sql count(*) vs count(1)

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MS SQL 123

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SELECT COUNT (*) FROM mytable 一定會使用 Table Scan?

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NO

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SQL 會用最小 i/o 成本的方式選擇最小的 Page Number

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DEMO

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CREATE TABLE CTest (c1 INT IDENTITY, c2 BIGINT DEFAULT 1, c3 CHAR (1000) DEFAULT 'a');GOSET NOCOUNT ON;GOINSERT INTO CTest DEFAULT VALUES;GO 10000

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開啟實際的執行計畫 Enable execution plan viewers in SSMS

Include Actual Execution Plan

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SELECT COUNT (*)

SELECT COUNT (*) from CTest;

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加入 NonClustered Index – c2 (BIGINT)

CREATE NONCLUSTERED INDEX CTest_1 ON CTest (c2);

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SELECT COUNT (*) SELECT COUNT (*) from CTest;

變成了 Index Scan 因為使用 CTest_1 index 會比使用 Table Scan 使用更少的 i/o

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加入 NonClustered Index – c1 (INT)

CREATE NONCLUSTERED INDEX CTest_2 ON CTest (c1);

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SELECT COUNT (*) SELECT COUNT (*) from CTest;

跟 CTest_1 Index 相比 CTest_2 Index 使用更少的 i/o

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看一下每個 Index 的 Page CountSELECT a.[index_id], a.[page_count], b.name AS index_nameFROM sys.dm_db_index_physical_stats(DB_ID (), OBJECT_ID ('CTest'), NULL, NULL, 'LIMITED') aINNER JOIN sys.indexes bON a.object_id = b.object_id AND a.index_id = b.index_id;

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Count(*) or

Count(1)

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50/50

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GUID是一個好的Cluster Key?

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NO

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RandomIndex fragmentation

16 bytes long

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DEMO

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建立測試的資料表create table tblSAMPLE_1(xnewid uniqueidentifier default(newid()) primary key, xname varchar(10) );go

insert into tblSAMPLE_1(xname) values('rainmaker')go 500

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查看 Index 的破碎狀況

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大於 30% 就 Rebuild Index

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查看 Table 各 Index 的破碎狀況SELECT a.index_id, name, avg_fragmentation_in_percentFROM sys.dm_db_index_physical_stats (DB_ID(N'tempdb'), OBJECT_ID(N'tblSAMPLE_1'), NULL, NULL, NULL) AS a JOIN sys.indexes AS b ON a.object_id = b.object_id AND a.index_id = b.index_id;