SQL Timeouts in CRM - Generic SQL Error

I often find myself answering forum posts about SQL errors (the most common error is 'Generic SQL Error'). By far the most likely cause of this error is a timeout when accessing SQL server. If this is a case your options are to increase the timeout, or to try and ensure the query does not take so long.

Increase the Timeout
The SQL timeout is controlled by a registry value on each CRM server:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSCRM\OLEDBTimeout

This value does not exist by default, and if it does not exist then the default value will be 30 seconds. To change it, add the registry value (of type DWORD), and put in the value you want (measured in seconds). Then you have to recycle the CrmAppPool application pool for the change to take effect (this is a step most Microsoft documentation omits to mention); do this we IISReset, or less drastically via iisapp.vbs

Reduce the time taken by the query
This may not be so simple, as you may have little control over the query. If the query is run as a result of your code (e.g. through a RetrieveMultiple request), then you may be able to make useful changes. For example, RetrieveMultiple requests on activities are not necessarily processed very efficiently by CRM (the problem is the way that is accesses the activity parties), and I've been able to make significant improvements by using a FetchXml query instead, which gives closer control over the joins used.

Otherwise, the other query optimisation option is to add indexes in SQL Server. This is a massive topic in its own right (I used to deliver 5 day training courses just on this topic), so I'm not going to go into detail here. The general steps are:

1. Identify the SQL query that takes the time - I use CRM tracing for this - http://support.microsoft.com/kb/907490
2. Use the SQL Management Studio and SQL Profiler to identify the query execution plan and to get recommendations about possible indexes

There are 2 important things to take into account:

1. Although adding an index may improve the performance of one query, it can adversely affect other SQL operations - most obviously data updates. There is no easy solution to this, though the SQL Profiler can help you if you capture and analyse a representative sample of SQL operations
2. Some out-dated CRM documentation suggested that it is unsupported to add indexes to the MSCRM database. However, adding indexes is supported, providing the index does not implement any constraints (i.e. it's not a UNIQUE index)