Re: PGparam proposal - Mailing list pgsql-hackers

Andrew Chernow <ac@esilo.com> writes:
> This proposal extends libpq by adding a printf style functions for
> sending and recveiving through the paramterized interface.

I think a printf-style API is fundamentally a bad idea in this context.
printf only works well when the set of concepts (datatypes, format
specifiers, etc) is small and fixed; neither of which adjectives
describe PG's set of datatypes.  You've already had to go to
two-character format codes in order to have even slightly mnemonic codes
for the most commonly used built-in types; that doesn't look like it's
going to scale up for long.  And what are you going to do about add-on
data types, such as contrib stuff, PostGIS and other add-on projects,
or user-defined types?

> PQputf offers a way of packing pgtypes for use with the parameterized
> functions.  One or more values can be put at the same time.  The params
> are stored within the PGconn struct as a PGparam structure (internal
> API only). The paramspec describes the pgtypes that you want to put.
> In the paramspec, anything other than a valid conversion specifiers is
> ignored.  "%n4, -@#= %n8" is treated the same way as "%n4%n8".
> Once all params have been put, one of four paramterized functions that
> are aware of PGparam can be used:

I find the idea of embedding state like that into the PGconn to be
pretty horrid, as well.  It makes the design non-reentrant --- consider
the example of wanting to execute a query during the process of
computing parameters for a later query.  If there's merit in the idea
at all, expose PGparam as a separate (but opaque) data structure that is
explicitly passed into and out of the functions that are concerned with
it.

> * PQexecParams
> * PQexecPrepared
> * PQsendQueryParams
> * PQsendQueryPrepared

You can't just randomly change the behavior of existing API functions.

> Date and time types:
>   dt  time, timetz
>   dd  date
>   dT  timestamp, timestamptz
>   di  interval

I'm not sure whether timestamp/timestamptz can or should be treated
as interchangeable; but time and timetz definitely are not.

BTW, how will this code react to the inevitable future changes in
binary formats?  As examples, show what you'd do with

* the 8.2-to-8.3 change in the width of type money

* the likely future change to type timestamptz to store original timezone explicitly

* the likely future change to type text to store encoding/collation info explicitly

If the answer is that libpq will be unable to deal with these
events, I think the proposal is dead in the water.  There's a reason
why we aren't pushing client-side use of binary formats very hard:
in many cases those formats are subject to change.

There might be some value in the concept of building up parameter
values in a PGparam object before passing it to an eventual PQexec-like
function.  However, I see no reason to tie that concept to the
use of binary parameter format.
        regards, tom lane