Performance problem using distributed query

Question

Hi,
Can anyone provide an explanation/solution to the following problem encountered on an 8.1.7 database:
SELECT /*+ DRIVING_SITE(vs) */
vs.col2
,user_defined_function
FROM v_remote_table_1 vs
,v_remote_table_2 vc
,local_table_1 s
WHERE vp.col1 = vs.col1
AND s.col1 = vs.col2
AND vs.col2 = &some_val
This generalised query joins two tables in a remote database schema to a table in the local database schema and returns a single row. Without the call to the local user defined function, my specific query returns the row in less than a second. As soon as I include any local user defined function call, even a call to a dummy function which simply returns a string, my query takes several minutes to return its row. Explain plan gives exactly the same plan for both versions of the query, but the statistics are different:
Version without call to function
================================
8 recursive calls
0 db block gets
7 consistent gets
0 physical reads
0 redo size
235 bytes sent via SQL*Net to client
311 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
Version with call to function
=============================
93 recursive calls
4 db block gets
109 consistent gets
1998 physical reads
0 redo size
288 bytes sent via SQL*Net to client
312 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
2 sorts (memory)
2 sorts (disk)
1 rows processed
I'm assuming that, when calling a local function, the DRIVING_SITE hint is ignored, but I don't know why. It's quite important that we are able to include the results of function calls in the query. Any ideas as to the cause of the degraded performance and possible solutions to it would be most welcome!
Thanks.....

Answer 1

I would try something like:
SELECT r.col2,user_defined_function
FROM (SELECT vs.col2
      FROM v_remote_table_1 vs, v_remote_table_2 vp
      WHERE vp.col1 = vs.col1 AND
            vs.col2 = &some_val) r,
     local_table_1 s
WHERE s.col1 = r.col2Thismay allow Oracle to better seperate the calls to the two databases.
HTH
John