From: | Sebastien FLAESCH <sf(at)4js(dot)com> |
---|---|
To: | pgsql-interfaces(at)postgresql(dot)org |
Subject: | Type identification with libpq / PQFmod() when using aggregates (SUM) |
Date: | 2014-04-14 10:06:39 |
Message-ID: | 534BB32F.4020503@4js.com |
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Thread: | |
Lists: | pgsql-interfaces |
Hello,
In a libpq client application, we need to properly identify the data type
when fetching data produced from a SELECT, therefore we use the PQftype()
and PQfmod() APIs ...
Consider a column defined with the following data type:
interval hour to second(0)
When executing a SELECT query using directly the columns values (no aggregate),
the libpq APIs (PQftype and PQfmod) return clear type information.
With an interval hour to second(0), we get:
PQftype() = 1186
PQfmode() = 469762048 (pgprec=7168, pgscal=65532, pgleng=469762044)
where precision, scale and length are computed as follows:
#define VARHDRSZ 4
int pgfmod = PQfmod(st->pgResult, i);
int pgprec = (pgfmod >> 16);
int pgscal = ((pgfmod - VARHDRSZ) & 0xffff);
int pgleng = (pgfmod - VARHDRSZ);
But when using an aggregate function like SUM(), PQfmode() function returns
"no information available" (-1) ...
Is the type of the result of an aggregate function (or even more complex
expressions) not known by the server?
Is this considered as bug or is it expected?
I found not much information in the PQfmod() description.
A workaround is to cast the result of the aggregate function:
SELECT CAST( SUM(mycol) AS INTERVAL HOUR TO SECOND(0) ) FROM ...
But I just wonder that the type of the result is not just the same as
the type of the source column...
Thanks!
Seb
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