52.7. pg_attribute
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## 52.7. pg_attribute #

The catalog pg_attribute stores information about table columns. There will be exactly one pg_attribute row for every column in every table in the database. (There will also be attribute entries for indexes, and indeed all objects that have pg_class entries.)

The term attribute is equivalent to column and is used for historical reasons.

Table 52.7. pg_attribute Columns

Column Type Description
`attrelid` oid (references pg_class.`oid`) The table this column belongs to
`attname` name The column name
`atttypid` oid (references pg_type.`oid`) The data type of this column (zero for a dropped column)
`attlen` int2 A copy of `pg_type.typlen` of this column's type
`attnum` int2 The number of the column. Ordinary columns are numbered from 1 up. System columns, such as `ctid`, have (arbitrary) negative numbers.
`atttypmod` int4 `atttypmod` records type-specific data supplied at table creation time (for example, the maximum length of a varchar column). It is passed to type-specific input functions and length coercion functions. The value will generally be -1 for types that do not need `atttypmod`.
`attndims` int2 Number of dimensions, if the column is an array type; otherwise 0. (Presently, the number of dimensions of an array is not enforced, so any nonzero value effectively means “it's an array”.)
`attbyval` bool A copy of `pg_type.typbyval` of this column's type
`attalign` char A copy of `pg_type.typalign` of this column's type
`attstorage` char Normally a copy of `pg_type.typstorage` of this column's type. For TOAST-able data types, this can be altered after column creation to control storage policy.
`attcompression` char The current compression method of the column. Typically this is `'\0'` to specify use of the current default setting (see default_toast_compression). Otherwise, `'p'` selects pglz compression, while `'l'` selects LZ4™ compression. However, this field is ignored whenever `attstorage` does not allow compression.
`attnotnull` bool This column has a (possibly invalid) not-null constraint.
`atthasdef` bool This column has a default expression or generation expression, in which case there will be a corresponding entry in the pg_attrdef catalog that actually defines the expression. (Check `attgenerated` to determine whether this is a default or a generation expression.)
`atthasmissing` bool This column has a value which is used where the column is entirely missing from the row, as happens when a column is added with a non-volatile `DEFAULT` value after the row is created. The actual value used is stored in the `attmissingval` column.
`attidentity` char If a zero byte (`''`), then not an identity column. Otherwise, `a` = generated always, `d` = generated by default.
`attgenerated` char If a zero byte (`''`), then not a generated column. Otherwise, `s` = stored, `v` = virtual. A stored generated column is physically stored like a normal column. A virtual generated column is physically stored as a null value, with the actual value being computed at run time.
`attisdropped` bool This column has been dropped and is no longer valid. A dropped column is still physically present in the table, but is ignored by the parser and so cannot be accessed via SQL.
`attislocal` bool This column is defined locally in the relation. Note that a column can be locally defined and inherited simultaneously.
`attinhcount` int2 The number of direct ancestors this column has. A column with a nonzero number of ancestors cannot be dropped nor renamed.
`attcollation` oid (references pg_collation.`oid`) The defined collation of the column, or zero if the column is not of a collatable data type
`attstattarget` int2 `attstattarget` controls the level of detail of statistics accumulated for this column by ANALYZE. A zero value indicates that no statistics should be collected. A null value says to use the system default statistics target. The exact meaning of positive values is data type-dependent. For scalar data types, `attstattarget` is both the target number of “most common values” to collect, and the target number of histogram bins to create.
`attacl` aclitem[] Column-level access privileges, if any have been granted specifically on this column
`attoptions` text[] Attribute-level options, as “keyword=value” strings
`attfdwoptions` text[] Attribute-level foreign data wrapper options, as “keyword=value” strings
`attmissingval` anyarray This column has a one element array containing the value used when the column is entirely missing from the row, as happens when the column is added with a non-volatile `DEFAULT` value after the row is created. The value is only used when `atthasmissing` is true. If there is no value the column is null.

In a dropped column's pg_attribute entry, `atttypid` is reset to zero, but `attlen` and the other fields copied from pg_type are still valid. This arrangement is needed to cope with the situation where the dropped column's data type was later dropped, and so there is no pg_type row anymore. `attlen` and the other fields can be used to interpret the contents of a row of the table.

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