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The Purge SQL Agent Job for MDW takes a long time to complete

I use the dbWarden alerts to inform me if a SQL job is taking longer to complete than normal and I got one this morning:
 
Click to enlarge

I noticed by looking at the history this purge job was gradually taking longer and longer to complete each day since I installed it again (see mylast post on this):

RunDate
RunTime
Duration
ExecutionStatus
JobName
04/10/2013
02:00:01
00:00:45
Succeded
mdw_purge_data_[MDW]
05/10/2013
02:00:00
00:13:27
Succeded
mdw_purge_data_[MDW]
06/10/2013
02:00:00
00:17:03
Succeded
mdw_purge_data_[MDW]
07/10/2013
02:00:01
00:30:58
Succeded
mdw_purge_data_[MDW]
08/10/2013
02:00:01
01:04:25
Succeded
mdw_purge_data_[MDW]
09/10/2013
02:00:01
01:41:39
Succeded
mdw_purge_data_[MDW]
10/10/2013
02:00:01
02:35:17
Succeded
mdw_purge_data_[MDW]
11/10/2013
02:00:00
03:34:21
Succeded
mdw_purge_data_[MDW]
12/10/2013
02:00:01
04:12:51
Succeded
mdw_purge_data_[MDW]
13/10/2013
02:00:01
04:54:04
Succeded
mdw_purge_data_[MDW]
14/10/2013
02:00:00
05:38:58
Succeded
mdw_purge_data_[MDW]
15/10/2013
02:00:01
07:07:13
Succeded
mdw_purge_data_[MDW]
16/10/2013
02:00:00
08:21:11
Cancelled
mdw_purge_data_[MDW]

The last job I cancelled so I could figure out what was going on. A quick Google took me to this page which recommended adding a couple of indexes. Before I did so I took a look at the size of both the tables mentioned in the article:

sp_spaceused  '[snapshots].[query_stats]'

name
rows
reserved
data
index_size
unused
query_stats
5,702,294
599072 KB
587616 KB
11320 KB
136 KB

sp_spaceused '[snapshots].[notable_query_text]'

name
rows
reserved
data
index_size
unused
notable_query_text
26,383
34872 KB
33616 KB
272 KB
984 KB

...And the execution plan of the following query before I made any changes:

SELECT qt.[sql_handle]
INTO #tmp_notable_query_text_test
FROM snapshots.notable_query_text AS qt
WHERE NOT EXISTS (
   SELECT snapshot_id
   FROM snapshots.query_stats AS qs
   WHERE qs.[sql_handle] = qt.[sql_handle])

Execution Plan Before

Click to enlarge

You can clearly see that the scan of CIDX_query_stats costs by far the most of any operation in the query.
So I applied the following indexes as recommended in the article and looked at the execution plan of the query again:

USE [MDW]
GO

SET ANSI_PADDING ON
GO

CREATE NONCLUSTERED INDEX [IX_sql_handle]
    ON [snapshots].[query_stats]
          (
          [sql_handle] ASC
          )
       GO

CREATE NONCLUSTERED INDEX [IX_sql_handle]
    ON [snapshots].[notable_query_text]
          (
          [sql_handle] ASC
          )  
GO




Execution Plan After

Click to enlarge

Now various changes have happened to the query play including a Nested Loop operator replacing the Hash Match and an Index Seek replacing the Index Scan.

I ran the job manually and it completed much faster :

RunDate
RunTime
Duration
ExecutionStatus
JobName
16/10/2013
10:41:56
00:05:32
Succeded
mdw_purge_data_[MDW]

But much of the hard work would have already been done from the previous night’s schedule, so I will need to wait until tonight to see if the time had be reduced.

**Update**

It looks like the queries have done their job. The  duration has greatly improved:

RunDate
RunTime
Duration
ExecutionStatus
JobName
17/10/2013
02:00:00
00:21:00
Succeded
mdw_purge_data_[MDW]


A word of warning

Even though the MDW has been active for only two weeks and I am monitoring only one medium-busy server, nearly 14GB of data has been generated:

Click to enlarge




Comments

  1. Very useful. I have many customers that simply LOVE the Data Collector interface. Unfortunately, DC is full of problems (random errors in collection jobs, database growing too much, report errors).

    ReplyDelete

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