Internally, Salesforce stores Standard Object's field data and custom field data for a given record in two separate physical Oracle tables. So, while you don't see it, when you execute a SOQL query to return a mixture of both standard and custom fields, this internally requires an Oracle SQL query, which requires an internal database join to be made.
If Salesforce Support determines that avoiding this join would speed up a given set of queries, they can create a skinny table. Such a table is not visible to the Force.com developer and is kept in sync with your object records automatically by the platform, including any standard or custom indexes.
A skinny table can contain commonly used standard and custom fields that you, the subscriber, and Salesforce deem appropriate, all in the one Oracle table, thus, it avoids the join (all at the internal cost of duplicating the record data). If a SOQL, Report, or List View query utilizes the fields or a subset, the platform will automatically...
