Re: Bug in libpq implentation and omission in documentation?

Hey Jim,

2012/8/8 Jim Vanns <james(dot)vanns(at)framestore(dot)com>

> Hello PG hackers. Yesterday I began diagnosing a peculiar bug in some
> production code that has been happily running for months. I finally got
> to the bottom of it despite the rather misleading error message. Anyway,
> within a section of code we are making a DELETE call to the database via
> the libpq call PQexecParams(). It failed with this message:
>
> 'ERROR: bind message has 32015 parameter formats but 1 parameters'
>
> This was just plain wrong. In fact, the # of parameters was more like
> 80,000. The area of code is quite clear. Despite this being a
> particularly large number of parameters (as you can imagine this query
> is built dynamically based on arbitrarily sized input) the data type for
> nParams for is a plain old 4-byte int. Upon further and deeper
> inspection I find that this 4 byte int is truncated to two bytes just
> before going down the wire.
>
> There is no mention of any restriction in the 9.1.4 documentation:
>
> http://www.postgresql.org/docs/9.1/static/libpq-exec.html
>
> And the interface quite clearly accepts a 4 byte int however, the
> PQsendQueryGuts()
> function on line 1240 of src/interfaces/libpq/fq-exec.c just blatantly
> truncates the
> integer - it's calls pqPutInt() for nParams with a literal 2 rather than
> 4. It does this
> several times, in fact.
>
> Unless I'm barking mad, surely this should either
>
> a) Be fixed and send 4 with nParams for pqPutInt() rather than 2
> b) Documented (and the type changed) as only being a 2 byte int
> and therefore having a restriction on the number of parameters
> permitted in PQexecParams().
>
> Could someone either verify or correct me before I submit an official bug
> report!?
>
> Regards,
>
> Jim Vanns
>
AFAIK, this is a limitation of the frontend/backend protocol which allows
you to bind no more than 2^16 parameters.
See the description of the Bind (F) message here
http://www.postgresql.org/docs/9.2/static/protocol-message-formats.html

--
// Dmitriy.