Salesforce creates skinny tables to contain frequently used fields and to avoid joins, and it keeps the skinny tables in sync with their source tables when the source tables are modified. To enable skinny tables, contact Salesforce Customer Support.

For each object table, Salesforce maintains other, separate tables at the database level for standard and custom fields. This separation ordinarily necessitates a join when a query contains both kinds of fields. A skinny table contains both kinds of fields and does not include
soft-deleted records.

Skinny tables are most useful with tables containing millions of records. They can be created on custom objects, and on Account, Contact, Opportunity, Lead, and Case objects. In addition, they can enhance performance for reports, list views, and SOQL.

Skinny tables can contain the following types of fields.

• Checkbox
• Date
• Date and time
• Email
• Number
• Percent
• Phone picklist
• Picklist (multi-select)
• Text
• Text area
• Text area (long)
• URL


Considerations
• Skinny tables can contain a maximum of 100 columns.
• Skinny tables cannot contain fields from other objects.
• For Full sandboxes: Skinny tables are copied to your Full sandbox organizations, as of the Summer ’15 release. Previously, they were not.

For other types of sandboxes: Skinny tables are still not copied to your sandbox organizations. To have production skinny tables activated for sandbox types other than Full sandboxes, contact Salesforce Customer Support.