sql with underscore is slow !!

Hi all,
the env is oracle 9.2.0.4/Aix 5.2L.

I find something strange with a query.it joins 11 tables .

the predicate part has something like

"AND papr.payroll_name = 'AAQPTESTQB'"

Note that the underscore Q above is a double byte character.

the PAPR table has 2 records for the
value 'AAQPTESTQB'. This query takes a long time and waits indefinitely for 'direct path write' event.

whereas when i change the predicate part to

"AND papr.payroll_name like 'AA%PTEST%B'"

the response is instantaneous.I have replaced the underscore with "%" and used LIKE operator.

the database character set is JA16SJIS.

I wonder why it happens so ? Have anyone faced such issues while working with multibyte charcters.

Regards,
Prem.

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