55.7. `pg_attribute`
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## 55.7. `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 55.7. `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)
`attstattarget` `int4` `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 negative 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.
`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.
`attndims` `int4` 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”.)
`attcacheoff` `int4` Always -1 in storage, but when loaded into a row descriptor in memory this might be updated to cache the offset of the attribute within the row
`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`.
`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 represents a 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. (Other values might be added in the future.)
`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` `int4` 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
`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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