Dear Sql Server experts:
First off, I am no sql server expert :)
A few months ago I put a database into a production environment.
Recently, It was brought to my attention that a particular query that
executed quite quickly in our dev environment was painfully slow in
production. I analyzed the the plan on the production server (it
looked good), and then tried quite a few tips that I'd gleaned from
reading newsgroups. Nothing worked. Then on a whim I performed an
UPDATE STATISTICS on a few of the tables that were being queried. The
query immediately went from executing in 61 seconds to under 1 second.
I checked to make sure that statistics were being "auto updated" and
they were.
Why did I need to run UPDATE STATISTICS? Will I need to again?
A little more background info:
The database started empty, and has grown quite rapidly in the last
few months. One particular table grows at a rate of about 300,000
records per month. I get fast query times due to a few well placed
indexes.
A quick question:
If I add an index, do statistics get automatically updated for this
new index immediately?
Thanks in advance for any help,
Felix
17  14069 
In article <80************ **************@ posting.google. com>, fe************* ************@ya hoo.com says... A few months ago I put a database into a production environment.
Recently, It was brought to my attention that a particular query that
executed quite quickly in our dev environment was painfully slow in
production. I analyzed the the plan on the production server (it
looked good), and then tried quite a few tips that I'd gleaned from
reading newsgroups. Nothing worked. Then on a whim I performed an
UPDATE STATISTICS on a few of the tables that were being queried. The
query immediately went from executing in 61 seconds to under 1 second.
I checked to make sure that statistics were being "auto updated" and
they were.
I've seen this type of thing many times, developers are not exposed to
the production system long enough to build a maintenance plan, and the
DBA doesn't get enough time to monitor the DB to build a maintenance
plan specific to the database.
In most cases, I create generic maintenance plans that will auto select
20% of the tables per night and reindex them, same with marking stored
procedures for recompile.
If you do something like this, or if you just reindex/recompile them all
on a weekend you should be able to maintain your performance.
Just so you know, this is a problem in Oracle too - had a team of
developers build a inventory system, worked great for almost a year, but
no one really noticed it getting slower every day until the reports
started failing. They tried for three days to fix it before calling me.
My first clue was no schedule maintenance plan, second was that it
worked for almost a year, and then that there are about 20K inserts in a
single table per day with no deletes.... Reindexed two tables and it
returned to the same level of performance as when it was developed.
Also, when you mark a sproc for recompile, the first time it executes it
will be SLOW. While tables are reindexing they will also be slow or
could even be locked, so you want to do this in the off-hours.
--
-- sp*********@rro hio.com
(Remove 999 to reply to me)
Felix (fe************ *************@y ahoo.com) writes: A few months ago I put a database into a production environment.
Recently, It was brought to my attention that a particular query that
executed quite quickly in our dev environment was painfully slow in
production. I analyzed the the plan on the production server (it
looked good), and then tried quite a few tips that I'd gleaned from
reading newsgroups. Nothing worked. Then on a whim I performed an
UPDATE STATISTICS on a few of the tables that were being queried. The
query immediately went from executing in 61 seconds to under 1 second.
I checked to make sure that statistics were being "auto updated" and
they were.
Why did I need to run UPDATE STATISTICS? Will I need to again?
If you only performed UPDATE STATISTICS it seems a little funny, if
there is autostatistics on. If you added WITH FULLSCAN, then there
is a possible explanation.
Whether you will need to do it again, I cannot tell, but as Leythos
discusses, the most critical part is when you start empty and volume
grows. Once you are over a certain level a lot of execution plans
becomes moot.
It the query was parameterized - including auto-parameterized or in
a stored procedure - there is another possible explanation, that the
cached plan was for an atypical parameter. Recall that when SQL Server
builds a query plan for a procedure, it uses the current value of
the parameters to build the plan. If the value used happens to be an
atypical one, then you may get a bad plan for common values to linger.
If this is the case, an sp_recompile on a table involved in the query
will do.
A quick question:
If I add an index, do statistics get automatically updated for this
new index immediately?
Yes, as I recall, SQL Server creates statistics when it builds an
index.
--
Erland Sommarskog, SQL Server MVP, so****@algonet. se
Books Online for SQL Server SP3 at http://www.microsoft.com/sql/techinf...2000/books.asp
The query was not parameterized, nor in a stored procedure.
And I did not run UPDATE STATISTICS WITH FULLSCAN.
What do you mean by "Once you are over a certain level a lot of
execution plans becomes moot."?
Anyway, I remain puzzled.
Thanks,
Felix
Erland Sommarskog <so****@algonet .se> wrote in message news:<Xn******* *************** @127.0.0.1>... Felix (fe************ *************@y ahoo.com) writes: A few months ago I put a database into a production environment.
Recently, It was brought to my attention that a particular query that
executed quite quickly in our dev environment was painfully slow in
production. I analyzed the the plan on the production server (it
looked good), and then tried quite a few tips that I'd gleaned from
reading newsgroups. Nothing worked. Then on a whim I performed an
UPDATE STATISTICS on a few of the tables that were being queried. The
query immediately went from executing in 61 seconds to under 1 second.
I checked to make sure that statistics were being "auto updated" and
they were.
Why did I need to run UPDATE STATISTICS? Will I need to again?
If you only performed UPDATE STATISTICS it seems a little funny, if
there is autostatistics on. If you added WITH FULLSCAN, then there
is a possible explanation.
Whether you will need to do it again, I cannot tell, but as Leythos
discusses, the most critical part is when you start empty and volume
grows. Once you are over a certain level a lot of execution plans
becomes moot.
