Select Query Hangs / No DB Locks / Query Plan is good

Question

I have below query in application which hangs ( Only sometimes ) and there are some session which also try to insert data into this tables also starts getting hanged.
SELECT DISTINCT F2 || ' - ' || F3 || ' - ' || F4
FROM Table1
WHERE F4 = 6160
AND F1|| F2|| F3 NOT IN (
SELECT F2|| F3 || F4
FROM Table2
WHERE F1 = 181)
F2 - varchar(4)
F3- varchar(100)
F4- varchar(20)
F1,F1 are NUMBER
Table1 has one unique index comprising all fields in ORDER F1,F2, X,F3, F4,Y - X,Y are some different field.
Table2 has one unique index comprising all fields in ORDER F1,F3, F4,F2 are some different field.
Below are facts from DBA ----
1- Session Query Plan was excellent -
2 - No tables were locked in DB
3 - Statistics on this table Table1 was calculated on Feb, 2008 and shows 8 millions rows but actual current row count is 13 million. DBA says sometime analyzing statistics again may cause poor query performance.
4- There were 8 session in DB, 1 was for above select statement and other 7 were trying to insert data into above table.
Can you please advise what could else be verified to root cause issue as it has been hanging many times - hence causing other session to hang ?
Many Thanks

Answer 1

Can you clarify exactly what the DBA meant by "No tables were locked in DB"? What was the exact query that was used to determine this? Was it hitting V$LOCK (GV$LOCK in a RAC system)? Or the DBA_BLOCKERS and DBA_WAITERS tables? Or something else? The specific query here is important because there is a world of difference between a row-level lock and a table-level lock. Of course, locks would never prevent a SELECT statement from running, but it could cause problems for the INSERT statements.
I strongly concur with Old DBA's suggestion about refactoring the query to not do the concatenation in the NOT IN clause.
Are the F4 = 6160 and F1 = 181 conditions really using hard-coded numeric literals? Or are these bind variables? Is the data in F1 or F4 skewed-- that is, are there certain values that are more likely than other values?
How many rows would you expect to be in Table1 with an F4 value of 6160? How many rows in Table2 with a F1 value of 181?
Justin