Page read statistics

Improving Query Performance in SQL Server

Dean Smith

Founder, Atamai Analytics

Table data pages

All data, in either memory or on the disk, is stored in 8 kilobyte size "pages"
One page can store many rows or one value could span multiple pages
A page can only belong to one table
SQL Server works with pages cached in memory
If a page is not cached in memory it is read from disk and cached in memory

Customers: data pages

Page 1

| ALFKI | Alfreds Futterkiste | Maria Anders | Sales Representative | … 
| ANATR | Ana Trujillo Emparedados y helados | Ana Trujillo | Owner | … 
| ANTON | Antonio Moreno Taqueria | Antonio Moreno | Owner | … 
| AROUT | Around the Horn | Thomas Hardy | Sales Representative | … 
| BERGS | Berglunds snabbkop | Christina Berglund | Order Administrator | … 
| BLAUS | Blauer See Delikatessen | Hanna Moos | Sales Representative | … 
| BLONP | Blondesddsl pere et fils | Frederique Citeaux | Marketing Manager | …

Page 2

| BOLID | Bolido Comidas preparadas | Martin Sommer | Owner | … 
| BONAP | Bon app  | Laurence Lebihan | Owner | … 
| BOTTM | Bottom-Dollar Markets | Elizabeth Lincoln | Accounting Manager | … 
| BSBEV | B s Beverages | Victoria Ashworth | Sales Representative | … 
| CACTU | Cactus Comidas para llevar | Patricio Simpson | Sales Agent | … 
| CENTC | Centro comercial Moctezuma | Francisco Chang | Marketing Manager | … 
| CHOPS | Chop-suey Chinese | Yang Wang | Owner | …

Page 3

| COMMI | Comercio Mineiro | Pedro Afonso | Sales Associate | … 
| CONSH | Consolidated Holdings | Elizabeth Brown | Sales Representative | … 
| DRACD | Drachenblut Delikatessen | Sven Ottlieb | Order Administrator | … 
| DUMON | Du monde entier | Janine Labrune | Owner | … 
| EASTC | Eastern Connection | Ann Devon | Sales Agent | … 
| ERNSH | Ernst Handel | Roland Mendel | Sales Manager | … 
| FAMIA | Familia Arquibaldo | Aria Cruz | Marketing Assistant | …

Page 4

| FISSA | FISSA Fabrica Inter. Salchichas S.A. | Diego Roel | Accounting Manager | … 
| FOLIG | Folies gourmandes | Martine Rance | Assistant Sales Agent | … 
| FOLKO | Folk och fa HB | Maria Larsson | Owner | … 
| FRANK | Frankenversand | Peter Franken | Marketing Manager | … 
| FRANR | France restauration | Carine Schmitt | Marketing Manager | … 
| FRANS | Franchi S.p.A. | Paolo Accorti | Sales Representative | … 
| FURIB | Furia Bacalhau e Frutos do Mar | Lino Rodriguez | Sales Manager | …

STATISTICS IO in SSMS

SET STATISTICS IO ON

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

(88 rows affected)
Table 'Worktable'. Scan count 1, logical reads 104568, physical reads 0, read-ahead reads 0, lob logical reads 0, ...
Table 'Cities'. Scan count 2, logical reads 548, physical reads 0, read-ahead reads 281, lob logical reads 0, lob physical reads 0, ...
Table 'Nations'. Scan count 1, logical reads 3, physical reads 1, read-ahead reads 0, lob logical reads 0, lob physical reads 0, ...

Logical reads

(88 rows affected)
Table 'Cities'. ... , logical reads 548, ... , ...
Table 'Nations'. ... , logical reads 3, ... , ...

logical reads: number of 8 kilobyte pages read per table

Example: query 1

SELECT UNStatisticalRegion, 
       CountryName,
       Capital,
          (SELECT MAX(Magnitude) 
          FROM Earthquakes e 
          WHERE n.Capital = e.NearestPop
              AND n.Code2 = e.Country) MaxMagnitude
FROM Nations n
WHERE Capital IN 
        (SELECT NearestPop 
         FROM Earthquakes e
         WHERE n.Capital = e.NearestPop 
            AND n.Code2 = e.Country)
                AND Capital IN 
                   (SELECT NearestPop
                    FROM Earthquakes 
                    WHERE Magnitude >= 7.5);

