---
title: Schema
---

## What is a Schema?

Schema defines the shape of the data in your database.

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[Tables](/concepts/sql/table) are the core unit of schema. Tables have columns and rows. Each column in a
table has a [type](/concepts/sql/types). A table can have one or more [primary keys](/concepts/sql/primary-key), the
combination of which identify the row and must be unique. Columns can also be assigned additional
[constraints](/concepts/sql/constraints), including foreign key constraints which are references to other
tables in the database.

Schema also includes [views](/concepts/sql/views). Views look like tables but the data in them is generated
using SQL stored in the view definition. The data is stored in the tables the views reference not
the view itself.

[Secondary Indexes](/concepts/sql/indexes) are a part of schema. An index allows read query performance to be
improved at the expense of write performance and increased storage.

Finally, schema includes [triggers](/concepts/sql/triggers) and [procedures](/concepts/sql/procedures). Triggers and
procedures are code stored in your database that executes on specific conditions or when a user
asks, respectively.

## How to use Schema

Schema is the core of database design. You use schema to explain to database users the shape of the
data in the database. What values are allowed in this column? What data is allowed to be duplicated
in multiple rows? Can a value exist in this table without existing in this other table as well?

Schema design also effects the performance of queries against the database. Defining primary keys
and indexes correctly can make your database perform for large databases or complex queries.

Changing schema can be a costly operation. For instance, adding an index to a column on a running
database requires scanning the entire table and writing a new index artifact, usually while also
restricting writes to that table.

## Difference between Postgres Schema and Doltgres Schema

Doltgres supports [most Postgres schema elements at least
partially](/reference/sql-support/supported-types), with more support being built out
now.

## Interaction with Doltgres Version Control

Doltgres versions your schema and data. So, if you want to see the difference between the schema of
two different versions, Doltgres provides this using `diff` functionality. See individual
SQLconcepts for how Doltgres handles each individual schema element with regards to versioning.