Current advice for the most efficient way to compare two large result/row sets seems to be to use the EXCEPT
operator. This self contained SQL script below gets very inefficient as row sizes increase (change @last values). I have tried to find unique entries in a combined table but with no improvement.
DECLARE @first AS INT, @step AS INT, @last AS INT;
-- This script is comparing two record sets using EXCEPT
-- I want to find additions from OLD to NEW
-- As number of rows increase performance gets terrible
-- I don't have to use two tables. I could use one combined table but I want the same result as quickly as possible
-- Compare 100 to 110 rows - 0 seconds
-- Compare 1000 to 1010 rows - 1 seconds
-- Compare 10000 to 10010 rows - 16 seconds
-- Compare 100000 to 100010 rows - ABORT after 8 minutes (tables are populated in 18 seconds)
DECLARE @temptableOLD TABLE ([Result1] int);
SET @step = 1; SET @first = 1; SET @last = 100000
WHILE(@first <= @last) BEGIN INSERT INTO @temptableOLD VALUES(@first) SET @first += @step END
DECLARE @temptableNEW TABLE ([Result1] int);
SET @step = 1; SET @first = 1; SET @last = 100010
WHILE(@first <= @last) BEGIN INSERT INTO @temptableNEW VALUES(@first) SET @first += @step END
select * from @temptableNEW
except
select * from @temptableOLD
Best Answer
EXCEPT
implies aDISTINCT
operation.I would use
NOT EXISTS
if this is not actually required.However the problem you are having is likely that you are getting a nested loops on an unindexed table due to the poor cardinality estimates associated with table variables.
Will be able to take account that the tables have 100K rows each and give a different plan.
In SQL Server 2012 you can only add indexes to table variables via constraints. If the values are unique you could use
to add an index. If done on both tables the plan (after the recompile hint is added) will likely use a merge join instead. Without any indexes I would expect a hash join.