Another behavior that follows schemas, query plan reuse

When I discuss separation of users and schemas in SQL Server 2005, I usually think of ownership chains (that follow the object owner which is always the schema owner unless you change it) or object resolution (which now goes: special name, then default_schema/procedure schema, then DBO). Another item that now follows schemas rather than users is query plan reuse with 1-part object names. Create two users:

— logins already created
CREATE USER mary for LOGIN mary
GRANT SELECT ON authors TO bob, mary

The query plan for the following query (executed by bob or mary)

USE pubs
SELECT * FROM authors — note the 1-part object name

would not normally be reused for both bob and mary. In SQL Server 2005, it can be reused if bob and mary have the same default schema. BTW, this is adhoc SQL, rules differ in procedural objects like stored procs. To see the reason for the query reuse, you can obtain the plan handle (reference this blog entry), but leave out querying the plan itself because you don't need it here) and pass it in to the sys.dm_exec_plan_attributes dynamic management function. This function shows the attributes of the plan and includes a second column, is_cache_key. If the cache_keys match, the plan is reused.

One of the rows produced by sys.dm_exec_plan_attributes is named user_id. Interestingly, for the query plan for bob or mary against SELECT * FROM authors, the user_id is 1. DBO. Hmmm… User_id is a misleading name for this column. It's not the user_id of the user who executed the query (and caused the plan to be created) but that user's DEFAULT_SCHEMA's owner's user_id. Looking at the CREATE USER DDL statements above, because I didn't specify a DEFAULT_SCHEMA, both users' DEFAULT_SCHEMA is DBO. So, for adhoc SQL, all other things being equal, the plan will be resued for two users as long as they have the same DEFAULT_SCHEMA. The rules for query plan reuse follow the rules for object resolution, which makes sense, once you think about it.

Of course, now that you know this, ALWAYS use 2-part names. User_id with 2-part names' plans has a special value of -2 which ensures the plan is resued regardless of DEFAULT_SCHEMA of the user who executes it.

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