我有几个案例，我的情结CTE ( Common Table Expressions ) 比使用 SQL Server 中的临时表的相同查询慢十倍.
我的问题是关于如何 SQL Server处理CTE查询，看起来它尝试连接所有单独的查询，而不是存储每个查询的结果，然后尝试运行以下查询。所以这可能就是使用临时表时速度如此之快的原因。
例如:
查询 1:使用 Common Table Expression :
;WITH Orders AS
(
SELECT
ma.MasterAccountId,
IIF(r.FinalisedDate IS NULL, 1, 0)) [Status]
FROM
MasterAccount ma
INNER JOIN
task.tblAccounts a ON a.AccountNumber = ma.TaskAccountId
AND a.IsActive = 1
LEFT OUTER JOIN
task.tblRequisitions r ON r.AccountNumber = a.AccountNumber
WHERE
ma.IsActive = 1
AND CAST(r.BatchDateTime AS DATE) BETWEEN @fromDate AND @toDate
AND r.BatchNumber > 0
),
StockAvailability AS
(
SELECT sa.AccountNumber,
sa.RequisitionNumber,
sa.RequisitionDate,
sa.Lines,
sa.HasStock,
sa.NoStock,
CASE WHEN sa.Lines = 0 THEN 'Empty'
WHEN sa.HasStock = 0 THEN 'None'
WHEN (sa.Lines > 0 AND sa.Lines > sa.HasStock) THEN 'Partial'
WHEN (sa.Lines > 0 AND sa.Lines <= sa.HasStock) THEN 'Full'
END AS [Status]
FROM
(
SELECT
r.AccountNumber,
r.RequisitionNumber,
r.RequisitionDate,
COUNT(rl.ProductNumber) Lines,
SUM(IIF(ISNULL(psoh.AvailableStock, 0) >= ISNULL(rl.Quantity, 0), 1, 0)) AS HasStock,
SUM(IIF(ISNULL(psoh.AvailableStock, 0) < ISNULL(rl.Quantity, 0), 1, 0)) AS NoStock
FROM task.tblrequisitions r
INNER JOIN task.tblRequisitionLines rl ON rl.RequisitionNumber = r.RequisitionNumber
LEFT JOIN ProductStockOnHandSummary psoh ON psoh.ProductNumber = rl.ProductNumber
WHERE dbo.fn_RemoveUnitPrefix(r.BatchNumber) = 0
AND r.UnitId = 1
AND r.FinalisedDate IS NULL
AND r.RequisitionStatus = 1
AND r.TransactionTypeNumber = 301
GROUP BY r.AccountNumber, r.RequisitionNumber, r.RequisitionDate
) AS sa
),
Available AS
(
SELECT ma.MasterAccountId,
SUM(IIF(ma.IsPartialStock = 1, CASE WHEN sa.[Status] IN ('Full', 'Partial') THEN 1 ELSE 0 END,
CASE WHEN sa.[Status] = 'Full' THEN 1 ELSE 0 END)) AS AvailableStock,
SUM(IIF(sa.[Status] IN ('Full', 'Partial', 'None'), 1, 0)) AS OrdersAnyStock,
SUM(IIF(sa.RequisitionDate < dbo.TicksToTime(ma.DailyOrderCutOffTime, @toDate),
IIF(ma.IsPartialStock = 1, CASE WHEN sa.[Status] IN ('Full', 'Partial') THEN 1 ELSE 0 END,
CASE WHEN sa.[Status] = 'Full' THEN 1 ELSE 0 END), 0)) AS AvailableBeforeCutOff
FROM MasterAccount ma
INNER JOIN StockAvailability sa ON sa.AccountNumber = ma.TaskAccountId
GROUP BY ma.MasterAccountId, ma.IsPartialStock
),
Totals AS
(
SELECT
o.MasterAccountId,
COUNT(o.MasterAccountId) AS BatchedOrders
FROM Orders o
GROUP BY o.MasterAccountId
)
SELECT a.MasterAccountId,
ISNULL(t.BatchedOrders, 0) BatchedOrders,
ISNULL(t.PendingOrders, 0) PendingOrders,
ISNULL(av.AvailableStock, 0) AvailableOrders,
ISNULL(av.AvailableBeforeCutOff, 0) AvailableCutOff,
ISNULL(av.OrdersAnyStock, 0) AllOrders
FROM MasterAccount a
LEFT OUTER JOIN Available av ON av.MasterAccountId = a.MasterAccountId
LEFT OUTER JOIN Totals t ON t.MasterAccountId = a.MasterAccountId
WHERE a.IsActive = 1
查询 2:使用临时表:
DROP TABLE IF EXISTS #Orders
CREATE TABLE #Orders (MasterAccountId int, [Status] int);
INSERT INTO #Orders
SELECT
ma.MasterAccountId,
dbo.fn_GetBatchPickingStatus(ma.BatchPickingOnHold,
iif(r.GroupNumber > 0, 1, 0),
iif(r.FinalisedDate is null, 1, 0)) [Status]
FROM MasterAccount ma (nolock)
INNER JOIN wh3.dbo.tblAccounts a (nolock) on a.AccountNumber = dbo.fn_RemoveUnitPrefix(ma.TaskAccountId) and a.IsActive = 1
LEFT OUTER JOIN wh3.dbo.tblRequisitions r (nolock) on r.AccountNumber = a.AccountNumber
WHERE cast(r.BatchDateTime as date) between @fromDate and @toDate
AND r.BatchNumber > 0
AND ma.IsActive = 1
DROP TABLE IF EXISTS #StockAvailability
