Oversigt:
Jeg kørte hver forespørgsel 10 gange hver ved at bruge nedenstående testdatasæt...
- Et meget stort underforespørgselsresultatsæt (100.000 rækker)
- Duplikér rækker
- Nul rækker
For alle ovenstående scenarier, både IN og EXISTS udført på identisk måde.
Nogle oplysninger om Performance V3-database bruges til test.20000 kunder har 1000000 ordrer, så hver kunde duplikeres tilfældigt (i et interval på 10 til 100) i ordretabellen.
Udførelsesomkostninger, Tid:
Nedenfor er et skærmbillede af begge forespørgsler, der kører. Overhold hver forespørgsel relative omkostninger.
Hukommelsesomkostninger:
Hukommelsestildelingen for de to forespørgsler er også den samme..Jeg tvang MDOP 1 for ikke at spilde dem til TEMPDB..
CPU-tid, læses:
For Exists:
Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Customers'. Scan count 1, logical reads 109, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 1, logical reads 3855, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
(1 row(s) affected)
SQL Server Execution Times:
CPU time = 469 ms, elapsed time = 595 ms.
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 0 ms.
For IN:
(20000 row(s) affected)
Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Customers'. Scan count 1, logical reads 109, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 1, logical reads 3855, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
(1 row(s) affected)
SQL Server Execution Times:
CPU time = 547 ms, elapsed time = 669 ms.
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 0 ms.
I hvert tilfælde er optimeringsværktøjet smart nok til at omarrangere forespørgslerne.
Jeg plejer at bruge EXISTS kun dog (min mening). Ét use case for at bruge EXISTS er, når du ikke ønsker at returnere et andet tabelresultatsæt.
Opdater i henhold til forespørgsler fra Martin Smith:
Jeg kørte nedenstående forespørgsler for at finde den mest effektive måde at hente rækker fra den første tabel, for hvilken der findes en reference i den anden tabel.
SELECT DISTINCT c.*
FROM Customers c
JOIN Orders o ON o.custid = c.custid
SELECT c.*
FROM Customers c
INNER JOIN (SELECT DISTINCT custid FROM Orders) AS o ON o.custid = c.custid
SELECT *
FROM Customers C
WHERE EXISTS(SELECT 1 FROM Orders o WHERE o.custid = c.custid)
SELECT *
FROM Customers c
WHERE custid IN (SELECT custid FROM Orders)
Alle ovenstående forespørgsler deler samme pris med undtagelse af 2. INNER JOIN , Planlæg at være den samme for resten.
Hukommelsestilskud:
Denne forespørgsel
SELECT DISTINCT c.*
FROM Customers c
JOIN Orders o ON o.custid = c.custid
påkrævet hukommelsesbevilling af
Denne forespørgsel
SELECT c.*
FROM Customers c
INNER JOIN (SELECT DISTINCT custid FROM Orders) AS o ON o.custid = c.custid
påkrævet hukommelsesbevilling på ..
CPU-tid, læses:
Til forespørgsel:
SELECT DISTINCT c.*
FROM Customers c
JOIN Orders o ON o.custid = c.custid
(20000 row(s) affected)
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Workfile'. Scan count 48, logical reads 1344, physical reads 96, read-ahead reads 1248, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 5, logical reads 3929, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Customers'. Scan count 5, logical reads 322, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 1453 ms, elapsed time = 781 ms.
For forespørgsel:
SELECT c.*
FROM Customers c
INNER JOIN (SELECT DISTINCT custid FROM Orders) AS o ON o.custid = c.custid
(20000 row(s) affected)
Table 'Customers'. Scan count 5, logical reads 322, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 5, logical reads 3929, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 1499 ms, elapsed time = 403 ms.