How to optimize cursor in a stored procedure - sql-server

Question

I'm having problems with a stored procedure that iterates over a table, it works fine with a few hundred rows however when the table is over the thousands it saturates the memory and crashes.
The procedure should iterate row by row and fill a column with a value which is calculated from another column in the row. I suspect it is the cursor that crashes the procedure and in other questions I've read to use a while loop but I'm no expert in sql and the examples I tried from those answers didn't work.
CREATE PROCEDURE [dbo].[GenerateNewHashes]
AS
BEGIN
SET NOCOUNT ON;
DECLARE #module BIGINT = 382449983
IF EXISTS(SELECT 1 FROM dbo.telephoneSource WHERE Hash IS NULL)
BEGIN
DECLARE hash_cursor CURSOR FOR
SELECT a.telephone, a.Hash
FROM dbo.telephoneSource AS a
OPEN hash_cursor
FETCH FROM hash_cursor
WHILE ##FETCH_STATUS = 0
BEGIN
UPDATE dbo.telephoneSource
SET Hash = CAST(telephone AS BIGINT) % #module
WHERE CURRENT OF hash_cursor
FETCH NEXT FROM hash_cursor
END
CLOSE hash_cursor
DEALLOCATE hash_cursor
END
END
Basically the stored procedure is intended to fill a new column called Hash that was added to the existing table, when the script that updates the table ends the new column is filled with NULL values and then this stored procedure is supposed to fill each null value with the operation telephone number (which is a bigint) % module variable (big int as well).
Is there anything besides changing to a while loop that I can do to make it use less memory or just don't crash? Thanks in advance.

Answer 1

You could do the following:
WHILE 1=1
BEGIN
UPDATE TOP (10000) dbo.telephoneSource
SET Hash = CAST(telephone AS BIGINT)%#module
WHERE Hash IS NULL;
IF ##ROWCOUNT = 0
BEGIN
BREAK;
END;
END;
This will update Hash as long as there are NULL values and will exit once there have been no records updated.
Adding a filtered index could be useful as well:
CREATE NONCLUSTERED INDEX IX_telephoneSource_Hash_telephone
ON dbo.telephoneSource (Hash)
INCLUDE (telephone)
WHERE Hash IS NULL;
It will speed up lookups in order to update it. But this might be not needed.

Answer 2

Here is example of code to do it in loops from my comment above with out using a cursor, and if you add where your field you are updating IS NOT NULL into the inner loop it wont update ones that were already done (in case you need to restart the process or something.
I didnt include your specific tables in there but if you need me to I can add it in there.
DECLARE #PerBatchCount as int
DECLARE #MAXID as bigint
DECLARE #WorkingOnID as bigint
Set #PerBatchCount = 1000
--Find range of IDs to process using yoru tablename
SELECT #WorkingOnID = MIN(ID), #MAXID = MAX(ID)
FROM YouTableHere WITH (NOLOCK)
WHILE #WorkingOnID <= #MAXID
BEGIN
-- do an update on all the ones that exist in the offer table NOW
--DO YOUR UPDATE HERE
-- include this where clause where ID is your PK you are looping through
WHERE ID BETWEEN #WorkingOnID AND (#WorkingOnID + #PerBatchCount -1)
set #WorkingOnID = #WorkingOnID + #PerBatchCount
END
SET NOCOUNT OFF;

Answer 3

I would simply add computed column:
ALTER TABLE dbo.telephoneSource
ADD Hash AS (CAST(telephone AS BIGINT)%382449983) PERSISTED;