The catalog pg_attrdef stores column default values. The main information about columns is stored in pg_attribute (see below). Only columns that explicitly specify a default value (when the table is created or the column is added) will have an entry here.
Table 47-6. pg_attrdef Columns
|oid||oid||Row identifier (hidden attribute; must be explicitly selected)|
|adrelid||oid||pg_class.oid||The table this column belongs to|
|adnum||int2||pg_attribute.attnum||The number of the column|
|adbin||pg_node_tree||The internal representation of the column default value|
|adsrc||text||A human-readable representation of the default value|
The adsrc field is historical,
and is best not used, because it does not track outside changes
that might affect the representation of the default value.
Reverse-compiling the adbin field
pg_get_expr for example) is a
better way to display the default value.
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