Performance issue using IsNull function in the Select statement - sql

Question

I have a financial application. I have ViewHistoricInstrumentValue which has rows like this
instrument1, date1, price, grossValue, netValue
instrument2, date1, price, grossValue, netValue
...
instrument1, date2, price, grossValue, netValue
...
My views are complicated but the db itself is small (4000 transactions). ViewHistoricInstrumentValue was executed in less than 1 second before I added the next CTE to the view. After that it takes 26s. ActualEvaluationPrice is the price for instrumentX at dateY. If this value is missing from HistoricPrice table then I find the previous price for instrumentX.
, UsedEvaluationPriceCte AS (
SELECT *
, isnull(ActualEvaluationPrice,
(select top 1 HistoricPrice.Price -- PreviousPrice
from HistoricPrice JOIN ValidDate
on HistoricPrice.DateId = ValidDate.Id
and HistoricPrice.InstrumentId = StartingCte.InstrumentId
and ValidDate.[Date] < StartingCte.DateValue
order by ValidDate.[Date]))
as UsedEvaluationPrice
FROM StartingCte
)
My problem is that the execution time increased needlessly. Right now the HistoricPrice table has no missing value so ActualEvaluationPrice is never null, so the previous price should be never determined.
ViewHistoricInstrumentValue returns 1815 rows. One other mystery is that the first query takes 26s, but the second only 2s.
SELECT * FROM [ViewHistoricInstrumentValue]
SELECT top(2000) * FROM [ViewHistoricInstrumentValue]
Appendix
The execution plan: https://www.dropbox.com/s/5st69uhjkpd3b5y/IsNull.sqlplan?dl=0
The same plan: https://www.brentozar.com/pastetheplan/?id=rk9bK1Wiv
The view:
ALTER VIEW [dbo].[ViewHistoricInstrumentValue] AS
WITH StartingCte AS (
SELECT
HistoricInstrumentValue.DateId
, ValidDate.Date as DateValue
, TransactionId
, TransactionId AS [Row]
, AccountId
, AccountName
, ViewTransaction.InstrumentId
, ViewTransaction.InstrumentName
, OpeningDate
, OpeningPrice
, Price AS ActualEvaluationPrice
, ClosingDate
, Amount
, isnull(ViewTransaction.FeeValue, 0) as FeeValue
, HistoricInstrumentValue.Id AS Id
FROM ViewBriefHistoricInstrumentValue as HistoricInstrumentValue
JOIN ValidDate on HistoricInstrumentValue.DateId = ValidDate.Id
JOIN ViewTransaction ON ViewTransaction.Id = HistoricInstrumentValue.TransactionId
left JOIN ViewHistoricPrice ON ViewHistoricPrice.DateId = HistoricInstrumentValue.DateId AND
ViewHistoricPrice.InstrumentId = ViewTransaction.InstrumentId
)
, UsedEvaluationPriceCte AS (
SELECT *
, isnull(ActualEvaluationPrice,
(select top 1 HistoricPrice.Price -- PreviousPrice
from HistoricPrice JOIN ValidDate
on HistoricPrice.DateId = ValidDate.Id
and HistoricPrice.InstrumentId = StartingCte.InstrumentId
and ValidDate.[Date] < StartingCte.DateValue
order by ValidDate.[Date]))
as UsedEvaluationPrice
FROM StartingCte
)
, GrossEvaluationValueCte AS (
SELECT *
, Amount * UsedEvaluationPrice AS GrossEvaluationValue
, (UsedEvaluationPrice - OpeningPrice) * Amount AS GrossCapitalGains
FROM UsedEvaluationPriceCte
)
, CapitalGainsTaxCte AS (
SELECT *
, dbo.MyMax(GrossCapitalGains * 0.15, 0) AS CapitalGainsTax
FROM GrossEvaluationValueCte
)
, IsOpenCte AS (
SELECT
DateId
, DateValue
, TransactionId
, [Row]
, AccountId
, AccountName
, InstrumentId
, InstrumentName
, OpeningDate
, OpeningPrice
, ActualEvaluationPrice
, UsedEvaluationPrice
, ClosingDate
, Amount
, GrossEvaluationValue
, GrossCapitalGains
, CapitalGainsTax
, FeeValue
, GrossEvaluationValue - CapitalGainsTax - FeeValue AS NetEvaluationValue
, GrossCapitalGains - CapitalGainsTax - FeeValue AS NetUnrealizedGains
, CASE WHEN ClosingDate IS NULL OR DateValue < ClosingDate
THEN CAST(1 AS BIT)
ELSE CAST(0 AS BIT)
END
AS IsOpen
, convert(NVARCHAR, DateValue, 20) + cast([Id] AS NVARCHAR(MAX)) AS Temp
, Id
FROM CapitalGainsTaxCte
)
Select * from IsOpenCte

Answer 1

I have no idea what your query is supposed to be doing. But this process:
ActualEvaluationPrice is the price for instrumentX at dateY. If this value is missing from HistoricPrice table then I find the previous price for instrumentX.
is handled easily with lag():
select vhiv.*
coalesce(vhiv.ActualEvaluationPrice,
lag(vhiv.ActualEvaluationPrice) over (partition by vhiv.InstrumentId order by DateValue)
) as UsedEvaluationPrice
from ViewHistoricInstrumentValue vhiv;
Note: If you need to filter out certain dates by joining to ValidDates, you can include the JOIN in the query. However, that is not part of the problem statement.