> For the complete documentation index, see [llms.txt](https://vikram-bajaj.gitbook.io/cs-gy-6083-principles-of-database-systems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://vikram-bajaj.gitbook.io/cs-gy-6083-principles-of-database-systems/chapter1/i-nstances-and-schemas.md).

# Instances and Schemas

Instances and schemas are analogous to variables and types (respectively) in programming languages.

## Schema

A schema is the logical structure of the database.

* A **physical schema** describes the database design at the physical level
* A **logical schema** describes the database design at the logical level

The ability to modify the physical schema without changing the logical schema is called **physical data independence**. This is possible because applications only depend on the logical schema, and there are well-defined interfaces between the levels to prevent the changes made in one layer from having a significant impact on the other layers.

## Instance

An instance is the actual content of the database at a given point in time.