How to Optimize SQL Server Performance

This is a simple baseline query to see where SQL Server has been spending time before changing indexes or configuration.

SELECT TOP (10)
    wait_type,
    waiting_tasks_count,
    wait_time_ms / 1000.0 AS wait_time_seconds,
    max_wait_time_ms / 1000.0 AS max_wait_seconds,
    signal_wait_time_ms / 1000.0 AS signal_wait_seconds
FROM sys.dm_os_wait_stats
WHERE wait_type NOT LIKE 'SLEEP%'
  AND wait_type NOT IN ('CLR_SEMAPHORE', 'LAZYWRITER_SLEEP', 'XE_TIMER_EVENT')
ORDER BY wait_time_ms DESC;

When estimates are clearly wrong, targeted statistics work is usually better than broad, blind maintenance.

UPDATE STATISTICS dbo.SalesOrderHeader
WITH FULLSCAN;

SELECT
    s.name AS stats_name,
    sp.last_updated,
    sp.rows,
    sp.rows_sampled,
    sp.modification_counter
FROM sys.stats AS s
CROSS APPLY sys.dm_db_stats_properties(s.object_id, s.stats_id) AS sp
WHERE s.object_id = OBJECT_ID('dbo.SalesOrderHeader')
ORDER BY sp.last_updated DESC;

Use this to identify the queries worth investigating before you start rewriting indexes or stored procedures.

SELECT TOP (20)
    qs.execution_count,
    qs.total_worker_time / 1000 AS total_cpu_ms,
    qs.total_elapsed_time / 1000 AS total_duration_ms,
    qs.total_logical_reads,
    qs.total_logical_writes,
    DB_NAME(st.dbid) AS database_name,
    OBJECT_NAME(st.objectid, st.dbid) AS object_name,
    SUBSTRING(
        st.text,
        (qs.statement_start_offset / 2) + 1,
        (
            (CASE qs.statement_end_offset
                WHEN -1 THEN DATALENGTH(st.text)
                ELSE qs.statement_end_offset
            END - qs.statement_start_offset
            ) / 2
        ) + 1
    ) AS statement_text
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
ORDER BY qs.total_worker_time DESC;