Function build_column_default

src/backend/rewrite/rewriteHandler.c:1250–1331

* Make an expression tree for the default value for a column. * * If there is no default, return a NULL instead. */

 * If there is no default, return a NULL instead.
 */
Node *
build_column_default(Relation rel, int attrno)
{
	TupleDesc	rd_att = rel->rd_att;
	Form_pg_attribute att_tup = TupleDescAttr(rd_att, attrno - 1);
	Oid			atttype = att_tup->atttypid;
	int32		atttypmod = att_tup->atttypmod;
	Node	   *expr = NULL;
	Oid			exprtype;

	if (att_tup->attidentity)
	{
		NextValueExpr *nve = makeNode(NextValueExpr);

		nve->seqid = getIdentitySequence(RelationGetRelid(rel), attrno, false);
		nve->typeId = att_tup->atttypid;

		return (Node *) nve;
	}

	/*
	 * If relation has a default for this column, fetch that expression.
	 */
	if (att_tup->atthasdef)
	{
		if (rd_att->constr && rd_att->constr->num_defval > 0)
		{
			AttrDefault *defval = rd_att->constr->defval;
			int			ndef = rd_att->constr->num_defval;

			while (--ndef >= 0)
			{
				if (attrno == defval[ndef].adnum)
				{
					/* Found it, convert string representation to node tree. */
					expr = stringToNode(defval[ndef].adbin);
					break;
				}
			}
		}
		if (expr == NULL)
			elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
				 attrno, RelationGetRelationName(rel));
	}

	/*
	 * No per-column default, so look for a default for the type itself.  But
	 * not for generated columns.
	 */
	if (expr == NULL && !att_tup->attgenerated)
		expr = get_typdefault(atttype);

	if (expr == NULL)
		return NULL;			/* No default anywhere */

	/*
	 * Make sure the value is coerced to the target column type; this will
	 * generally be true already, but there seem to be some corner cases