFO] End getting the total counts to be deleted, total count:
1721586
[01/12/2015 16:06:25 > 2f578e: INFO] Max delete count per iteration: 5000, Batch delete count
1000, Total iterations: 345
[01/12/2015 16:06:25 > 2f578e: INFO] Start deleting in iteration 1
[01/12/2015 16:09:50 > 2f578e: INFO] Successfully finished deleting in iteration 1. Elapsed time:
00:03:25.2410404
[01/12/2015 16:09:50 > 2f578e: INFO] Start deleting in iteration 2
[01/12/2015 16:13:07 > 2f578e: INFO] Successfully finished deleting in iteration 2. Elapsed time:
00:03:16.5033831
[01/12/2015 16:13:07 > 2f578e: INFO] Start deleting in iteration 3
[01/12/2015 16:16:41 > 2f578e: INFO] Successfully finished deleting in iteration 3. Elapsed time:
00:03:336439434
Per the log, SQL azure takes more than 3 mins to delete 5000 records in each iteration, and the total time is around
11 hours.
Any suggestion to improve the deleting records performance?

Answer 1

This is one approach:
Assume:
1. There is an index on 'createtime'
2. Peak time insert (avgN) is N times more than average (avg). e.g. supposed if average per hour is 10,000 and peak time per hour is 5 times more, that gives 50,000. This doesn't have to be precise.
3. Desirable maximum record to be deleted per batch is 5,000, don't have to be exact.
Steps:
1. Find count of records more than 3 days old (TotalN), say 1,000,000.
2. Divide TotalN (1,000,000) with 5,000 gives the number of deleted batches (200) if insert is very even. But since it is not even and maximum inserts can be 5 times more per period, set number of deleted batches should be 200 * 5 = 1,000.
3. Divide 3 days (4,320 minutes) with 1,000 gives 4.32 minutes.
4. Create a delete statement and a loop that deletes record with creation day < today - (3 days ago - 3.32 * I minutes). (I is the number of iterations from 1 to 1,000)
In this way the number of records deleted in each batch is not even and not known but should mostly within 5,000 and even you run a lot more batches but each batch will be very fast.
Frank