Default Configuration of SQL Server (and query hints)

Throughout the years, I have become convinced that the default settings used in SQL Server are often wrong for systems that need scale or are properly managed. There is a series of settings I find myself consistently changing when I audit or install a new server. They are “my defaults”

I thought I would share those here. Bear in mind that these are setting that assume a certain level of rational behaviour in the organisation you find yourself in. If you are working in a bank, they may not apply to you.

Server Configuration

Here is what I always do

Configuration	Reason
Grant Lock Pages in Memory	Paging the buffer pool is bad.
Set Max Memory	So you don’t take too many of above
Grant Perform Volume Maintenance tasks to the SQL Server account	Allow instant file initialisation. See: http://technet.microsoft.com/en-us/library/ms175935(v=sql.105).aspx
tempdb #files = #cores	I have shown that this is the most scalable configuration
model in simple mode and autogrowth off	Prevents new users from allocating a database that he has not consciously made good backup choices on
Add new filegroup named DATA to model, set this to the default	All my database have at least two filegroups (more about this later) As pointed out in comments (Thanks commenters), this has to be done the hard way, during database creation time
Enabled Async Stats in Model	I have yet to find a case where async stats are not better than the default
Named Pipes off, TCP/IP on	I never found a real use case for named pipes. Turning them off is one less surface area for malicious attackers to exploit
max worker threads	On a modern 2 socket, I generally set this to 8192. Typically, you can run more threads than the default configuration allows. Threads that are un-used only burn a bit of memory, which is cheap.
-T1118	Mixed extends are a joke. Off with them!
-T1117	If you are managing you data files right, you need them to grow at the same rate. Failure to do so means you are doing something wrong – and I don’t configure for people who do things wrong
-T4199	Always enable the latest optimiser fixes. There is enough bad stuff happening with the optimiser all the time (nature of the beast). Being on the latest version is much better than being stuck in the past
-T3226	Logging successful backups to the error log is a royal pain. That log is reserved for error conditions and other things you need to carefully manage. Logging of backup status belongs in the agent or in the backup system you run.
-T835	Enable Locked Pages on Standard Edition

For a documentation of many of these trace flags, see this: http://technet.microsoft.com/en-us/library/ms188396.aspx

Database / Table Configuration

In addition to server configuration, there are a series of settings I automatically change on databases and tables

Setting	Reason
Async stats	As described above. In my experience, always the superior choice. You need a good reason to have it off, not the other way around.
New filegroup: DATA, set it to default	I never put objects in the PRIMARY filegroup. PRIMARY contains the metadata, and needs extra protection (I normally put it on the TLOG drives). The additional filegroups allow me to freely reallocate new tables and move things around between disk
sp_autostats OFF on large tables	In nearly every case I have seen on large tables, auto stats are harmful. They always kick in at the most inconvenient times. And when they kick in, the cause the one thing that is worse than bad performance and data loss: Unpredictability. I leave auto create stats enabled as this happens rarely enough to be useful and not disruptive.
Table lock escalation OFF on large tables	Same problem as auto stats, when lock escalation kicks in it nearly always spells trouble. If you are running out of locks, you are doing something wrong that needs to be addressed. Lock escalation is not the solution – it typically makes the problem worse or hides the underlying issue. See background details on the trade offs: http://technet.microsoft.com/en-us/library/ms184286(v=sql.105).aspx
ALLOW_PAGE_LOCKS = OFF, ALLOW_ROW_LOCKS = ON for small, and sometimes large, tables	The lock manager often becomes the bottleneck at scale. For small tables, getting rid of page locks help. For large tables, make sure you can live with the the impact on table scans before turning it off
#files = #cores	Yes, even for user databases!

 

On Important Queries and Hinting

I would like to wrap this post up with another place where I deviate from the standard configuration and “best practices” in the SQL Server world.

If I find a stored procedure or query that either

A) Gets run often… or

B) Is very important and needs to be consistent (“important” is nearly always the same as consistent)

… I simply don’t trust the optimiser with these types of queries. Mission critical queries are much too important to leave in the hands of an unpredictable process like that.

Once I have seen a good plan for the query (and validated that I can’t make a better one myself) I force the query using hints. I don’t wait for it to go wrong, I do it when I see the query the first time (even if the plan is good). This has already saved our production systems from trouble and I am teaching this technique to my colleagues.

However, I do restrict myself to these hints:

Hint	Reason
OPTION (<join type> JOIN)	Pretty safe, even under changing index conditions. In OLTP system, if OPTION (LOOP JOIN) cant give you a good plan, you are likely missing an index (and it will be obvious which one when you see the forced plan)
LEFT/INNER <join type> JOIN	Allows me to force ordering and really control execution to harvest filters in the right order.
FORCESCAN	The optimiser often gets this wrong for reporting queries. I find that it is a little too fond of seeking when a scan is better. Scans are often “safer” in the sense that you know they are proportional with the size of the table scanning. Seeks gone wrong (especially if they cause table spools) can really wreck havoc.
FORCESEEK	While this hint can be risky (because it assumes the existence of an index) – there are rare cases where the optimiser needs a bit of manhandling. This is a hint of last resort if the join strategies don’t give me enough
CROSS APPLY	Yes, I use this as a hint. If I know that a result will return a specific number of rows (and the optimiser does not) I can do a CROSS APPLY to SELECT TOP 1. A good example of this is BETWEEN joins on dates
OPTIMIZE FOR UNKNOWN	My favourite hint. Most of the times, I don’t want a plan to assuming anything about the boundary values of stats in another table (my fondness for GUID helps here as they are more likely to be nicely spread out). This hint is great for stability under changing conditions
MAXDOP	Very few queries (except those tuned by master Machanic) takes advantage of more than 8-16 cores.
OPTION (HASH GROUP)	Sometimes, the optimiser will trigger a massive sort before going into an aggregate operation. If I know that the aggregate will have a small result (often the case) I force the hashing strategy instead to avoid the excessive memory usage of the sort.

Hints I avoid:

Hint	Reason
Index hints	I don’t want queries to break if I changes indexes. In my entire career, I think I have found only 1-2 cases where I needed an index hint and forcing of join orders wasn’t good enough. Index hints make queries very brittle – which I suspect is why query hints in general have such a bad rap.
FORCE ORDER	If I force order, I also force the join strategy (as above). So I just haven’t found a need for this hint.
OPTIMISE FOR (@var = x)	Making something dependent on the contents of a column is dangerous. Data changes faster than code. However, Adam Machanic has some really cool ways to force some interesting side effects by using this hint together with a TOP (@var) (See his blog: http://sqlblog.com/blogs/adam_machanic/)