Teoretycznie będą używać tych samych planów i działać prawie w tym samym czasie.
W praktyce
SELECT TOP 1 Id FROM Table1 ORDER BY Id DESC
będzie bardziej prawdopodobnie użyć PRIMARY KEY INDEX
.
Również ten jest bardziej rozszerzalny, jeśli zdecydujesz się wybrać inną kolumnę wraz z id
.
Rzeczywisty Plan na MAX()
mówi:
SELECT <- AGGREGATE <- TOP <- CLUSTERED INDEX SCAN
, podczas gdy plan TOP 1
mówi:
SELECT <- TOP <- CLUSTERED INDEX SCAN
, i. mi. aggregate
jest pominięty.
Agregacja właściwie nic tu nie zrobi, ponieważ jest tylko jeden rząd.
PS Jak @Mehrdad Afshari
i @John Sansom
zauważyć, na polu non-indeksowanej MAX
jest nieco szybciej (oczywiście, że nie 20
razy jak mówi optymalizator):
-- 18,874,368 rows
SET LANGUAGE ENGLISH
SET STATISTICS TIME ON
SET STATISTICS IO ON
PRINT 'MAX'
SELECT MAX(id) FROM master
PRINT 'TOP 1'
SELECT TOP 1 id FROM master ORDER BY id DESC
PRINT 'MAX'
SELECT MAX(id) FROM master
PRINT 'TOP 1'
SELECT TOP 1 id FROM master ORDER BY id DESC
PRINT 'MAX'
SELECT MAX(id) FROM master
PRINT 'TOP 1'
SELECT TOP 1 id FROM master ORDER BY id DESC
PRINT 'MAX'
SELECT MAX(id) FROM master
PRINT 'TOP 1'
SELECT TOP 1 id FROM master ORDER BY id DESC
PRINT 'MAX'
SELECT MAX(id) FROM master
PRINT 'TOP 1'
SELECT TOP 1 id FROM master ORDER BY id DESC
Changed language setting to us_english.
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
MAX
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 20 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, physical reads 0, read-ahead reads 447, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 5452 ms, elapsed time = 2766 ms.
TOP 1
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, physical reads 0, read-ahead reads 2, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 6813 ms, elapsed time = 3449 ms.
MAX
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, physical reads 0, read-ahead reads 44, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 5359 ms, elapsed time = 2714 ms.
TOP 1
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, 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 = 6766 ms, elapsed time = 3379 ms.
MAX
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, 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 = 5406 ms, elapsed time = 2726 ms.
TOP 1
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, 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 = 6780 ms, elapsed time = 3415 ms.
MAX
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, physical reads 0, read-ahead reads 85, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 5392 ms, elapsed time = 2709 ms.
TOP 1
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, physical reads 0, read-ahead reads 10, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 6766 ms, elapsed time = 3387 ms.
MAX
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, 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 = 5374 ms, elapsed time = 2708 ms.
TOP 1
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 1 ms.
(строк обработано: 1)
Table 'master'. Scan count 3, logical reads 32655, 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 = 6797 ms, elapsed time = 3494 ms.
Od odpowiedzi tutaj, z indeks nie ma różnicy w wydajności. Jednak znalazłem 'MAX' /' MIN' ma kilka zalet. Po pierwsze, jak wspomniano poniżej, bez indeksu 'MAX' będzie bardziej wydajny, to również staje się znaczące, gdy masz bardziej złożone połączenia w miejscu. Po drugie, jeśli są NULL, to 'SELECT MIN' zwraca wartość, ale' SELECT TOP 1 ... ASC' zwraca 'NULL' – icc97