Sql Server Schema and scope
This morning I spent 20 minutes completely puzzled on a stored procedure in Sql Server. This stored procedure is not too complex, it moves data incrementally on a denormalized table to speed up some searches. The concept is simple, I run a series of queries to obtain a list of the ids of modified entity since the last run of the stored, then I update those lines and insert the new ones.
I ran all the queries inside SSMS, verified that everything is good and then copy/paste T-SQL code inside a stored procedure. Now I reset the last run date, execute the stored and… surprise … no row was loaded in the table. I was puzzled, so I open stored procedure’s code, selected the whole T-SQL content (except the ALTER PROCEDURE), pressed F5 and magically the table gets populated, then I execute the stored from another query windows and still no row was inserted… how it is possible that a query behave differently when you execute the T-SQL code and when the very same T-SQL code is executed inside a stored procedure?
The problem is subtle and originated from the design of the database: I have a view called CampaignView, that is really slow (it has many join and lot of subqueries) so I decided to create a table called datawarehouse.CampaignView to store the materialized content of the view. The code that populates the table at a certain point issue a query like this one
Basically I simply insert into the datawarehouse table selecting rows from the original slow view with a WHERE condition that identify all the new rows to be inserted.
When you execute the code it works perfectly, because the SELECT clause selects from the * dbo.CampaignCategoryView *view. When the very same code gets executed inside a stored called * [datawarehouse].[CampaignCategoryViewIncrementalPopulation]*the default schema is datawarehouse because it is the schema where the stored lives in. This completely changes how the previous SQL snippet gets executed, because the SELECT FROM part is now selecting from * [datawarehouse].[CampaignCategoryView]. *This means that nothing get inserted in the table because it is inserting data from itself. The solution was to specify the schema for the source table
This problem taught me two different lessons: the first one is that in SQL isalways a good idea to specify SCHEMA of objects and the second one is,not to create two object in a database with the very same name but in different SCHEMA, because if you forget the first lesson, the code executes differently depending on context in witch it runs.
Tags: Sql Server