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A field
is either an attribute of a given class or one of the following:
oid
stands for the unique identifier of an instance which is added by
Postgres to all instances automatically. Oids are not reused and are 32
bit quantities.
xmin
The identity of the inserting transaction.
xmax
The identity of the deleting transaction.
cmin
The command identifier within the transaction.
cmax
The identity of the deleting command.
For further information on these fields consult
Stonebraker, Hanson, Hong, 1987.
Times are represented internally as instances of the
abstime
data type. Transaction and command identifiers are 32 bit quantities.
Transactions are assigned sequentially starting at 512.
instance
identifies a particular class and can be thought of as standing for
the instances of that class. An instance variable is either a class
name, a surrogate for a class defined by means of a FROM clause,
or the keyword NEW or CURRENT.
NEW and CURRENT can only appear in the action portion of a rule, while
other instance variables can be used in any SQL statement.
composite_field
is a field of of one of the Postgres composite types,
while successive composite fields address attributes in the
class(s) to which the composite field evaluates. Lastly,
field
is a normal (base type) field in the class(s) last addressed. If
field
is of type array,
then the optional number
designator indicates a specific element in the array. If no number is
indicated, then all array elements are returned.
