Question

in a query, is there any performance hit by doing a where clause like so

select bunchofstuff from sometable where column like '%'

vs

select bunchofstuff from sometable

Building some dynamic queries, just curious. Thanks.

Answer 1

On SQL 2005, SP3 running the following

set statistics io on
select * from sys.columns where name like '%'
set statistics io off

set statistics io on
select * from sys.columns
set statistics io off

suggests that the cost will be almost identical, since both return the same result set:

(1134 row(s) affected)
Table 'syscolpars'. Scan count 1, logical reads 50, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Switching on the query plans and/or statistics IO shows that select * from sys.columns where name like '%' is marginally more expensive (51% of query cost vs. 49% for select * from sys.columns), since it appears that the filter is parsed and applied even though it has no effect.