Checking who is connected to the database

sp_who


SELECT
NULL as [Connections by Database],
[host_name] AS [Client Machine],
db.name AS [Database],
[program_name] AS [Client Program],
COUNT(*) AS [Open Connections]
FROM sys.dm_exec_sessions s (nolock)
LEFT JOIN sys.dm_exec_requests r (nolock) ON r.session_id = s.session_id
LEFT JOIN sys.databases db (nolock) ON db.database_id = r.database_id
WHERE s.session_id >= 50 -- Ignore SQL Server processes
GROUP BY [host_name], db.name, [program_name]
ORDER BY [Client Machine], [Database], [Client Program]


Adjusting connection pooling settings

By default pool size is 100 per database with a command timeout (effectively a connection timeout) of 15 seconds. To increase add following to the connection string:

Connection Timeout=30;Pooling='true';Max Pool Size=200

Checking the size of column's contents

To check how much data the column contains:

SELECT *, DATALENGTH([column]) AS ColumnSizeInBytes FROM table

Using SQL Server profiler

Profiler is a powerful way to debug application performance problems, to check query execution time. If you're developing any application using SQL Server as a database, running it at least once for every application release can help with pinpointing possible performance issues. Remember the rule - there should be a constant number of queries regardless of the amount of records fetched - that way application will scale better. Sometimes it's better to replace a single complex query with more simpler queries because for each result row, columns shared by joins are repeated and delivered to client for processing many times.