sql-serverdata-warehousedatamart

Force trusted foreign key SQL Server to allow join culling in view


I'm trying to create a star schema view of fact to it's dimension.

eg.

if the view is

Select  
    _fact.A, _Dim.B
from 
    _fact
inner join 
    _dim on _Fact.dim_sk = _dim.Dim_sk

and I query

Select _Fact.A 
from _view

It will ignore the join in the _dim .

One way of doing this is to create trusted foreign keys to the dimension. However this means I have to create and enable check constraints which will slow down my inserts.

https://www.brentozar.com/archive/2015/05/do-foreign-keys-matter-for-insert-speed/

  1. Is there a better way to allow join culling in a view?

  2. Is there a way to force SQL server to mark foreign keys as trusted?

e.g. something along the lines of

update sys.foreign_keys
set is_not_trusted = 0

Solution

  • First, you really may want enforced FKs in your schema. You can disable them and check them at the end of your ETL if you want, and they are useful.

    At some scale, however, you want to leave the FKs as NOCHECK for performance.

    So the alternative is to use a LEFT JOIN, and then so long as the DIM table has a unique index on the join column, SQL knows that the join can't change the cardinality of the result, and will eliminate the join if don't reference the dimension columns. eg:

        use tempdb
    
        go
    
        drop table if exists _fact 
        drop table if exists _dim
        go
        create table _dim(Dim_sk int primary key, B int)
    
        create table _fact( A int, dim_sk int references _dim)
    
        go
    
        create or alter view _view
        as
        Select  
            _fact.A, _Dim.B
        from 
            _fact
        left join 
            _dim on _Fact.dim_sk = _dim.Dim_sk
    
        go
        set statistics io on
        go
        Select A 
        from _view
        go
        set statistics io off
    

    outputs

    Table '_fact'. Scan count 1, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.