As the main article mentions, when members of our DBA team were preparing our data for graphing, we executed some preliminary queries to pull data from the System Monitor—generated CounterData and CounterDetails tables and received some interesting results. First, we found that pulling data from the default table structures was slow. Then, we added a calculated field and index to CounterData and found that queries performed significantly faster when CounterDateTime was an indexed datetime field rather than a non-indexed char(24) field. (We appreciate the assistance the SQL Server Magazine technical editors gave us in figuring this out.) But when we modified the structure of the CounterData table with the appropriate indexes and calculated fields, System Monitor wouldn't log the data at all, although our queries performed somewhat better. It turns out that System Monitor tries to recreate the tables when it finds structural changes in them. We also tried creating an INSTEAD OF trigger to route the data entry into another table. However, when we did so, SQL Server bulk-loaded the data and ignored triggers. We thought about modifying the tables, but you can't expect assistance from Microsoft if you change the system tables, so we recommend that you don't alter them.

In the Microsoft Platform Software Development Kit (SDK) under the Performance Monitor heading (at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/perfmon/base/performance_data.asp), Microsoft describes the fields of the CounterData table as Table A shows.

However, the description of CounterDateTime is incorrect. If you investigate the System Monitor tables CounterData and CounterDetails, you'll find that the counter names are stored in CounterDetails and counter values are stored in CounterData, using one column for every counter and logged one row at a time. For example, if you logged the 12 counters for 2 minutes, CounterDetails would contain 12 records for the names of the counters, whereas CounterData would contain 24 entries for each minute the data was logged. One way to make pulling data from this format more efficient and effective is to transform the data into a pivot-table format in which one column exists for the date and time and additional columns exist for each counter whose data you want to view. Interestingly, this is the same format that a System Monitor CSV file uses.