Updating SQL Database Table with 30M Rows - sql

Question

I have a MS SQL table with approx 30M rows that I need to update a field based on the previous records. Here is an update that works but it is taking an incredible amount of time:
UPDATE AccountTransaction
SET EndingBalance = (SELECT COALESCE(SUM(b.amount), 0)
FROM AccountTransaction AS b
WHERE b.AccountId = AccountTransaction.AccountId
and b.Date <= AccountTransaction.Date
and (b.Date != AccountTransaction.Date
or b.CreatedDate < AccountTransaction.CreatedDate))
+ Amount
Here is the full DDL:
CREATE TABLE [dbo].[AccountTransaction](
[AccountTransactionId] [uniqueidentifier] NOT NULL,
[AccountId] [uniqueidentifier] NOT NULL,
[Amount] [decimal](16, 2) NOT NULL,
[EndingBalance] [decimal](16, 2) NOT NULL,
[Date] [date] NOT NULL,
[CreatedDate] [datetime2](3) NOT NULL,
CONSTRAINT [PkAccountTransaction] PRIMARY KEY CLUSTERED
(
[AccountTransactionId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX [IxAccountTransaction_AccountId_Date_CreatedDate] ON [dbo].[AccountTransaction]
(
[AccountId] ASC,
[Date] ASC,
[CreatedDate] ASC
)
INCLUDE ([AccountTransactionId],
[Amount],
[EndingBalance]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX [IxAccountTransaction_AccountId] ON [dbo].[AccountTransaction]
(
[AccountId] ASC
)
INCLUDE ([AccountTransactionId],
[Amount],
[EndingBalance],
[Date],
[CreatedDate]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

Answer 1

The following should yield much better performance and will be able to take advantage of the IxAccountTransaction_AccountId_Date_CreatedDate index...
WITH
cte_Runningtotal AS (
SELECT
at1.EndingBalance,
NewEB = SUM(at1.Amount) OVER (PARTITION BY at1.AccountId ORDER BY at1.[Date] ROWS UNBOUNDED PRECEDING)
FROM
dbo.AccountTransaction at1
)
UPDATE rt SET
rt.EndingBalance = rt.NewEB
FROM
cte_Runningtotal rt;