UPDATE table via ODBC fails.

I have a fairly straightforward routine which uses an ODBC connection to
do 4 steps:

* DROP TABLE IF EXISTS tmp_rxpurge
* CREATE TABLE tmp_rxpurge (rxnum VARCHAR(300),dt_wrt DATE,dt_lastfill
DATE,dt_lastact DATE, b_purge BOOLEAN)
* INSERT INTO tmp_rxpurge (rxnum,dt_wrt,b_purge) SELECT
"RXNUM","DATE",'f' FROM rx
* UPDATE tmp_rxpurge as t SET dt_lastfill = rf.dt_lastfill FROM
(SELECT "RXNUM" AS rxnum, Max("DATE") as dt_lastfill FROM
transactions GROUP BY "RXNUM") as rf WHERE rf.rxnum=t.rxnum

The first three commands are executed perfectly as expected.
The update fails to do anything.
There is no error generated.
I have enabled 'logging' in the ODBC connector - and I don't see
anything out of the ordinary in the short-log. (The long-log - I have no
idea what all that stuff means.)

If I copy that last command into a psql window - it executes - and works
fine.

But - it will not work via the ODBC connector.

What's going on?

Ken