pg_collation
The catalog pg_collation describes the
available collations, which are essentially mappings from an SQL
name to operating system locale categories.
See Section 23.2 for more information.
Table 52.12. pg_collation Columns
| Name | Type | References | Description |
|---|---|---|---|
oid | oid | Row identifier (hidden attribute; must be explicitly selected) | |
collname | name | Collation name (unique per namespace and encoding) | |
collnamespace | oid | | The OID of the namespace that contains this collation |
collowner | oid | | Owner of the collation |
collprovider | char | Provider of the collation: d = database
default, c = libc, i = icu | |
collencoding | int4 | Encoding in which the collation is applicable, or -1 if it works for any encoding | |
collcollate | name | LC_COLLATE for this collation object | |
collctype | name | LC_CTYPE for this collation object | |
collversion | text | Provider-specific version of the collation. This is recorded when the collation is created and then checked when it is used, to detect changes in the collation definition that could lead to data corruption. |
Note that the unique key on this catalog is (collname,
collencoding, collnamespace) not just
(collname, collnamespace).
PostgreSQL generally ignores all
collations that do not have collencoding equal to
either the current database's encoding or -1, and creation of new entries
with the same name as an entry with collencoding = -1
is forbidden. Therefore it is sufficient to use a qualified SQL name
(schema.name) to identify a collation,
even though this is not unique according to the catalog definition.
The reason for defining the catalog this way is that
initdb fills it in at cluster initialization time with
entries for all locales available on the system, so it must be able to
hold entries for all encodings that might ever be used in the cluster.
In the template0 database, it could be useful to create
collations whose encoding does not match the database encoding,
since they could match the encodings of databases later cloned from
template0. This would currently have to be done manually.