> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/the-sql-language/ddl/ddl-system-columns.md).

# 5.6. 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.

`tableoid` [#](#DDL-SYSTEM-COLUMNS-TABLEOID) :

```
The OID of the table containing this row. This column is
particularly handy for queries that select from partitioned
tables (see [Section 5.12](ddl-partitioning.md)) or inheritance
hierarchies (see [Section 5.11](ddl-inherit.md)), 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` [#](#DDL-SYSTEM-COLUMNS-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` [#](#DDL-SYSTEM-COLUMNS-CMIN) :

```
The command identifier (starting at zero) within the inserting
transaction.
```

`xmax` [#](#DDL-SYSTEM-COLUMNS-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` [#](#DDL-SYSTEM-COLUMNS-CMAX) :

```
The command identifier within the deleting transaction, or zero.
```

`ctid` [#](#DDL-SYSTEM-COLUMNS-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` should not be used as a 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 24](/server-administration/maintenance.md) 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.

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/ddl-system-columns.html)（英文原文，待翻譯）


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