Re: 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/

Teodor Sigaev <teodor(at)sigaev(dot)ru> writes:

> 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.
> */
...
> 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?

I think that comes into play in cases like the following:

postgres=# create table qux as (select 'foo'::bpchar, 'foo'::varchar, 0::numeric);
SELECT
postgres=# \d qux
Table "public.qux"
Column | Type | Modifiers
---------+-------------------+-----------
bpchar | character(3) |
varchar | character varying |
numeric | numeric |

Note that unlike most of the built-in types bpchar doesn't actually make much
sense without a typmod. NUMERIC, VARCHAR, etc can all exist without a typmod
and behave sensibly but bpchar without a typmod would just be a varchar. The
default for CHARACTER without a typmod is CHAR(1) which is what happens if you
do ::CHAR but I guess we don't want to do that for ::bpchar.

On the other hand I can manually create a table with a column of type bpchar
and it does behave like a varchar with strange comparison semantics so I guess
you could argue bpchar without a typmod isn't completely meaningless.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> Teodor Sigaev <teodor(at)sigaev(dot)ru> writes:
>> I'm working on user-defined typmod and try to move all typmod calculations into
>> type-specific functions. But there is a strange place:

> Note that unlike most of the built-in types bpchar doesn't actually make much
> sense without a typmod.

You may be reading too much into it. Looking at the patch that
introduced exprTypmod(), I think I may have just been interested
in avoiding an unnecessary length-coercion function call when assigning
a constant that was already of the correct length to a CHAR(N) column.
I concur with Teodor that embedding this type-specific knowledge into
exprTypmod probably isn't all that great an idea.

regards, tom lane