Question

I have two tables with the following columns:

Table1
{   ID            NUMBER(15),
    ROLL_NUM      VARCHAR2(9),
    BATCH_NUM     VARCHAR2(6),
    ACCT_BALANCE  NUMBER(15,2)
}

Table2
{   Table1_ID      NUMBER(15) REFERENCES TABLE1.ID,
    SEQ_NUM       NUMBER(2),
    TRANS_NUM     VARCHAR2(10),
    TRANS_AMT     NUMBER(8,2),
    TRANS_DT      DATE
}

Table1 has 200,000 records and Table2 has 500,000 records

I have simple joins as follows:

SQL #1:

SELECT A.ROLL_NUM, A.ACCT_BALANCE, B.TRANS_NUM, TRANS_AMT, TRANS_DT
FROM   TABLE1 A, TABLE2 B
WHERE  B.Table1_ID = A.ID
AND    A.BATCH_NUM = 400012

SQL #2:

SELECT A.ROLL_NUM, A.ACCT_BALANCE, B.TRANS_NUM, TRANS_AMT, TRANS_DT
FROM   TABLE1 A, TABLE2 B
WHERE  B.Table1_ID = A.ID
AND    A.BATCH_NUM = '400012'

SQL #3:

SELECT A.ROLL_NUM, A.ACCT_BALANCE, B.TRANS_NUM, TRANS_AMT, TRANS_DT
FROM   TABLE1 A, TABLE2 B
WHERE  B.Table1_ID = A.ID
AND    A.BATCH_NUM = TO_NUMBER('400012')

The expected result from the count should be 500,000 if every BATCH_NUM from Table1 is ‘400012’ and all the ID’s match up in Table2.

When I run these queries in Oracle (v11 or v10), SQL #2 seems to take forever and I had to stop the query from running after 10 to 15 minutes. SQL #1 and #3 seems to come back with results instantaneously with the full 500,000 records in less than a second. At first, I thought it was an indexing issue, but adding the index doesn’t do anything to resolve the issue. I tried this query in TOAD and in SQL Developer with the same results.

I’m at a loss here because the BATCH_NUM column in Table1 is a VARCHAR2 and you’d think that an implicit conversion of the data types would cause the query to be slower, not faster than a non-converted comparison. Can someone explain this?

Answer 1

I agree that use of implicit conversion would prevent an index access path.

To be sure what’s really going on, do the following:

explain plan for
select ......
/

Then, immediately following that, do:

select * from table(dbms_xplan.display);

and post the results.

Without looking at the execution plan, anything I say would be guesswork.

Oh, and please post the definitions of all indexes that exist on the tables.

-Mark