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Databases - Practical PostgreSQL
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System Columns

PostgreSQL defines a series of system columns in all tables, which are normally invisible to the user (e.g., they will not be shown by queries unless explicitly requested). These columns contain meta-data about the content of the table's rows. Many of these contain data that can help to differentiate between tuples (an individual state of a row) when working with transaction blocks. (See Chapter 7 for more about transactions.)

As a result of these system-defined columns, in addition to the user-defined columns of a table, any inserted row will have values in each of the columns described in Table 3-25.

Table 3-25. System columns

Column

Description

oid (object identifier)

The unique object identifier of a row. PostgreSQL automatically adds this 4-byte number to all rows. It is never re-used within the same table.

tableoid (table object identifier)

The oid of the table that contains a row. The name and oid of a table are related by the pg_class system table.

xmin (transaction minimum)

The transaction identifier of the inserting transaction of a tuple.

cmin (command minimum)

The command identifier, starting at 0, associated with the inserting transaction of a tuple.

xmax (transaction maximum)

The transaction identifier of a tuple's deleting transaction. If a tuple is visible (has not been deleted) this is set to zero.

cmax (command maximum)

The command identifier associated with the deleting transaction of a tuple. Like xmax, if a tuple is visible, this is set to zero.

ctid (tuple identifier)

The identifier which describes the physical location of the tuple within the database. A pair of numbers are represented by the ctid: the block number, and tuple index within that block.

Databases - Practical PostgreSQL
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