It the query was parameterized - including auto-parameterized or in
a stored procedure - there is another possible explanation, that the
cached plan was for an atypical parameter. Recall that when SQL Server
builds a query plan for a procedure, it uses the current value of
the parameters to build the plan. If the value used happens to be an
atypical one, then you may get a bad plan for common values to linger.
If this is the case, an sp_recompile on a table involved in the query
will do.
A quick question:
If I add an index, do statistics get automatically updated for this
new index immediately?
Yes, as I recall, SQL Server creates statistics when it builds an
index.
Felix (fe************ *************@y ahoo.com) writes: The query was not parameterized, nor in a stored procedure.
SQL Server can do autoparameteriz ation, although I believe this
mainly happen for simple queries.
What do you mean by "Once you are over a certain level a lot of
execution plans becomes moot."?
Say that you have a query which involves two tables. One table is a
Products table, and one is an Orders table. When you start the system,
you have a lot of products, but you have no orders. But after running
the system for six months, you have a tenfold more orders than products,
and this ratio is only going to increase. Thus, all plans that are built
on the assumption that the Orders table is smaller are no longer of
interest.
--
Erland Sommarskog, SQL Server MVP, so****@algonet. se
Books Online for SQL Server SP3 at http://www.microsoft.com/sql/techinf...2000/books.asp
Erland Sommarskog <so****@algonet .se> wrote in message news:<Xn******* *************** @127.0.0.1>... Felix (fe************ *************@y ahoo.com) writes: The query was not parameterized, nor in a stored procedure.
SQL Server can do autoparameteriz ation, although I believe this
mainly happen for simple queries.
What do you mean by "Once you are over a certain level a lot of
execution plans becomes moot."?
Say that you have a query which involves two tables. One table is a
Products table, and one is an Orders table. When you start the system,
you have a lot of products, but you have no orders. But after running
the system for six months, you have a tenfold more orders than products,
and this ratio is only going to increase. Thus, all plans that are built
on the assumption that the Orders table is smaller are no longer of
interest.
Ah, thanks. That makes a lot of sense. But since the stats are being
auto updated (in the default case and in my case), this should not be
a problem, right?
Felix (fe************ *************@y ahoo.com) writes: Ah, thanks. That makes a lot of sense. But since the stats are being
auto updated (in the default case and in my case), this should not be
a problem, right?
Right. For this typical case that I outlined, SQL Server handles the case,
and you are not likely to see any problems.
But then there might be more sensitive cases. Say that you have a complex
query with many tables involved. A minor error in the statistics of one of
the innermost tables in the query, can lead to gross errors in the
estimates. And, of course, there are cases where even with completely
accurate statistics the optimizer will go astray.
--
Erland Sommarskog, SQL Server MVP, so****@algonet. se
Books Online for SQL Server SP3 at http://www.microsoft.com/sql/techinf...2000/books.asp
In article <80************ *************@p osting.google.c om>, felix666007 _n************* *@yahoo.com says... Ah, thanks. That makes a lot of sense. But since the stats are being
auto updated (in the default case and in my case), this should not be
a problem, right?
A good example of AUTO for things is the standard MS Drive Fragmentation
- if you leave it to the OS your drive will be fragmented, same if using
DiskKeeper, there will still be some fragmentation. If you take it
offline you can fully defragment the drive.
This same thought holds try for stats - I've seen hundreds of servers
that benefited from having tables manually reindexed, stored procs
recompiled, etc....
While the automation works well, it leaves a lot to be desired. You can
write your own scripts to automate the process of the reindexing,
recompiling, etc... and then schedule them on a nightly basis (do about
20% of the objects each evening if possible).
In one system, where they insert about 500,000 new records a day, the
system was overly slow, a nightly reindex on the common insert tables
brought back performance to the level of the development days.
--
-- sp*********@rro hio.com
(Remove 999 to reply to me)
Erland Sommarskog (so****@algonet .se) writes: Felix (fe************ *************@y ahoo.com) writes: Ah, thanks. That makes a lot of sense. But since the stats are being
auto updated (in the default case and in my case), this should not be
a problem, right?
Right. For this typical case that I outlined, SQL Server handles the case,
and you are not likely to see any problems.
But then there might be more sensitive cases. Say that you have a complex
query with many tables involved. A minor error in the statistics of one of
the innermost tables in the query, can lead to gross errors in the
estimates. And, of course, there are cases where even with completely
accurate statistics the optimizer will go astray.
And, oh, just because the statistics are updated, does not mean that the
query plan is. I had a case recently where this happened. It's not really
a usuaul situation, because I'm running a long job that converts data
from a competitor's system to our system. I'm running eight years worth
of transaction in one long stored procedure which runs for days. The
job runs by handling each business day there has been over the years,
one at a time. Anyway, there was one procedure in the job, which for a
long time ran with nothing to do, as there was no data to convert, but
from one day there is data every day. By time the procedure went from
a few seconds, to half a minute. With an sp_recompile on the table that
this procedure loads, the procedure quickly fell back to a few seconds.
--
Erland Sommarskog, SQL Server MVP, so****@algonet. se
Books Online for SQL Server SP3 at http://www.microsoft.com/sql/techinf...2000/books.asp
Leythos (vo**@nowhere.c om) writes: This same thought holds try for stats - I've seen hundreds of servers
that benefited from having tables manually reindexed, stored procs
recompiled, etc....
But reindexing is another thing. There is no autoreindex, so of course
running reindex manually may have some effect.
--
Erland Sommarskog, SQL Server MVP, so****@algonet. se
Books Online for SQL Server SP3 at http://www.microsoft.com/sql/techinf...2000/books.asp This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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