SQL Serverのパフォーマンスが期待していたものではない

Question

私は非常に単純なテーブルクエリを手に入れました。私はうんざりしている。私が選択トップから200の代わりに50に変更すると、はるかに速くなります。テーブルには約100万行があります。

私は上記のすべての列にインデックスを持っています。 ORDER BYと一緒SELECT TOP xを使用すると、すべて同じ結果を保証するために、一般的に推奨されている間

CREATE TABLE dbo.EmailDetails (
     Id int IDENTITY(1, 1) NOT NULL, 
     AttendeesId int NOT NULL, 
     SponsorListContactId int NULL, 
     MessageUniqueId nvarchar(512) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     EmailSendStatus nvarchar(16) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     EmailSendStartTime datetime NULL, 
     EmailSendFinishTime datetime NULL, 
     EmailSendLogMessage nvarchar(max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     Subject nvarchar(2048) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     BodyText nvarchar(max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     SentDateTime datetime NULL, 
     EmailFrom nvarchar(128) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     EmailTo nvarchar(128) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     EmailDetailsTopicId int NULL, 
     EmailDetailsGuid uniqueidentifier NULL, 
     EmailReadCount int NULL, 
     EmailReadDate datetime NULL, 
     EmailSendStatusChange datetime NULL, 
     EmailSendPriority int NULL, 
     TextTo varchar(64) COLLATE SQL_Latin1_General_CP1_CI_AS NULL, 
     CONSTRAINT EmailDetails_pk PRIMARY KEY CLUSTERED (Id) 
     WITH (
      PAD_INDEX = OFF, IGNORE_DUP_KEY = OFF, STATISTICS_NORECOMPUTE = OFF, 
      ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) 
    ) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_AttendeesIds_idx ON dbo.EmailDetails 
     (AttendeesId) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_idx ON dbo.EmailDetails 
     (EmailDetailsGuid) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_idx2 ON dbo.EmailDetails 
     (EmailDetailsTopicId) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_idx3 ON dbo.EmailDetails 
     (SentDateTime) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_priority_status_idx4 ON dbo.EmailDetails 
     (EmailSendPriority, EmailSendStatus) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_SentStatus_idx ON dbo.EmailDetails 
     (EmailSendStatus) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_topicid_attendeeid ON dbo.EmailDetails 
     (EmailDetailsTopicId, AttendeesId) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

    CREATE NONCLUSTERED INDEX EmailDetails_topicid_sponsorcontactid_idx ON dbo.EmailDetails 
     (EmailDetailsTopicId, SponsorListContactId) 
    WITH (
     PAD_INDEX = OFF, 
     DROP_EXISTING = OFF, 
     STATISTICS_NORECOMPUTE = OFF, 
     SORT_IN_TEMPDB = OFF, 
     ONLINE = OFF, 
     ALLOW_ROW_LOCKS = ON, 
     ALLOW_PAGE_LOCKS = ON) 
    ON [PRIMARY] 
    GO 

Answer 1

：

とフルDDLは次のとおりです。

SELECT TOP 50 dbo.EmailDetails.Id, 
FROM dbo.EmailDetails WITH (NOLOCK) 
WHERE (dbo.EmailDetails.EmailSendStatus = 'NEEDTOSEND' OR 
    dbo.EmailDetails.EmailSendStatus = 'NEEDTOTEXT') AND 
    dbo.EmailDetails.EmailDetailsTopicId IS NOT NULL 
ORDER BY dbo.EmailDetails.EmailSendPriority, 
    dbo.EmailDetails.Id DESC 

プランの生成は、 ORDER BYは結果セット全体をソートし、その後にTOP xをfそれをロム。

SELECT TOP xの代わりにSELECT COUNT(*)を実行すれば、どうなるか分かりますか？

インデックスを使用すると高速ソートが保証されますが、このクエリの正確なORDER BYをカバーするインデックスはであり、インデックスはではありません。その上、WHERE句とSELECT句のフィールドを取得するには、おそらくディスク全体に広がるデータページに入る必要があります。このクエリだけを最適化するには、WHERE句とORDER BY句とSELECT句を新しいインデックスに（その順序で）追加することができます。もちろん、重複はありません。かなり大きなインデックスが作成されるため、挿入/更新が遅くなる可能性がありますが、SELECTを非常に高速にすることができます。これは、データページに入らずにインデックス全体を使用できるためです。