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)
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.