Question

I want to identify all rows whose content in a clob column is not unique.

The query I use is:

select
  id,
  clobtext
from
  table t
where 
    (select count(*) from table innerT where dbms_lob.compare(innerT.clobtext, t.clobtext) = 0)>1

However this query is very slow. Any suggestions to speed it up? I already tried to use the dbms_lob.getlength function to eliminate more elements in the subquery but I didn’t really improve the performance (feels the same).

To make it more clear an example:

table

ID | clobtext

1 | a

2 | b

3 | c

4 | d

5 | a

6 | d

After running the query. I’d like to get (order doesn’t matter):

1 | a

4 | d

5 | a

6 | d

Answer 1

In the past I’ve generated checksums (in my C# code) for each clob.

Whilst this will inccur a one off increase in io (to generate the checksum)
subsequent scans will be quicker, and you can index the value too

TK has a good PL\SQL example here:
Ask Tom