時間統計

改進 SQL Server 中的查詢效能

Dean Smith

Founder, Atamai Analytics

SQL Server Management Studio

SQL Server Management Studio 啟動畫面

SSMS 中的 STATISTICS TIME

SELECT UNStatisticalRegion, 
        CountryName,
        Capital
FROM Nations
WHERE Capital IN 
        (SELECT CityName 
        FROM Cities)
            AND Capital IN 
                (SELECT CityName 
                    FROM Cities 
                    WHERE Pop2017 > 1000000);

STATISTICS TIME - 指令會回報剖析、編譯與執行查詢所需的毫秒數。

SQL Server parse and compile time: 
   CPU time = 16 ms, elapsed time = 21 ms.

(88 rows affected)

 SQL Server Execution Times:
   CPU time = 390 ms,  elapsed time = 382 ms.

SQL Server 執行時間

 SQL Server Execution Times:
   CPU time = 390 ms,  elapsed time = 382 ms.

CPU time：伺服器處理器用於處理查詢的時間
Elapsed time：查詢的總耗時

範例：查詢 1

SELECT UNStatisticalRegion, 
       CountryName,
       Capital
FROM Nations
WHERE Capital IN 
        (SELECT CityName -- 1st sub-query
        FROM Cities) 
            AND Capital IN 
                (SELECT CityName -- 2nd sub-query
                 FROM Cities 
                 WHERE Pop2017 > 1000000);

範例：查詢 1

SET STATISTICS TIME ON

SELECT UNStatisticalRegion, 
       CountryName,
       Capital
FROM Nations
WHERE Capital IN 
        (SELECT CityName -- 1st sub-query
        FROM Cities) 
            AND Capital IN 
                (SELECT CityName -- 2nd sub-query
                 FROM Cities 
                 WHERE Pop2017 > 1000000);

SQL Server Execution Times:
   CPU time = 391 ms,  elapsed time = 381 ms.

範例：查詢 2

SELECT UNStatisticalRegion, 
       CountryName,
       Capital 
FROM Nations n
WHERE EXISTS 
    (SELECT 1 
     FROM  Cities c 
     WHERE n.Capital = c.CityName
        AND Pop2017 > 1000000);

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 2 ms.

範例：查詢 2

SELECT UNStatisticalRegion, 
       CountryName,
       Capital 
FROM Nations n
WHERE EXISTS 
    (SELECT 1 
     FROM  Cities c 
     WHERE n.Capital = c.CityName
        AND Pop2017 > 1000000);

SET STATISTICS TIME OFF

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 2 ms.

比較查詢

第一個查詢含兩個子查詢

SELECT UNStatisticalRegion, 
       CountryName,
       Capital
FROM Nations
WHERE Capital IN 
        (SELECT CityName 
        FROM Cities)
            AND Capital IN 
                (SELECT CityName 
                 FROM Cities 
                 WHERE Pop2017 > 1000000);

SQL Server Execution Times:
   CPU time = 391 ms,  elapsed time = 381 ms.

第二個查詢使用 EXISTS

SELECT UNStatisticalRegion, 
       CountryName,
       Capital 
FROM Nations n
WHERE EXISTS 
    (SELECT 1 
     FROM  Cities c 
     WHERE n.Capital = c.CityName
        AND Pop2017 > 1000000);

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 2 ms.

Elapsed time 與 CPU time

Elapsed time

在分析查詢時間統計時可能會有變動
衡量最快查詢的最佳時間指標

CPU time

在分析查詢時間統計時應該變動不大
若伺服器處理器平行執行，可能不具參考價值

取平均值

不要只看一次量測，取平均較可靠。

elapsed time = 2032 ms.

elapsed time = 2060 ms.

elapsed time = 1915 ms.

elapsed time = 4009 ms.

elapsed time = 3511 ms.

Average elapsed time = 2705 ms.

一起來練習吧！

改進 SQL Server 中的查詢效能