Question

I have a query:

select count(1) CNT
from file_load_params a
where a.doc_type = (select b.doc_type
                    from file_load_header b
                    where b.indicator = 'XELFASI')
 order by a.line_no

Which explain plan is:

-----------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                     |     1 |     7 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE               |                     |     1 |     7 |            |          |
|*  2 |   TABLE ACCESS FULL           | FILE_LOAD_PARAMS    |    15 |   105 |     2   (0)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| FILE_LOAD_HEADER    |     1 |    12 |     1   (0)| 00:00:01 |
|*  4 |     INDEX UNIQUE SCAN         | FILE_LOAD_HEADER_UK |     1 |       |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

I thought that I could optimize this query and write this one:

select count(1) CNT
from file_load_params a,file_load_header b
where  b.indicator = 'XELFASI'
and a.doc_type = b.doc_type
order by a.line_no

Its explain plan is:

-----------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                     |     1 |    19 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE               |                     |     1 |    19 |            |          |
|   2 |   NESTED LOOPS                |                     |    15 |   285 |     3   (0)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| FILE_LOAD_HEADER    |     1 |    12 |     1   (0)| 00:00:01 |
|*  4 |     INDEX UNIQUE SCAN         | FILE_LOAD_HEADER_UK |     1 |       |     0   (0)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL          | FILE_LOAD_PARAMS    |    15 |   105 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

Is it good? I think not,but I expected better result…Do you have any idea?

Answer 1

One of the possible optimizations i see from your explain plan is

TABLE ACCESS FULL           | FILE_LOAD_PARAMS  

This seems to indicate that table file_load_params possibly does not have any index on doc_type

If that is the case, can you add an index for doc_type. If you already have indexes, can you post your table schema for file_load_params