Why is UseDeclareFetch so slow?

[Rainer Bauer <usenet(at)munnin(dot)com>]

Hello,

based on the problem I had here
<http://archives.postgresql.org/pgsql-performance/2007-06/msg00461.php>
I thought it would be interesting to know why UseDeclareFetch is slowing my
query down dramatically.

I have a table containing 50 tuples and retrieve all 50 tuples with a bound
column select statement:
SELECT * FROM tblItem WHERE intItemIDCnt = ?

This takes 8 seconds (over a slow DSL connection with 150ms ping). Tom Lane
suggested enabling UseDeclareFetch and the performance drop was significant:
Instead of 8 seconds, the query now takes 30-32 seconds. I also tested
enabling UseServerSidePrepare (without UseDeclareFetch) and the query was
executed in 7 seconds.

After rewriting the query to use an IN clause it now works perfect:
SELECT * FROM tblItem WHERE intItemIDCnt IN (...)
The result is retrieved in 400ms (UseServerSidePrepare=950ms and
UseDeclareFetch=1242ms).

The only thread I found discussing the UseDeclareFetch/UseServerSidePrepare
options was <http://archives.postgresql.org/pgsql-odbc/2006-10/msg00051.php>.

So my questions are
1) Is there any chance the driver itself could be enhanced like Dimitri
suggested here:
<http://archives.postgresql.org/pgsql-performance/2007-06/msg00512.php>?
2) When should UseServerSidePrepare be used?
3) When should UseDeclareFetch be used?

Rainer