# Databases : PostgreSQL : Advanced : System Tables & Columns TODO ``` 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. tableoid The OID of the table containing this row. This column is particularly handy for queries that select from partitioned tables or inheritance hierarchies, since without it, it's difficult to tell which individual table a row came from. The tableoid can be joined against the oid column of pg_class to obtain the table name. xmin The 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.) cmin The command identifier (starting at zero) within the inserting transaction. xmax The 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. cmax The command identifier within the deleting transaction, or zero. ctid The physical location of the row version within its table. Note that although the ctid can be used to locate the row version very quickly, a row's ctid will change if it is updated or moved by VACUUM FULL. Therefore ctid is useless as a long-term row identifier. Transaction and Command IDs are 32b, and values will eventually be re-used. ```