Strange query / explain plan

Question

(I posted this in the Oracle Text forum, but I think it's as appropriate here.)
Consider the following queries:
:the_filter := 'FILE';
(1)
SELECT COUNT(*)
FROM FOO
WHERE CONTAINS(search_col,:the_filter) > 0;
(2)
SELECT COUNT(*)
FROM FOO
WHERE 'FILE' IS NULL OR CONTAINS(search_col,:the_filter) > 0;
(3)
SELECT COUNT(*)
FROM FOO
WHERE :the_filter IS NULL OR CONTAINS(search_col,:the_filter) > 0;
(1) and (2) use the text index and run normally, but (3) does a full table scan, but I don't understand why.
The reason behind my using a query of the form (3) is to say 'keep the record if the filter is null or the record contains the filter'. I'm using the filter as a switch, and (3) is actually a simplified form of a complex query with several tables and filters. How can I do this without the query using a full table scan?
Below is the script and a record of the full session:
SET ECHO ON;
SET TIMING ON;
DROP TABLE FOO;
CREATE TABLE FOO (search_col VARCHAR2(100));
-- Loads of dummy data
INSERT INTO FOO (search_col) SELECT OBJECT_NAME || ' ' || OBJECT_TYPE
FROM ALL_OBJECTS WHERE ROWNUM <= 20000;
COMMIT;
CREATE INDEX I1 ON FOO(search_col) INDEXTYPE IS CTXSYS.CONTEXT
PARAMETERS(' section group ctxsys.html_section_group');
variable the_filter varchar2(100);
begin
:the_filter := 'FILE';
end;
SET AUTOTRACE ON EXPLAIN;
SELECT COUNT(*)
FROM FOO
WHERE CONTAINS(search_col,:the_filter) > 0;
SELECT COUNT(*)
FROM FOO
WHERE 'FILE' IS NULL OR CONTAINS(search_col,:the_filter) > 0;
SELECT COUNT(*)
FROM FOO
WHERE :the_filter IS NULL OR CONTAINS(search_col,:the_filter) > 0;
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.6.0 - Production
With the Partitioning, Oracle Label Security, OLAP and Oracle Data Mining options
JServer Release 9.2.0.6.0 - Production
SQL*Plus: Release 10.2.0.1.0 - Production on Tue Nov 21 11:21:22 2006
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.6.0 - Production
With the Partitioning, Oracle Label Security, OLAP and Oracle Data Mining options
JServer Release 9.2.0.6.0 - Production
SQL> SET TIMING ON;
SQL> DROP TABLE FOO;
Table dropped.
Elapsed: 00:00:00.73
SQL> CREATE TABLE FOO (search_col VARCHAR2(100));
Table created.
Elapsed: 00:00:00.03
SQL> -- Loads of dummy data
SQL> INSERT INTO FOO (search_col) SELECT OBJECT_NAME || ' ' || OBJECT_TYPE
2 FROM ALL_OBJECTS WHERE ROWNUM <= 20000;
20000 rows created.
Elapsed: 00:00:01.92
SQL> COMMIT;
Commit complete.
Elapsed: 00:00:00.01
SQL> CREATE INDEX I1 ON FOO(search_col) INDEXTYPE IS CTXSYS.CONTEXT
2 PARAMETERS(' section group ctxsys.html_section_group');
Index created.
Elapsed: 00:00:10.76
SQL> variable the_filter varchar2(100);
SQL> begin
2 :the_filter := 'FILE';
3 end;
4 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.00
SQL> SET AUTOTRACE ON EXPLAIN;
SQL> SELECT COUNT(*)
2 FROM FOO
3 WHERE CONTAINS(search_col,:the_filter) > 0;
COUNT(*)
29
Elapsed: 00:00:00.07
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=59)
1 0 SORT (AGGREGATE)
2 1 DOMAIN INDEX OF 'I1' (Cost=0 Card=1 Bytes=59)
SQL> SELECT COUNT(*)
2 FROM FOO
3 WHERE 'FILE' IS NULL OR CONTAINS(search_col,:the_filter) > 0;
COUNT(*)
29
Elapsed: 00:00:00.08
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=59)
1 0 SORT (AGGREGATE)
2 1 DOMAIN INDEX OF 'I1' (Cost=0 Card=1 Bytes=59)
SQL> SELECT COUNT(*)
2 FROM FOO
3 WHERE :the_filter IS NULL OR CONTAINS(search_col,:the_filter) > 0;
COUNT(*)
29
Elapsed: 00:00:04.06
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=34482 Card=1 Bytes=5
9)
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'FOO' (Cost=34482 Card=453 Bytes=
26727)

Answer 1

Nothing is really strange when you are attempting to integrate with legacy systems! =)
If no numerical processing is done why should numerical values be stored as numbers?