Example: query 1

SELECT UNStatisticalRegion, 
       CountryName,
       Capital,
          (SELECT MAX(Magnitude) -- 1st sub-query
          FROM Earthquakes e 
          WHERE n.Capital = e.NearestPop
              AND n.Code2 = e.Country) MaxMagnitude
FROM Nations n
WHERE Capital IN 
        (SELECT NearestPop 
         FROM Earthquakes e
         WHERE n.Capital = e.NearestPop 
            AND n.Code2 = e.Country)
                AND Capital IN 
                   (SELECT NearestPop
                    FROM Earthquakes 
                    WHERE Magnitude >= 7.5);

Example: query 1

SELECT UNStatisticalRegion, 
       CountryName,
       Capital,
          (SELECT MAX(Magnitude) -- 1st sub-query
          FROM Earthquakes e 
          WHERE n.Capital = e.NearestPop
              AND n.Code2 = e.Country) MaxMagnitude
FROM Nations n
WHERE Capital IN 
        (SELECT NearestPop -- 2nd sub-query
         FROM Earthquakes e
         WHERE n.Capital = e.NearestPop 
            AND n.Code2 = e.Country)
                AND Capital IN 
                   (SELECT NearestPop
                    FROM Earthquakes 
                    WHERE Magnitude >= 7.5);

Example: query 1

SELECT UNStatisticalRegion, 
       CountryName,
       Capital,
          (SELECT MAX(Magnitude) -- 1st sub-query
          FROM Earthquakes e 
          WHERE n.Capital = e.NearestPop
              AND n.Code2 = e.Country) MaxMagnitude
FROM Nations n
WHERE Capital IN 
        (SELECT NearestPop -- 2nd sub-query
         FROM Earthquakes e
         WHERE n.Capital = e.NearestPop 
            AND n.Code2 = e.Country)
                AND Capital IN 
                   (SELECT NearestPop -- 3rd sub-query
                    FROM Earthquakes 
                    WHERE Magnitude >= 7.5);

Example: query 1

SET STATISTICS IO ON

SELECT UNStatisticalRegion, 
       CountryName,
       Capital,
          (SELECT MAX(Magnitude) -- 1st sub-query
          FROM Earthquakes e 
          WHERE n.Capital = e.NearestPop
              AND n.Code2 = e.Country) MaxMagnitude
FROM Nations n
WHERE Capital IN 
        (SELECT NearestPop -- 2nd sub-query
         FROM Earthquakes e
         WHERE n.Capital = e.NearestPop 
            AND n.Code2 = e.Country)
                AND Capital IN 
                   (SELECT NearestPop -- 3rd sub-query
                    FROM Earthquakes 
                    WHERE Magnitude >= 7.5);

Table 'Earthquakes'. ..., logical reads 54, ...
'Nations'. ..., logical reads 3, ...

Example: query 2

SELECT n.UNStatisticalRegion, 
       n.CountryName,
       n.Capital,
       MAX(e.Magnitude)
FROM Nations n
INNER JOIN Earthquakes e
     ON n.Capital = e.NearestPop 
         AND n.Code2 = e.Country
WHERE e.Magnitude >=7.5
GROUP BY n.UNStatisticalRegion, 
        n.CountryName,
        n.Capital

Example: query 2

SELECT n.UNStatisticalRegion, 
       n.CountryName,
       n.Capital,
       MAX(e.Magnitude)
FROM Nations n
INNER JOIN Earthquakes e
     ON n.Capital = e.NearestPop 
         AND n.Code2 = e.Country
WHERE e.Magnitude >=7.5
GROUP BY n.UNStatisticalRegion, 
        n.CountryName,
        n.Capital

SET STATISTICS IO OFF

Table 'Earthquakes'. ..., logical reads 18, ...
'Nations'. ..., logical reads 3, ...

Comparing queries

First query containing three sub-queries

Table 'Earthquakes'. ..., logical reads 54, ...
'Nations'. ..., logical reads 3, ...

SQL Server parse and compile time: 
   CPU time = 29 ms, elapsed time = 29 ms.

Second query using an INNER JOIN

Table 'Earthquakes'. ..., logical reads 18, ...
'Nations'. ..., logical reads 3, ...

SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 3 ms.

Let's practice!

Improving Query Performance in SQL Server