Thread: Re: Is there a way to describe precision and scale f

Thanks for the comments. They were very helpful.  

However, I am still not sure about the second problem 
i.e. of describing the prepared  statement. Does some 
other api exposes it? Can I use some sql command to get 
around it (I couldn't find any)? any other ideas?

Brijesh Shrivastav

-----Original Message-----
From: L J Bayuk [mailto:ljb220@mindspring.com]
Sent: Sunday, June 06, 2004 5:44 PM
To: Bshrivastav@esri.com
Cc: pgsql-interfaces@postgresql.org
Subject: Re: [INTERFACES] Is there a way to describe precision and scale
for columns in a s


Brijesh Shrivastav wrote:
> 
> Hi! All,
> 
> I am a new postgres developer and as is the case with many of us I am
> porting
> an existing application to postgres database. One of the requirement for
us
> is to be able to determine the scale and precision for select columns so
we
> can
> accordingly allordingly allocate memory or inform client applications. 
> 
> I looked at libpq api and found following api to describe a select
statement
> 
> PQfname()
> PQftype()
> PQfsize()
> 
> However, PQfsize() gives me the space alloced for this column in database
> row. How
> can I get precision and scale for numeric columns?

It's a little tricky. Use PQfmod() on the column.  Call the result F.
The precision is the high 16 bits, and the scale is the low 16 bits,
but with an offset of 4 first:    precision = ((F - 4)  >> 16) & 0xffff;    scale = (F - 4) & 0xffff;

> I have run into another problem. I execute PREPARE statement with pgexec
to
> prepare 
> the query before executing it multiple time with different parameter
values.
> It 
> seems I can't get the select column information untill I perform an
> PQexecPrepared().
> Is there any way around it?

I don't think so. The protocol supports it ("Describe" message on a named
prepared statement, without Bind or Execute), and I assume the backend
would play along, but I don't think libpq has any way to let you do it
in the current release.

Brijesh Shrivastav wrote:
> 
> Thanks for the comments. They were very helpful.  
> 
> However, I am still not sure about the second problem 
> i.e. of describing the prepared  statement. Does some 
> other api exposes it? Can I use some sql command to get 
> around it (I couldn't find any)? any other ideas?

Since libpq doesn't expose it, no API based on libpq will let you at it.
There are a few non-libpq interfaces, but I doubt any have this advanced
feature.  My pgin.tcl doesn't.

If your prepared query has a parameter which you can use to efficiently
select 0 (or a small number of rows), you might be able to use that as a
'dummy' query just to get column information. This is similar to the trick
of using "SELECT ... LIMIT 0" to get an empty result set just so you can
look at the column info.