The information schema consists of a set of views that contain information about the objects defined in the current database. The information schema is defined in the SQL standard and can therefore be expected to be portable and remain stable — unlike the system catalogs, which are specific to PostgreSQL™ and are modeled after implementation concerns. The information schema views do not, however, contain information about PostgreSQL™-specific features; to inquire about those you need to query the system catalogs or other PostgreSQL™-specific views.
When querying the database for constraint information, it is possible for a standard-compliant query that expects to return one row to return several. This is because the SQL standard requires constraint names to be unique within a schema, but PostgreSQL™ does not enforce this restriction. PostgreSQL™ automatically-generated constraint names avoid duplicates in the same schema, but users can specify such duplicate names.
This problem can appear when querying information schema views such
as check_constraint_routine_usage
,
check_constraints
, domain_constraints
, and
referential_constraints
. Some other views have similar
issues but contain the table name to help distinguish duplicate
rows, e.g., constraint_column_usage
,
constraint_table_usage
, table_constraints
.
The information schema itself is a schema named
information_schema
. This schema automatically
exists in all databases. The owner of this schema is the initial
database user in the cluster, and that user naturally has all the
privileges on this schema, including the ability to drop it (but
the space savings achieved by that are minuscule).
By default, the information schema is not in the schema search path, so you need to access all objects in it through qualified names. Since the names of some of the objects in the information schema are generic names that might occur in user applications, you should be careful if you want to put the information schema in the path.