5.5. System Columns
Every table has several system columns that are implicitly defined by the system. Therefore, these names cannot be used as names of user-defined columns. (Note that these restrictions are separate from whether the name is a key word or not; quoting a name will not allow you to escape these restrictions.) You do not really need to be concerned about these columns; just know they exist.
tableoidThe OID of the table containing this row. This column is particularly handy for queries that select from inheritance hierarchies (see Section 5.10), since without it, it's difficult to tell which individual table a row came from. The
tableoidcan be joined against theoidcolumn ofpg_classto obtain the table name.xminThe identity (transaction ID) of the inserting transaction for this row version. (A row version is an individual state of a row; each update of a row creates a new row version for the same logical row.)
cminThe command identifier (starting at zero) within the inserting transaction.
xmaxThe identity (transaction ID) of the deleting transaction, or zero for an undeleted row version. It is possible for this column to be nonzero in a visible row version. That usually indicates that the deleting transaction hasn't committed yet, or that an attempted deletion was rolled back.
cmaxThe command identifier within the deleting transaction, or zero.
ctidThe physical location of the row version within its table. Note that although the
ctidcan be used to locate the row version very quickly, a row'sctidwill change if it is updated or moved byVACUUM FULL. Thereforectidis useless as a long-term row identifier. A primary key should be used to identify logical rows.
Transaction identifiers are also 32-bit quantities. In a long-lived database it is possible for transaction IDs to wrap around. This is not a fatal problem given appropriate maintenance procedures; see Chapter 23 for details. It is unwise, however, to depend on the uniqueness of transaction IDs over the long term (more than one billion transactions).
Command identifiers are also 32-bit quantities. This creates a hard limit of 232 (4 billion) SQL commands within a single transaction. In practice this limit is not a problem — note that the limit is on the number of SQL commands, not the number of rows processed. Also, only commands that actually modify the database contents will consume a command identifier.