Skinny Tables

• A skinny table is a custom table in the Force.com platform that contains a subset of fields from a standard or custom base Salesforce object.
• To enable skinny tables, contact salesforce.com Customer Support.
• The storage consumed by a skinny table does not count toward your org's storage.
• Skinny tables do not expire, and are automatically updated by the system as data is uploaded.
• Skinny tables are created by our DB Performance Engineering team upon analyzing each specific case, if it would improve the performance of reports, list views, or SOQL queries.

Benefits of Using Skinny Tables

• A skinny table is a custom table in the Force.com platform that contains a subset of fields from a standard or custom base Salesforce object. By having narrower rows and less data to scan than the base Salesforce object, skinny tables allow Force.com to return more rows per database fetch, increasing throughput when reading from a large object, as this diagram shows.
• Skinny tables do not include soft-deleted rows (i.e., records in the Recycle Bin with isDeleted = true), which could also reduce the table volume in some cases.
• The Force.com platform automatically synchronizes the rows between the base object and the skinny table, so the data is always kept current

Risks or Limitation Associated with Using Skinny Tables

• They don’t have the dynamic metadata flexibility you find in the base object. If you alter a field type (e.g., change a number field to a text field) the skinny table becomes invalid, and you must contact salesforce.com Customer Support to create a new skinny table.
• Skinny tables can contain a maximum of 100 columns.
• Although data for existing fields in the Skinny table are automatically updated, the skinny table will need to be recreated if a new field has been created.
• They cannot be created on Activities
• There's nothing visible to the customer when a skinny table exists. It's entirely transparent.

Note: While it is important to follow best practices for managing large volumes of data—and to execute archiving-and-purging strategies as much as possible—understanding skinny tables might help you further improve your Force.com performance. If you take proactive measures, clarify skinny tables’ possible effects on end users, and use skinny tables only when appropriate, you can keep your organization fit, smart, and fast.

Refer below Links:

https://help.salesforce.com/apex/HTViewSolution?urlname=Force-com-Query-Optimizer-FAQ

https://developer.salesforce.com/blogs/engineering/2013/03/long-and-short-term-approaches-for-tuning-force-com-performance.html