如何检查 SQL Server 中运行缓慢的查询

Question

I have a query which can either execute for a long time or a normal time. I want to find reasons for when it runs long.

What I've already tried:

1. Check locks for objects which used in query. It doesn't exist for the query

2. Check a plan. All events have normal costs, so costs is the same every time

Can you suggest some other steps I might try, or other explanations for the differences in runtime?

Answer 1

I've defined three points which should be investigated:

1. non-optimal query plan

In that step, you should find the most expansive operations in the execution plan (use option "Include Actual Execution Plan")

2. wait events. Here you need to find session_id for your query. If the query running then uses:

   SELECT * FROM sys.dm_exec_requests CROSS APPLY sys.dm_exec_sql_text(sql_handle)

   and investigate columns last_wait_type, wait_type, wait_time

3. locks

The query above also returns blocks information this information in the column BlkBy