![]() Cut and paste it into two tabs, switch into text mode in both tabs, and run them simultaneously. The following loop implements this logic. Let us insert or update rows in a loop using the following simple logic: if a row with given ID exists, update it, and otherwise insert a new one. IF EXISTS(…) THEN pattern frequently fails under high concurrency. Here is the table we shall be using: CREATE TABLE dbo.TwoINTs MERGE is the king - it holds up much better.Let us do some stress testing and see for ourselves. Patterns work as expected under high concurrency. ![]() You can even OUTPUT the rows affected by a MERGE into a table variable if you need to find out afterward what was done. If you're using SQL Server 2008 and need to perform any sequence of INSERT, UPDATE and DELETE depending on whether or not the row already exists - even if it's just one row - there is no excuse not to be using MERGE. WHEN NOT MATCHED THEN INSERT () VALUES () ![]() SQL Server 2008, however, finally introduced MERGE syntax, so now all you have to do is this: MERGE target WHERE t.id NOT IN (SELECT id FROM source)Īs I said, performance was pretty lousy on this, but still a lot better than the one-row-at-a-time approach. WHERE s.id NOT IN (SELECT id FROM target) Worst-case, this will still perform two operations for every transaction, but at least there's a chance of only performing one, and it also eliminates the race condition (kind of).īut the real issue is that this is still being done for each row in the source.īefore SQL Server 2008, you had to use an awkward 3-stage model to deal with this at the set level (still better than row-by-row): BEGIN TRAN One very minor (and I emphasize minor) optimization is to just attempt the UPDATE anyway if the row doesn't exist, will be 0 and you can then "safely" insert: - For each row in source This will have the largest (worst) impact of all on overall performance. Worst of all - it's following an iterative model, thinking about these problems at the level of a single row. For every transaction you have an extra operation being performed maybe it's trivial, but that depends entirely on how well you've indexed. ![]() The row can disappear between IF EXISTS and the subsequent DELETE or UPDATE. This is just about the worst thing you can do, for several reasons:
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