Best practices when querying SharePoint lists/libraries

1) Golden rule - Never iterate through the whole list to filter/check-on couple of items in the list. List operations are heavy. Filter the required items needed maybe through a CAML query.

2) Use the SPQuery class if your objective is to get items from a single list

3) In your CAML query set ViewFields to filter the fields which you need. In the same way you can use SPList.GetItemByIdSelectedFields() method to get selected fields. Setting ViewFields makes the query more efficient

4) Use the SPSiteDataQuery class to get results from multiple lists. Here you can specify the web and lists which you are targeting at.

Example:

SPSiteDataQuery myQuery = new SPSiteDataQuery();
myQuery.query = "Your CAML query goes here";
myQuery.Webs = @"<Webs Scope=""SiteCollection"" />"; //Returns data from all webs in the current site collection
//myQuery.Webs = @"<Webs Scope=""Recursive"" />"; // will return from current web and its child webs

myQuery.Lists = @"<Lists ServerTemplate=""101"" />"; //101 refers to document libraries
myQuery.ViewFields =  @"
                                            <FieldRef Name=""Field One"" />
                                            <FieldRef Name=""Field Two"" />
                                           ";