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

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