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 42-7. pg_attribute Columns
|attrelid||oid||pg_class.oid||The table this column belongs to|
|attname||name||The column name|
|atttypid||oid||pg_type.oid||The data type of this 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 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 may 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. It is possible to change this column to enable or disable the 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.|
|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 may 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.|
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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