I have to update 40 million rows in SQL Server [closed] - sql

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The code shown here is taking a very long time to update or populate 42673844. Could anyone help me on how to simplify this code and improve the performance.
I am trying to populate the Session ID's based on some conditions
Declaration of variables
declare #sval as INT
declare #maxRow as INT
declare #currvlt as int
declare #prevvlt as int
declare #cashamount as int
declare #sec as INT
declare #next_action as INT
declare #previous_action as INT
declare #rec as int
declare #currentRow as INT
-- assigning the maximum value to the #maxrow
set #maxRow = (select max(number) from dbo.stage3 as count)
print #maxRow
set #sval = 0
set #currentRow = 1
I am putting the conditions in a while loop to perform an update all the 42 million rows. The below select will assign the values to the declared variables
While (#currentRow <= #maxRow)
begin
select
#currvlt = vltid ,
#prevvlt = previous_vltid ,
#cashamount = isnull(playercashableamount,0),
#sec = seconds,
#next_action = next_action , --next record type
#previous_action = previous_action,
#rec = recordtype
from dbo.stage3
where number = #currentRow
if #currvlt <> #prevvlt or #prevvlt is null
begin
update dbo.stage3 set sessionid = #sval+1 where number = #currentRow
set #sval = #sval +1
end
else
if (#rec = '4' and #sec > 30 and #previous_action in( 1 ,5)) or
(#cashamount < 1 and #sec < 30 and #next_action in( 1 ,4 )) or
((#rec = '5' and #sec < 30 and #next_action = '4' ) or
#next_action = '4')
begin
update dbo.stage3 set sessionid = #sval where number = #currentRow
end
else
update dbo.stage3 set sessionid = #sval where number = #currentRow
set #currentRow = #currentRow +1
end

Answer 1

This might help. You should not need to do a row by row evaluation. Instead, address the entire set in one call when possible. Since you have provided no information about the business challenge you are trying to solve, this code is posted as-is. This may not be exactly what you're after, but it should get you going in the right direction. Please adjust for your own needs.
Also, any update on a table with 40 million rows will take a long time; even if you have perfectly tuned indexes on lightning fast storage. Instead of thinking "how do I get the answer I want?", try thinking "what am I asking the server to do for me?", and "is there a better way to make this request?" SQL Server runs best when dealing with a set of data in one pass. Analyzing row by row is not what the engine is built to do. It can be done, but should be used only when all other means have been exhausted.
The code below is a select statement. Run it, and if the results are what you're looking for, uncomment the UPDATE section, then comment out the SELECT section and run it.
SELECT CASE
WHEN recordtype = '4'
AND seconds > 30
AND previous_action IN (1, 4) THEN 0
WHEN s.cash_amount < 1
AND seconds < 30
AND next_action IN (1, 4) THEN 0
WHEN next_action = 4 THEN 0
WHEN (vltid <> previous_vltid)
OR previous_vltid IS NULL THEN s.RowNum
ELSE s.RowNum
END AS session_id
,s.vltid
,s.previous_vltid
,s.cash_amount
,s.recordtype
,s.previous_action
,s.seconds
,s.next_action
--UPDATE s
--SET sessionid = CASE
-- WHEN recordtype = '4'
-- AND seconds > 30
-- AND previous_action IN (1, 4) THEN 0
-- WHEN s.cash_amount < 1
-- AND seconds < 30
-- AND next_action IN (1, 4) THEN 0
-- WHEN next_action = 4 THEN 0
-- WHEN (vltid <> previous_vltid)
-- OR previous_vltid IS NULL THEN s.RowNum
-- ELSE s.RowNum
-- END
FROM (
SELECT sessionid
,ROW_NUMBER() OVER (PARTITION BY vltid
ORDER BY
vltid
) AS RowNum
,vltid
,previous_vltid
,ISNULL(playercashableamount, 0) AS cash_amount
,recordtype
,previous_action
,seconds
,next_action
FROM dbo.stage3
) AS s;