---
title: "pg_attribute"
id: catalog-pg-attribute
pg_version: "20devel"
---

<a id="catalog-pg-attribute"></a>

## 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](catalog-pg-class.md) entries.) 

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

**pg_attribute Columns**

| Column Type | Description | 
| --- | --- |
| `attrelid` `oid` (references [pg_class](catalog-pg-class.md).`oid`) | The table this column belongs to |
| `attname` `name` | The column name |
| `atttypid` `oid` (references [pg_type](catalog-pg-type.md).`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` (`enum`)](runtime-config-client.md#guc-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-pg-attrdef.md) 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](catalog-pg-collation.md).`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`](sql-analyze.md). 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](catalog-pg-type.md) 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.