Update/Select query to long to complete

Hi,

i have an requirement to update 250 thousands of records comparing againt another table that have 10 million records

The query is like

Update TABLE1 set COLUMN1 = value
Where Exists ( Select 'X' from TABLE2
Where InStr(sfc_GetPkeyPattern(TABLE1.COLUMN2), '''' || TABLE2.COLUMNX ||'''',1)>0 )

-- The user defined function sfc_GetPkeyPattern retruns a string ( max of 100 char. length ) pattern based on the input values.

if i run this query it takes about 3/4 hrs some time not returing the control at all.

when i tried to execute the query using select with join it took 1 minute to return the result, if use select withs sub query it took the same time ie 3/4 hrs

is there any way to fine tune this update query / what's wrong in this?

Thanks & Regards
Ram

The only thing I can think of is that the subquery is having to run 250,000 times. If the join is faster, go with the join.

Does it use indexes?