Oracle 11.2.0.2 and nchar optimizer problem

Question

Hello,
sorry in advance for not being able to give all details to narrow better this possible problem.
I admit it could be elsewhere.....
I have a complex query where in particular an nchar(2) field (say col1) of table tab is involved
I'm experimenting a sort of problem in optimizer estimating the number of rows it will get from this condition and so the corresponding overall cost of the query
the table contains about 300k records and this field has at this moment 2 different values
blankblank --> almost all records
AA --> about 30 records
there is at the moment a single index IND1 defined on this column (not a bitmap index)
Both table and index are analyzed
if my query contains the condition
AND TAB.COL1 = ' ' ---> single blank
Then the optimizer erroneously thinks with explain plan to get about 30 rows
|* 5 | INDEX RANGE SCAN     | IND1 |     29 |     |     3 (0)
| 00:00:01 |
and the overall about 98, but the query actually doesn't come to an end ... (waited 10 minutes)
if my query contains the condition
AND TAB.COL1 = ' ' --->double space
it doesn't use that wrong path and completes in about 1 second.
If I replicate the query with single space on a test db with 11.2.0.1 it behaves correctly.
If I force it to behave as the default wrong query in 11.2.0.2 (I have to use 3 hints to duplicate it)
I get
|* 5 | INDEX RANGE SCAN     | IND1 | 135K|     | 306 (1)
| 00:00:04 |
and overall cost is 304K so it is not used as a possible path...
ANyone knows if anything changed between 11.2.0.1 and 11.2.0.2 for nchar and these possible different behaviours?
(BTW: also in another 11.1.0.6 db I don't have this problem)
The query is created by application and I cannot put the amount of necessary spaces.....
And I'm afraid this problem could exist also for other tables where I have nchar(N) fields with N >2 too....
Thanks in advance,
Gianluca

Answer 1

1) do you have a frequency histogram on this column?
and
2) is the client application using bind variables or string literals when querying?