Oracle optimizer and heterogeneous services (Transparent Gateway for ODBC)

Question

Hi everyone!
We have the following configuration:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production.
Oracle Database Gateway for ODBC 11.2.0.2.0 on the same machine as Oracle DB.
Non-Oracle DB installed on a different hardware accessed through Oracle Gateway for ODBC via dblink.
Now, we have to query the remote non-Oracle system and retrieve some data. For the sake of brevity let's suppose we have the following table in our non-Oracle DB:
create table test (
     ip_address varchar(15),
     start_time datetime,
     end_time datetime
And we run the following SQL query against the remote table on our Oracle DB:
SELECT start_time,end_time,ip_address
FROM remote_table@db_link
WHERE
end_time >= to_date('2011-10-11 00:00:00','rrrr-mm-dd hh24:mi:ss')
OR (
end_time IS NULL AND start_time >= to_date('2011-10-11 00:00:00','rrrr-mm-dd hh24:mi:ss')
AND start_time <= to_date('2011-10-12 00:00:00','rrrr-mm-dd hh24:mi:ss')
AND ip_address = '127.0.0.1';
But, having query monitoring on the remote DB enabled, we see that the actual query on the remote site looks like this:
SELECT "START_TIME","END_TIME","IP_ADDRESS"
FROM "REMOTE_TABLE"
WHERE "IP_ADDRESS"='127.0.0.1' AND "START_TIME"<=?
All references to END_TIME column are gone from WHERE clause for whatever reason.
However, Oracle execution plan shows us the following:
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
| 0 | SELECT STATEMENT | | 1 | 27 | 200 (0)| 00:00:03 | | |
|* 1 | FILTER | | 1 | 27 | 200 (0)| 00:00:03 | | |
| 2 | REMOTE | REMOTE_TABLE | | | | | DB_LINK | R->S |
Predicate Information (identified by operation id):
1 - filter("END_TIME">=TO_DATE('2011-10-11 00:00:00','rrrr-mm-dd hh24:mi:ss') OR
"END_TIME" IS NULL AND "START_TIME">=TO_DATE('2011-10-11 00:00:00','rrrr-mm-dd hh24:mi:ss'))
Remote SQL Information (identified by operation id):
2 - SELECT "END_TIME","IP_ADDRESS","START_TIME" FROM "REMOTE_TABLE" WHERE
"IP_ADDRESS"='127.0.0.1' AND "START_TIME"<=:1 (accessing 'DB_LINK' )
This actually means that Oracle CBO decided that it will apply filter on END_TIME column after having retrieved the data from the remote site. We don't want this behaviour but have no idea how to affect execution plan in this case.
So, the question is: How can we force Oracle CBO to not change the initial query and pass it to the remote site "as is"? Is there any way we can achieve this goal?
Edited by: user12024904 on 28-Oct-2011 04:05

Answer 1

Hi,
Our Oracle DB and Gateway are running on the following system:
HP-UX B.11.31 U ia64
The following SQL code is executed in SQLPlus:
SELECT start_time,end_time,ip_address
FROM ip_acc_complete@gr_link
WHERE
END_TIME >= to_date('2010-10-21 00:00:00','rrrr-mm-dd hh24:mi:ss')
OR (
END_TIME is null AND START_TIME >= to_date('2010-10-21 00:00:00','rrrr-mm-dd hh24:mi:ss')
AND START_TIME <= to_date('2010-10-22 00:00:00','rrrr-mm-dd hh24:mi:ss')
AND IP_ADDRESS = '127.0.0.1';
Gateway debug trace shows the following:
Oracle Corporation --- TUESDAY NOV 01 2011 14:59:15.254
Heterogeneous Agent Release
11.2.0.2.0
Oracle Corporation --- TUESDAY NOV 01 2011 14:59:15.254
Version 11.2.0.2.0
HOSGIP for "HS_FDS_TRACE_LEVEL" returned "ON"
HOSGIP for "HS_FDS_SHAREABLE_NAME" returned "/usr/local/greenwich64/lib/libodbc.so.1"
HOSGIP for "HS_OPEN_CURSORS" returned "50"
HOSGIP for "HS_FDS_FETCH_ROWS" returned "100"
HOSGIP for "HS_LONG_PIECE_TRANSFER_SIZE" returned "65536"
HOSGIP for "HS_NLS_NUMERIC_CHARACTER" returned ".,"
HOSGIP for "HS_KEEP_REMOTE_COLUMN_SIZE" returned "OFF"
HOSGIP for "HS_FDS_DELAYED_OPEN" returned "TRUE"
HOSGIP for "HS_FDS_WORKAROUNDS" returned "0"
HOSGIP for "HS_FDS_MBCS_TO_GRAPHIC" returned "FALSE"
HOSGIP for "HS_FDS_GRAPHIC_TO_MBCS" returned "FALSE"
HOSGIP for "HS_FDS_RECOVERY_ACCOUNT" returned "RECOVER"
HOSGIP for "HS_FDS_TRANSACTION_LOG" returned "HS_TRANSACTION_LOG"
HOSGIP for "HS_FDS_TIMESTAMP_MAPPING" returned "DATE"
HOSGIP for "HS_FDS_DATE_MAPPING" returned "DATE"
HOSGIP for "HS_FDS_MAP_NCHAR" returned "TRUE"
HOSGIP for "HS_FDS_RESULTSET_SUPPORT" returned "FALSE"
HOSGIP for "HS_FDS_RSET_RETURN_ROWCOUNT" returned "FALSE"
HOSGIP for "HS_FDS_PROC_IS_FUNC" returned "FALSE"
HOSGIP for "HS_FDS_REPORT_REAL_AS_DOUBLE" returned "FALSE"
using query as default value for "HS_FDS_DEFAULT_OWNER"
HOSGIP for "HS_SQL_HANDLE_STMT_REUSE" returned "FALSE"
SQL text from hgopars, id=1, len=117 ...
00: 53454C45 43542022 454E445F 54494D45 [SELECT "END_TIME]
10: 222C2249 505F4144 44524553 53222C22 [","IP_ADDRESS","]
20: 53544152 545F5449 4D452220 46524F4D [START_TIME" FROM]
30: 20224950 5F414343 5F434F4D 504C4554 [ "IP_ACC_COMPLET]
40: 45222057 48455245 20224950 5F414444 [E" WHERE "IP_ADD]
50: 52455353 223D2731 32372E30 2E302E31 [RESS"='127.0.0.1]
60: 2720414E 44202253 54415254 5F54494D [' AND "START_TIM]
70: 45223C3D 3F [E"<=?]
Deferred open until first fetch.
Performing delayed open.
The non-Oracle DB is CopperEye Retrieval Server with its own custom ODBC driver.
CopperEye query log shows that the following query is received from Oracle Gateway:
01/11/2011 14:26:31 - NEW QUERY :
SELECT "END_TIME","IP_ADDRESS","START_TIME" FROM "IP_ACC_COMPLETE" WHERE "IP_ADDRESS"='127.0.0.1' AND "START_TIME"<=?
which complies with the information in Oracle Gateway trace file.
So, it seems that it's Oracle DB decision to split the original WHERE clause into two parts, delegate one part to the remote site, retrieve the data and then apply the remaining filter locally.