src/backend/parser/parse_expr.c:exprTypmod() question

I'm working on user-defined typmod and try to move all typmod calculations into
type-specific functions. But there is a strange place:

/*
* exprTypmod -
* returns the type-specific attrmod of the expression, if it can be
* determined. In most cases, it can't and we return -1.
*/
int32
exprTypmod(Node *expr)
{
<skip>
case T_Const:
{
/* Be smart about string constants... */
Const *con = (Const *) expr;

switch (con->consttype)
{
case BPCHAROID:
if (!con->constisnull)
{
int32 len =
VARSIZE(DatumGetPointer(con->constvalue)) - VARHDRSZ;

/* if multi-byte, take len and find # characters */
if (pg_database_encoding_max_length() > 1)
len =
pg_mbstrlen_with_len(VARDATA(DatumGetPointer(con->constvalue)), len);
return len + VARHDRSZ;
}
break;
default:
break;
}
}
break;

So, I can't understand why it's needed at all. First, it's returns length as
typmod, second, it looks like optimization, but I don't believe in significant
benefits... It's a constant coming from query. Am I missing something?

--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/