# 51.7. 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 51.7. `pg_attribute` Columns** | Name | Type | References | Description | | --------------- | ----------- | ----------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `attrelid` | `oid` | [`pg_class`](https://www.postgresql.org/docs/10/static/catalog-pg-class.html).oid | The table this column belongs to | | `attname` | `name` | | The column name | | `atttypid` | `oid` | [`pg_type`](https://www.postgresql.org/docs/10/static/catalog-pg-type.html).oid | The data type of this column | | `attstattarget` | `int4` | | `attstattarget` controls the level of detail of statistics accumulated for this column by [ANALYZE](https://www.postgresql.org/docs/10/static/sql-analyze.html). 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 `oid`, 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 | | `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. | | `attalign` | `char` | | A copy of `pg_type.typalign` of this column's type | | `attnotnull` | `bool` | | This represents a not-null constraint. | | `atthasdef` | `bool` | | This column has a default value, in which case there will be a corresponding entry in the `pg_attrdef` catalog that actually defines the value. | | `attidentity` | `char` | | If a zero byte (`''`), then not an identity column. Otherwise, `a` = generated always, `d` = generated by default. | | `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` | [`pg_collation`](https://www.postgresql.org/docs/10/static/catalog-pg-collation.html).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 | 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.postgresql.tw/internals/system-catalogs/pg_attribute.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.