Slow SQL Update using sub query - sql

Question

Hi all … Wonder if anyone out there can help me with this one please.
I am running a query to update product categories against sales lines and need to back file a few million records so I wrote the query below to run for a specific order ID
DECLARE #ID INT
SET #ID = 659483
UPDATE [TradeSpace].[TradeSpace].[dbo].[SalesLine]
SET [ProductCategory] = [curSync].[pc_Cat]
FROM (SELECT [SC_ID],
[pc_cat]
FROM [MW_MereSys].[dbo].[MWSLines]
INNER
JOIN [MW_MereSys].[dbo].[MWProductCats]
ON [MWSLines].[pc_catref] = [MWProductCats].[pc_catref]
WHERE [sh_id] = #ID
) AS [curSync]
WHERE [SalesLine].[slID] = [curSync].[sc_id]
AND [salesline].[soid] = #ID
The sub SELECT runs in less than one second but the update has yet to finished (have left it for an hour at most). Indexes exist for [slID] and [soid] .. a manual update for one line takes less than one seconds but run like this (10 lines) is desperately slow.
Does anybody have any clues please. I've written plenty of queries like this and never had a problem … stumped :(

Answer 1

Your query rewritten with no changes:
UPDATE s SET
ProductCategory = curSync.pc_Cat
FROM TradeSpace.TradeSpace.dbo.SalesLine s
INNER JOIN
(
SELECT [SC_ID], [pc_cat]
FROM [MW_MereSys].[dbo].[MWSLines] l
INNER JOIN [MW_MereSys].[dbo].[MWProductCats] c ON l.[pc_catref] = c.[pc_catref]
WHERE [sh_id] = #ID
) AS [curSync]
on s.[slID] = [curSync].[sc_id]
WHERE s.[soid] = #ID
Are your sure everything is correct here? That single row from SalesLine always matches only one row from subquery?
Try this then. Will fail if this is not true. Original query would silently update same row with different values in same situation.
UPDATE s SET
ProductCategory = (
SELECT [pc_cat]
FROM [MW_MereSys].[dbo].[MWSLines] l
INNER JOIN [MW_MereSys].[dbo].[MWProductCats] c ON l.[pc_catref] = c.[pc_catref]
WHERE [sh_id] = #ID
AND [sc_id] = s.[slID]
)
FROM TradeSpace.TradeSpace.dbo.SalesLine s
WHERE s.[soid] = #ID
And please check estimated execution plan. Does it hit indexes?

Answer 2

We need other detail like a I mention in comments.
Your update is slow because of very high cardinality estimate when update table is join with Sub query result.
It may be because of wrong join and where predicate.
you can put the sub query result in #Temp table and try.Also you can create same index in #temp table.
DECLARE #ID INT
SET #ID = 659483
create #temp table([SC_ID] int,[pc_cat] int)
insert into #temp
SELECT [SC_ID],
[pc_cat]
FROM [MW_MereSys].[dbo].[MWSLines]
INNER JOIN [MW_MereSys].[dbo].[MWProductCats]
ON [MWSLines].[pc_catref] = [MWProductCats].[pc_catref]
WHERE [sh_id] = #ID
UPDATE SalesLine
SET [ProductCategory] = [curSync].[pc_Cat]
FROM [TradeSpace].[TradeSpace].[dbo].[SalesLine] as SalesLine
inner join #temp AS [curSync]
WHERE [SalesLine].[slID] = [curSync].[sc_id]
AND [salesline].[soid] = #ID
drop table #temp