pg_enum catalog contains entries showing the values and labels for each enum type. The internal representation of a given enum value is actually the OID of its associated row in
The OID of the
The sort position of this enum value within its enum type
The textual label for this enum value
The OIDs for
pg_enum rows follow a special rule: even-numbered OIDs are guaranteed to be ordered in the same way as the sort ordering of their enum type. That is, if two even OIDs belong to the same enum type, the smaller OID must have the smaller
enumsortorder value. Odd-numbered OID values need bear no relationship to the sort order. This rule allows the enum comparison routines to avoid catalog lookups in many common cases. The routines that create and alter enum types attempt to assign even OIDs to enum values whenever possible.
When an enum type is created, its members are assigned sort-order positions 1..
n. But members added later might be given negative or fractional values of
enumsortorder. The only requirement on these values is that they be correctly ordered and unique within each enum type.