Create Table #StockAvailability (AccountNumber int, RequisitionNumber int, RequisitionDate datetime, Lines int, HasStock int, NoStock int);
Insert Into #StockAvailability
SELECT
r.AccountNumber,
r.RequisitionNumber,
r.RequisitionDate,
COUNT(rl.ProductNumber) Lines,
SUM(IIF(ISNULL(psoh.AvailableStock, 0) >= ISNULL(rl.Quantity, 0), 1, 0)) AS HasStock,
SUM(IIF(ISNULL(psoh.AvailableStock, 0) < ISNULL(rl.Quantity, 0), 1, 0)) AS NoStock
FROM WH3.dbo.tblrequisitions r (nolock)
INNER JOIN WH3.dbo.tblRequisitionLines rl (nolock) ON rl.RequisitionNumber = r.RequisitionNumber
LEFT JOIN ProductStockOnHandSummary psoh (nolock) ON psoh.ProductNumber = rl.ProductNumber -- Joined with View
WHERE r.BatchNumber = 0
AND r.FinalisedDate is null
AND r.RequisitionStatus = 1
AND r.TransactionTypeNumber = 301
GROUP BY r.AccountNumber, r.RequisitionNumber, r.RequisitionDate
DROP TABLE IF EXISTS #StockAvailability2
Create Table #StockAvailability2 (AccountNumber int, RequisitionNumber int, RequisitionDate datetime, Lines int, HasStock int, NoStock int, [Status] nvarchar(7));
Insert Into #StockAvailability2
SELECT sa.AccountNumber,
sa.RequisitionNumber,
sa.RequisitionDate,
sa.Lines,
sa.HasStock,
sa.NoStock,
CASE WHEN sa.Lines = 0 THEN 'Empty'
WHEN sa.HasStock = 0 THEN 'None'
WHEN (sa.Lines > 0 AND sa.Lines > sa.HasStock) THEN 'Partial'
WHEN (sa.Lines > 0 AND sa.Lines <= sa.HasStock) THEN 'Full'
END AS [Status]
FROM #StockAvailability sa
DROP TABLE IF EXISTS #Available
Create Table #Available (MasterAccountId int, AvailableStock int, OrdersAnyStock int, AvailableBeforeCutOff int);
INSERT INTO #Available
SELECT ma.MasterAccountId,
SUM(IIF(ma.IsPartialStock = 1, CASE WHEN sa.[Status] IN ('Full', 'Partial') THEN 1 ELSE 0 END,
CASE WHEN sa.[Status] = 'Full' THEN 1 ELSE 0 END)) AS AvailableStock,
SUM(IIF(sa.[Status] IN ('Full', 'Partial', 'None'), 1, 0)) AS OrdersAnyStock,
SUM(IIF(sa.RequisitionDate < dbo.TicksToTime(ma.DailyOrderCutOffTime, @toDate),
IIF(ma.IsPartialStock = 1, CASE WHEN sa.[Status] IN ('Full', 'Partial') THEN 1 ELSE 0 END,
CASE WHEN sa.[Status] = 'Full' THEN 1 ELSE 0 END), 0)) AS AvailableBeforeCutOff
FROM MasterAccount ma (NOLOCK)
INNER JOIN #StockAvailability2 sa ON sa.AccountNumber = dbo.fn_RemoveUnitPrefix(ma.TaskAccountId)
GROUP BY ma.MasterAccountId, ma.IsPartialStock
;WITH Totals AS
(
SELECT
o.MasterAccountId,
COUNT(o.MasterAccountId) AS BatchedOrders,
SUM(IIF(o.[Status] IN (0,1,2), 1, 0)) PendingOrders
FROM #Orders o (NOLOCK)
GROUP BY o.MasterAccountId
)
SELECT a.MasterAccountId,
ISNULL(t.BatchedOrders, 0) BatchedOrders,
ISNULL(t.PendingOrders, 0) PendingOrders,
ISNULL(av.AvailableStock, 0) AvailableOrders,
ISNULL(av.AvailableBeforeCutOff, 0) AvailableCutOff,
ISNULL(av.OrdersAnyStock, 0) AllOrders
FROM MasterAccount a (NOLOCK)
LEFT OUTER JOIN #Available av (NOLOCK) ON av.MasterAccountId = a.MasterAccountId
LEFT OUTER JOIN Totals t (NOLOCK) ON t.MasterAccountId = a.MasterAccountId
WHERE a.IsActive = 1
最佳答案
答案很简单。
SQL Server 不实现 CTE。正如您从执行计划中看到的那样，它内联了它们。
其他 DBMS 可能会以不同的方式实现它，一个众所周知的例子是 Postgres，它确实实现了 CTE(它本质上是在后台为 CTE 创建临时表)。
显式临时表中的中间结果的显式实现是否更快，取决于查询。
在复杂查询中，将中间数据写入和读取到临时表的开销可以通过优化器能够生成的更高效、更简单的执行计划来抵消。
另一方面，在 Postgres 中，CTE 是一个“优化栅栏”，引擎无法将谓词推送到 CTE 边界。
有时一种方法更好，有时另一种方法更好。一旦查询复杂性超过特定阈值，优化器就无法分析处理数据的所有可能方法，并且必须做出某种决定。例如，连接表的顺序。排列的数量随着可供选择的表的数量呈指数增长。优化器生成计划的时间有限，因此当所有 CTE 都内联时，它可能会做出糟糕的选择。当您手动将复杂查询分解为更小的更简单的查询时，您需要了解自己在做什么，但优化器有更好的机会为每个简单查询生成良好的计划。
关于sql - 与 SQL Server 中的临时表相比，为什么 CTE(通用表表达式)在某些情况下会减慢查询速度，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/51814383/