一：设置限制最大内存阈值（配置阈值，预防直接把服务器内存全部吃掉）

二：获取查询 SELECT 语句的执行次数排名

SELECT TOP (100) qs.execution_count
    ,qs.last_elapsed_time
    ,qs.min_elapsed_time
    ,qs.max_elapsed_time
    ,total_worker_time
    ,total_logical_reads
    ,SUBSTRING(qt.[text], qs.statement_start_offset / 2 + 1, (
            CASE 
                WHEN qs.statement_end_offset = - 1
                    THEN LEN(CONVERT(NVARCHAR(MAX), qt.[text])) * 2
                ELSE qs.statement_end_offset
                END - qs.statement_start_offset
            ) / 2) AS query_text
FROM sys.dm_exec_query_stats AS qs WITH (NOLOCK)
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS qt
ORDER BY qs.execution_count DESC
OPTION (RECOMPILE);

三：查询前10条性能最差的语句


SELECT TOP 10 TEXT AS 'SQL Statement'
    ,last_execution_time AS 'Last Execution Time'
    ,(total_logical_reads + total_physical_reads + total_logical_writes) / execution_count AS [Average IO]
    ,(total_worker_time / execution_count) / 1000000.0 AS [Average CPU Time (sec)]
    ,(total_elapsed_time / execution_count) / 1000000.0 AS [Average Elapsed Time (sec)]
    ,execution_count AS "Execution Count"
    ,qp.query_plan AS "Query Plan"
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.plan_handle) st
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp
ORDER BY total_elapsed_time / execution_count DESC

四：收集一个人人都是DBA 连接，分享大家

https://www.cnblogs.com/gaochundong/p/everyone_is_a_dba_query_info_collection.html#query_top_10_poor_performance_sql_statements