Dalam aplikasi kami, kami memiliki kotak di mana pengguna dapat halaman lebih dari sejumlah besar catatan (10-20 juta). Grid mendukung pengurutan dalam urutan menaik dan menurun dalam sejumlah kolom (20+). Banyak dari nilai-nilai ini juga tidak unik dan aplikasi juga mengurutkan oleh id sebagai tie-breaker untuk memastikan bahwa baris selalu muncul di halaman yang sama. Sebagai contoh, jika pengguna ingin mengurutkan berdasarkan ukuran widget (dimulai dengan yang terbesar), aplikasi menghasilkan kueri yang terlihat sedikit seperti ini:
SELECT TOP 30
* -- (Pretend that there is a list of columns here)
FROM Test
-- WHERE widgetSize > 100
ORDER BY
widgetSize DESC,
id ASC
Kueri ini membutuhkan waktu ~ 15 detik untuk dijalankan (dengan data cache), bagian utama dari biaya tampaknya adalah pengurutan ~ 1,3 juta baris menurut widgetSize. Dalam upaya untuk menyetel kueri ini, saya menemukan bahwa jika saya menambahkan WHERE
klausa yang dibatasi hanya pada ukuran widget terbesar (dikomentari dalam kueri di atas) kueri hanya membutuhkan ~ 800 ms (semua 50.000 hasil teratas memiliki ukuran widget> 100) .
Mengapa kueri tanpa WHERE
klausa jauh lebih lambat? Saya telah memeriksa statistik pada kolom widgetSize dan mereka menunjukkan bahwa 739 baris teratas memiliki WidgetSize> 506. Karena hanya 30 baris yang diperlukan, SQL server tidak dapat menggunakan informasi ini untuk menyimpulkan bahwa hanya perlu mengurutkan baris dengan ukuran widget. mana yang besar?
Saya tahu bahwa saya dapat membuat kueri spesifik ini bekerja lebih cepat dengan menambahkan indeks pada widgetSize
dan id
, namun indeks ini hanya berguna dalam skenario khusus ini, dan menjadi tidak bernilai jika (misalnya) pengguna membalikkan arah pengurutan. Tabel ini mengandung banyak kolom tambahan dan setiap indeks besar (~ 200mb) jadi saya tidak bisa benar-benar mampu menambahkan indeks untuk setiap urutan urutan yang mungkin.
Apakah ada cara agar kueri ini dapat dilakukan tanpa menambahkan indeks untuk setiap kemungkinan urutan? (pengguna dapat mengurutkan berdasarkan salah satu dari 20 kolom)
Script berikut membuat tabel di atas dan mengisinya dengan beberapa data representatif. Tabelnya jauh lebih sempit dari tabel sebenarnya, namun masih menunjukkan kinerja yang saya lihat. Di PC saya kueri dengan klausa where mengambil ~ 200ms sementara kueri tanpa caluse di mana ~ 800ms.
Peringatan: Basis data yang dihasilkan setelah menjalankan skrip ini adalah ~ 2Gb.
CREATE TABLE Test
(
id INT NOT NULL IDENTITY(1,1) PRIMARY KEY,
widgetSize INT NOT NULL
)
CREATE TABLE #Data
(
widgetSize INT NOT NULL,
recordCount INT NOT NULL
)
INSERT INTO #Data (widgetSize, recordCount)
VALUES
(40826,1),
(30317,1),
(28513,1),
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(-365105,1)
DECLARE c CURSOR FOR
SELECT widgetSize, recordCount FROM #Data
OPEN c
DECLARE @widgetSize INT
DECLARE @rowCount INT
FETCH NEXT FROM c INTO @widgetSize, @rowCount
WHILE @@FETCH_STATUS = 0
BEGIN
;WITH cte AS
(
SELECT rowNumber = 1
UNION ALL
SELECT rowNumber + 1
FROM cte
WHERE rowNumber < @rowCount
)
INSERT INTO Test
(
widgetSize
)
SELECT
@widgetSize
FROM cte
OPTION (MAXRECURSION 0)
FETCH NEXT FROM c INTO @widgetSize, @rowCount
END
CLOSE c
DEALLOCATE c
DROP TABLE #Data
CREATE STATISTICS WidgetSize
ON Test (WidgetSize) WITH FULLSCAN
(id, widgetSize)
? Jika urutan pencarian membalik dari ASC/DESC
indeks hanya dibaca kembali ke depan - itu tidak menjadi usang.
CREATE CLUSTERED INDEX CIX_id_widgetSize ON Test (id, widgetSize)
13773285 rows < 100
dan hanya 65717 rows > 100
, jadi Anda sebagian besar membatasi baris yang dipertanyakan dengan WHERE
. Apakah ada nilai lain yang bisa Anda filter? Jika Anda memiliki perusahaan, Anda dapat mempertimbangkan mempartisi tabel.
id
danwidgetsize
?