Very large IN-clause is slow, but how to rewrite it?

I've been profiling a PG database / mix of applications and found that
one statement which takes a very long time to execute is:

select e.keywordid, e.quantity, e.max_cpc, i.position
from bid3_events_impressions i, bid3_events e
where i.eventid = e.id and e.keywordid in ($1,$2,$3,$4,$5,$6,$7,
$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25,$26,
$27,$28,$29,$30,$31,$32,$33,$34,$35,$36,$37,$38,$39,$40,$41,$42,$43,$44,
$45,$46,$47,$48,
[... placeholders $49 thru $1908 omitted ...]
$1909,$1910,$1911,$1912,$1913,$1914,$1915,$1916,$1917,$1918,$1919,$1920,
$1921,$1922,$1923,$1924,$1925,$1926,$1927,$1928,$1929,$1930,$1931,$1932)
order by e.keywordid, e.creativeid, e.t

Needless to say this statement is being generated programatically.

The problem is that the code needs to execute selects of this sort on
various different number of keyword IDs quite frequently. I'm not
sure how to rewrite it. If I put the keyword IDs into a temporary
table then it is not at all clear that the overhead of doing each
individual INSERT to populate the table won't be just as slow (the
database is located across a network so there is a significant RTT,
and COPY isn't supported by my PG lib).

Has anyone got any suggestions?

Rich.

----------------------------------------------------------------------
explain select e.keywordid, e.quantity, e.max_cpc, i.position from bid3_events_impressions i, bid3_events e where i.eventid = e.id and e.keywordid in (1,2,3,4,5,6,7,8,9,10) order by e.keywordid, e.creativeid, e.t; QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=15795.79..15796.57 rows=312 width=34)
Sort Key: e.keywordid, e.creativeid, e.t
-> Hash Join (cost=11623.58..15782.87 rows=312 width=34)
Hash Cond: ("outer".eventid = "inner".id)
-> Seq Scan on bid3_events_impressions i (cost=0.00..3471.78 rows=136878 width=8)
-> Hash (cost=11622.35..11622.35 rows=489 width=34)
-> Seq Scan on bid3_events e (cost=0.00..11622.35 rows=489 width=34)
Filter: ((keywordid = 1) OR (keywordid = 2) OR (keywordid = 3) OR (keywordid = 4) OR (keywordid = 5) OR (keywordid = 6) OR (keywordid = 7) OR (keywordid = 8) OR (keywordid = 9) OR (keywordid = 10))
(8 rows)