Skinny tables and other LDV mitigation techniques
Skinny tables are an LDV mitigation technique that is useful when users experience slow read operations for queries against the Salesforce database. This is since the underlying (virtual) database schema for a standard object is actually a join across two tables. There are standard object standard fields and standard object custom fields. For example, a query for data from a standard object such as Account that includes both standard and custom fields of the Account object may benefit from skinny tables when performance is impacted.
As mentioned in Chapter 6, Understanding Large Data Volumes, skinny tables are for read operations only, in order to speed up queries for the fields that the skinny table contains. Whether a query should use a skinny table or not is determined behind the scenes by the Salesforce application layer. Skinny tables do not contain any soft-deleted records, and therefore can only be used to speed up the performance...
