Function ExecInitCoerceToDomain

src/backend/executor/execExpr.c:3219–3357

* Prepare evaluation of a CoerceToDomain expression. */

 * Prepare evaluation of a CoerceToDomain expression.
 */
static void
ExecInitCoerceToDomain(ExprEvalStep *scratch, CoerceToDomain *ctest,
					   ExprState *state, Datum *resv, bool *resnull)
{
	DomainConstraintRef *constraint_ref;
	Datum	   *domainval = NULL;
	bool	   *domainnull = NULL;
	ListCell   *l;

	scratch->d.domaincheck.resulttype = ctest->resulttype;
	/* we'll allocate workspace only if needed */
	scratch->d.domaincheck.checkvalue = NULL;
	scratch->d.domaincheck.checknull = NULL;

	/*
	 * Evaluate argument - it's fine to directly store it into resv/resnull,
	 * if there's constraint failures there'll be errors, otherwise it's what
	 * needs to be returned.
	 */
	ExecInitExprRec(ctest->arg, state, resv, resnull);

	/*
	 * Note: if the argument is of varlena type, it could be a R/W expanded
	 * object.  We want to return the R/W pointer as the final result, but we
	 * have to pass a R/O pointer as the value to be tested by any functions
	 * in check expressions.  We don't bother to emit a MAKE_READONLY step
	 * unless there's actually at least one check expression, though.  Until
	 * we've tested that, domainval/domainnull are NULL.
	 */

	/*
	 * Collect the constraints associated with the domain.
	 *
	 * Note: before PG v10 we'd recheck the set of constraints during each
	 * evaluation of the expression.  Now we bake them into the ExprState
	 * during executor initialization.  That means we don't need typcache.c to
	 * provide compiled exprs.
	 */
	constraint_ref = (DomainConstraintRef *)
		palloc(sizeof(DomainConstraintRef));
	InitDomainConstraintRef(ctest->resulttype,
							constraint_ref,
							CurrentMemoryContext,
							false);

	/*
	 * Compile code to check each domain constraint.  NOTNULL constraints can
	 * just be applied on the resv/resnull value, but for CHECK constraints we
	 * need more pushups.
	 */
	foreach(l, constraint_ref->constraints)
	{
		DomainConstraintState *con = (DomainConstraintState *) lfirst(l);
		Datum	   *save_innermost_domainval;
		bool	   *save_innermost_domainnull;

		scratch->d.domaincheck.constraintname = con->name;