August 1, 2018 at 5:40 am
Good afternoon,
I'm currently working for qn SAP consultacy firm, as a junior SAP technical consultant.
But due to my preveious work expereience most questions about SQL Server are thrown my way.
One of of our customers "recently" (12-13 May 2018) upgraded there underlying RDBMS from SQL Server 2012 to SQL Server 2016.
Since then there have been serious performance issues with 2 jobs.
UPDATE_STATS_XYZ_DAILY and UPDATE_STATS_XYZ
(XYZ being a replacement of the customers name)
Both run stored procedures with the same name setup on SQL Server 2012
Before the upgrade the daily job took 5 hours while the other one (run on sundays) took a day and 22 hours.
After the update the daily takes 12 to 13 hours to finish while the weekly can run over 3 full days.
Both jobs doe the following
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
-- Insert statements for procedure here
update statistics table1 with FULLSCAN
...
update statistics tableX with SAMPLE 15 PERCENT
...
update statistics tableZ with FULLSCAN
With the daily only doing a select number of tables and the weekly doing all 65 in the database.
The data is being mirrored to another server.
There is a backup of the log files every 15 minutes and a daily full backup.
Any help in improving performance or identifiying the a potential cause is greatly appreciated.
August 1, 2018 at 5:46 am
Interesting. Is the underlying hardware exactly the same as it was previously?
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.
August 1, 2018 at 5:50 am
Probably related to the new cardinality estimator.
Identify the queries that have degraded in performance, tune them or add the hint to force the legacy CE.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
August 1, 2018 at 6:52 am
Phil Parkin - Wednesday, August 1, 2018 5:46 AMInteresting. Is the underlying hardware exactly the same as it was previously?
The underlying hardware has remained the same.
GilaMonster - Wednesday, August 1, 2018 5:50 AMProbably related to the new cardinality estimator.Identify the queries that have degraded in performance, tune them or add the hint to force the legacy CE.
We will look into this direction.
August 2, 2018 at 4:49 am
And, since you're on 2016, you can take advantage of the query store to force plans. It won't help now, since the upgrade is done, but in the future, the way to upgrade from pre-2014 to 2016/2017/greater, is to first turn on Query Store, leave the compatibility mode to the old version, run the load for a time, then change compatibility mode. Any regressions can be dealt with through plan forcing.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning
August 8, 2018 at 7:23 am
OK we have determined the change in CE was the culprit.
However I got one question I can't find a satifactionary answer to.
Why was the logic of the CE changed
August 8, 2018 at 7:58 am
Resender - Wednesday, August 8, 2018 7:23 AMOK we have determined the change in CE was the culprit.However I got one question I can't find a satifactionary answer to.
Why was the logic of the CE changed
The cardinality estimation engine was written for SQL Server 7 in 1998... and then never updated or changed until 2014. It was old and needed some help. The new engine is much better... most of the time. Sometimes, it's not.
For example, the old estimator assumed that each column in a compound index added to the selectivity, not allowing for the fact that columns actually have relationships and therefore, each column added did not actually make the index more and more and more selective. So, they did a regressive calculation that made each additional column, a little less selective than the column before. This much more accurately reflects real data... in the overwhelming majority of cases, but not all. The old engine default to 1 row for any estimates outside the histogram. The new one uses an average of the histogram. Lots of other changes that make a lot of sense, but, sometimes, hurt. Same with any change in behavior.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning
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