postgresql中的默认列

语法

OptTableElementList->TableElementList->TypedTableElement->columnDef->ColQualList->ColConstraint->ColConstraintElem

default的语法规则在ColConstraintElem下，解析时会解析为Constraint,contype为CONSTR_DEFAULT 存在ColumnDef->constraints下

DDL创建表

transformCreateStmt

transformColumnDefinition

default语法表达式赋值给raw_default

DefineRelation

AddRelationNewConstraints

make_parsestate

addRangeTableEntryForRelation

addNSItemToQuery

cookDefault

StoreAttrDefault

语义分析对应constraint 并且把表达式结果序列化插入pg_attrdef表中

List *
AddRelationNewConstraints(Relation rel,
						  List *newColDefaults,
						  List *newConstraints,
						  bool allow_merge,
						  bool is_local,
						  bool is_internal,
						  const char *queryString)
{
	List	   *cookedConstraints = NIL;
	TupleDesc	tupleDesc;
	TupleConstr *oldconstr;
	int			numoldchecks;
	ParseState *pstate;
	ParseNamespaceItem *nsitem;
	int			numchecks;
	List	   *checknames;
	List	   *nnnames;
	Node	   *expr;
	CookedConstraint *cooked;

	/*
	 * Get info about existing constraints.
	 */
	tupleDesc = RelationGetDescr(rel);
	oldconstr = tupleDesc->constr;
	if (oldconstr)
		numoldchecks = oldconstr->num_check;
	else
		numoldchecks = 0;

	/*
	 * Create a dummy ParseState and insert the target relation as its sole
	 * rangetable entry.  We need a ParseState for transformExpr.
	 */
	pstate = make_parsestate(NULL);
	pstate->p_sourcetext = queryString;
	nsitem = addRangeTableEntryForRelation(pstate,
										   rel,
										   AccessShareLock,
										   NULL,
										   false,
										   true);
	addNSItemToQuery(pstate, nsitem, true, true, true);

	/*
	 * Process column default expressions.
	 */
	foreach_ptr(RawColumnDefault, colDef, newColDefaults)
	{
		Form_pg_attribute atp = TupleDescAttr(rel->rd_att, colDef->attnum - 1);
		Oid			defOid;

		expr = cookDefault(pstate, colDef->raw_default,
						   atp->atttypid, atp->atttypmod,
						   NameStr(atp->attname),
						   atp->attgenerated);

		/*
		 * If the expression is just a NULL constant, we do not bother to make
		 * an explicit pg_attrdef entry, since the default behavior is
		 * equivalent.  This applies to column defaults, but not for
		 * generation expressions.
		 *
		 * Note a nonobvious property of this test: if the column is of a
		 * domain type, what we'll get is not a bare null Const but a
		 * CoerceToDomain expr, so we will not discard the default.  This is
		 * critical because the column default needs to be retained to
		 * override any default that the domain might have.
		 */
		if (expr == NULL ||
			(!colDef->generated &&
			 IsA(expr, Const) &&
			 castNode(Const, expr)->constisnull))
			continue;

		defOid = StoreAttrDefault(rel, colDef->attnum, expr, is_internal);

		cooked = (CookedConstraint *) palloc(sizeof(CookedConstraint));
		cooked->contype = CONSTR_DEFAULT;
		cooked->conoid = defOid;
		cooked->name = NULL;
		cooked->attnum = colDef->attnum;
		cooked->expr = expr;
		cooked->is_enforced = true;
		cooked->skip_validation = false;
		cooked->is_local = is_local;
		cooked->inhcount = is_local ? 0 : 1;
		cooked->is_no_inherit = false;
		cookedConstraints = lappend(cookedConstraints, cooked);
	}
...

	/*
	 * Update the count of constraints in the relation's pg_class tuple. We do
	 * this even if there was no change, in order to ensure that an SI update
	 * message is sent out for the pg_class tuple, which will force other
	 * backends to rebuild their relcache entries for the rel. (This is
	 * critical if we added defaults but not constraints.)
	 */
	SetRelationNumChecks(rel, numchecks);

	return cookedConstraints;
}

插入数据

在重写阶段将表达式赋给对应value

rewriteTargetListIU