Time statistics

Improving Query Performance in SQL Server

Dean Smith

Founder, Atamai Analytics

SQL Server Management Studio

SQL Server Management Studio splash screen

STATISTICS TIME in SSMS

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 - command reports number of milliseconds, required to parse, compile, and execute a query.

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 Execution Times

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

CPU time: time taken by server processors to process the query
Elapsed time: total duration of the query

Example: query 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);

Example: query 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.

Example: query 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.

Example: query 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.

Comparing queries

First query containing two sub-queries

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.

Second query using 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 vs. CPU time

Elapsed time

May be variable when analyzing query time statistics
Time best time statistics measure for the fastest running query

CPU time

Should vary little when analyzing query time statistics
May not be a useful measure if server processors are running in parallel

Taking an average

Don't rely on one measurement, take an average.

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.

Let's practice!

Improving Query Performance in SQL Server