InternalError Exception

Greetings,

my recent multithreading problems have been solved (multiple connections 
scale well); now the next issue, not related to multithreading as it 
shows up in single-threaded mode as well.

I perform a query to determine whether a table contains a certain column 
(more elegant ways to detect this welcome). In the case it does not 
exist, I expect (and normally get) a ProgrammingError exception, which I 
catch.

However, with a certain query, I always get an InternalError exception - 
which I could handle in the same way as an ProgrammingError, but I think 
ProgrammingError would be the better exception.

psycopg2 is version 2.2.1; the last lines of the debug output is below. 
Full dump on request.

  thanks, Joachim

[25507] psyco_conn_cursor:     parameters: name = (null)
[25507] cursor_setup: init cursor object at 0x25c3c18
[25507] cursor_setup: parameters: name = (null), conn = 0x264f320
[25507] cursor_setup: good cursor object at 0x25c3c18, refcnt = 1
[25507] psyco_conn_cursor: new cursor at 0x25c3c18: refcnt = 1
[25507] psyco_conn_cursor: new cursor for connection at 0x264f320
[25507] psyco_conn_cursor:     parameters: name = (null)
[25507] cursor_setup: init cursor object at 0x25c3d00
[25507] cursor_setup: parameters: name = (null), conn = 0x264f320
[25507] cursor_setup: good cursor object at 0x25c3d00, refcnt = 1
[25507] psyco_conn_cursor: new cursor at 0x25c3d00: refcnt = 1
[25507] psyco_curs_execute: starting execution of new query
[25507] curs_execute: pg connection at 0x26d1250 OK
[25507] pq_begin_locked: pgconn = 0x26d1250, isolevel = 1, status = 1
[25507] pq_execute_command_locked: pgconn = 0x26d1250, query = BEGIN; 
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
[25507] cursor_dealloc: deleted cursor object at 0x2555d00, refcnt = 0
[25507] pq_execute: executing SYNC query:
[25507]     SELECT wtime_A FROM qt_1574 WHERE _pb_data_idx_=0
[25507] pq_execute: entering syncronous DBAPI compatibility mode
[25507] pq_fetch: pgstatus = PGRES_TUPLES_OK
[25507] pq_fetch: data from a SELECT (got tuples)
[25507] _pq_fetch_tuples: looking for cast 701:
[25507] _pq_fetch_tuples:     per-connection dict: (nil)
[25507] _pq_fetch_tuples:     global dict: 0x2552fc8
[25507] _pq_fetch_tuples: using cast at 0x2552fc8 (FLOAT) for type 701
[25507] pq_fetch: fetching done; check for critical errors
[25507] psyco_curs_execute: res = 1, pgres = 0x7f4b8c001640
[25507] psyco_curs_execute: starting execution of new query
[25507] curs_execute: pg connection at 0x26d1250 OK
[25507] pq_begin_locked: pgconn = 0x26d1250, isolevel = 1, status = 2
[25507] pq_begin_locked: transaction in progress
[25507] pq_execute: executing SYNC query:
[25507]     SELECT wtime_A FROM qt_1577 WHERE _pb_data_idx_=0
[25507] pq_execute: entering syncronous DBAPI compatibility mode
[25507] pq_fetch: pgstatus = PGRES_FATAL_ERROR
[25507] pq_fetch: uh-oh, something FAILED
[25507] pq_fetch: fetching done; check for critical errors
[25507] psyco_curs_execute: res = -1, pgres = (nil)
[25507] psyco_curs_execute: starting execution of new query
[25507] curs_execute: pg connection at 0x26d1250 OK
[25507] pq_begin_locked: pgconn = 0x26d1250, isolevel = 1, status = 2
[25507] pq_begin_locked: transaction in progress
[25507] pq_execute: executing SYNC query:
[25507]     SELECT wtime_B FROM qt_1574 WHERE _pb_data_idx_=0
[25507] pq_execute: entering syncronous DBAPI compatibility mode
[25507] pq_fetch: pgstatus = PGRES_FATAL_ERROR
[25507] pq_fetch: uh-oh, something FAILED
[25507] pq_fetch: fetching done; check for critical errors
[25507] psyco_curs_execute: res = -1, pgres = (nil)
#* Internal database error.
   current transaction is aborted, commands ignored until end of 
transaction block