The catalog pg_collation describes the available collations, which are essentially mappings from an SQL name to operating system locale categories. See Section 22.2 for more information.
Table 45-13. pg_collation Columns
|collname||name||Collation name (unique per namespace and encoding)|
|collnamespace||oid||pg_namespace.oid||The OID of the namespace that contains this collation|
|collowner||oid||pg_authid.oid||Owner of the collation|
|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|
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.