How bad is IN operator for SQL query perfomance?

Question

I had SQL query that was taking 9 hours for execution. See below:

Select Field1, Field2
From A
Where Field3 IN (45 unique values here) 

Whe

Answer 1

• What database engine are you using (Oracle, SQL Server, MySQL, etc.)?
• Can you post the query plan for both queries?

It sounds like the optimizer your database is using chose a very poor execution plan for the first query. Assuming that your database engine uses some sort of cost-based optimizer (most of the large vendors do), that generally implies that the optimizer's guess as to how many rows are returned was quite incorrect. Most likely, its estimates are equally incorrect whether there are 15 or 45 unique values specified but it happens that tripling the bad estimate leads the optimizer to believe that a different query plan would be appropriate. Generally, this means that the information you've given the optimizer in terms of statistics about table A are incorrect and need to be fixed.

That being said, how large is table A? If you are really querying from a single table, the optimizer has relatively few options to consider. It really only has to decide between doing a full table scan on A and using an index on Field3. Even if it decides to do a table scan rather than use an index on Field3, there is no way that it should take 9 hours to scan a single table barring really exceptional circumstances where table A is measured in at least hundreds of GB and the hardware is relatively pedestrian.