SQL & PL/SQL

Hi
when i use substr function in the join condition it loses index and the query is slow. How to fix this problem or tune this query. These are the lines in the query.
and substr(a.invoice_num,1,9) = l.invoice_num
and substr(c.invoice_num,1,9) = mgr_apprv_lst.user_key
and substr(c.invoice_num,1,9) = pbl_apprv_lst.user_key



select
pap.full_name employe_name,
k.SEGMENT1 ||'.' ||k.segment2 Cost_Center,
a.invoice_num Invoice_Number,
b.item_description Line_Item,
b.amount Amount,
cc.trx_id Corporate_Card_Transaction_Id,
cc.transaction_date Date_Charge_Incurred,
cc.posted_date Date_Charge_Posted_To_USBank,
cc.creation_date Date_Posted_To_IExpense,
a.creation_date Expense_Report_Creation_Date,
l.report_submitted_date Expense_Report_Submitted_Date,
mgr_apprv_lst.activity_begin_date Managers_Approval_Begin_Date,
mgr_apprv_lst.activity_end_date Managers_Approval_End_Date,
pbl_apprv_lst.activity_begin_date AP_Approval_Begin_Date,
pbl_apprv_lst.activity_end_date AP_Approval_End_Date,
e.check_date Payment_Date_To_USBank,
e.check_number Payment_Number,
mgr_apprv_lst.activity_result_display_name Managers_Process_Result,
pbl_apprv_lst.activity_result_display_name AP_Process_Result
from
ap_checks_all e,
ap_invoice_payments_all d,
ap_invoices_all c,
ap_expense_report_headers_all a,
ap_credit_card_trxns_all cc,
per_all_people_f pap,
ap_expense_report_headers_all l,
ap_expense_report_lines_all b,
gl_code_combinations k,
(select ias1.user_key,
ias1.activity_result_display_name,
ias1.activity_begin_date,
ias1.activity_end_date
from wf_item_activity_statuses_v ias1,
(select c1.invoice_num
from ap_checks_all e1,
ap_invoice_payments_all d1,
ap_invoices_all c1
where trunc(e1.check_date) between nvl(:From_Date, trunc(e1.check_date))
and nvl(:To_Date, trunc(e1.check_date))
and e1.org_id = 141
and e1.void_date IS null
and d1.check_id = e1.check_id
and c1.invoice_id = d1.invoice_id) inv_lst1
where ias1.item_type = 'APEXP'
and ias1.user_key = inv_lst1.invoice_num
and ias1.activity_name = 'AP_MANAGER_APPROVAL_PROCESS') mgr_apprv_lst,
(select ias2.user_key,
ias2.activity_result_display_name,
ias2.activity_begin_date,
ias2.activity_end_date
from wf_item_activity_statuses_v ias2,
(select c2.invoice_num
from ap_checks_all e2,
ap_invoice_payments_all d2,
ap_invoices_all c2
where trunc(e2.check_date) between nvl(:From_Date, trunc(e2.check_date))
and nvl(:To_Date, trunc(e2.check_date))
and e2.org_id = 141
and e2.void_date IS null
and d2.check_id = e2.check_id
and c2.invoice_id = d2.invoice_id) inv_lst2
where ias2.item_type = 'APEXP'
and ias2.user_key = inv_lst2.invoice_num
and ias2.activity_name = 'AP_PAYABLES_APPROVAL_PROCESS') pbl_apprv_lst
where
trunc(e.check_date) between nvl(:From_Date, trunc(e.check_date))
and nvl(:To_Date, trunc(e.check_date))
and e.org_id = 141
and e.void_date IS null
and d.check_id = e.check_id
and c.invoice_id = d.invoice_id
and a.invoice_num = c.invoice_num
and a.source = 'CREDIT CARD'
and a.employee_id = nvl(:Emp_id,a.employee_id)
and a.report_header_id = b.report_header_id
and cc.trx_id = b.credit_card_trx_id
and pap.person_id = a.employee_id
and pap.effective_start_date <= trunc(sysdate)
and pap.effective_end_date >= trunc(sysdate)
and k.code_combination_id = b.code_combination_id
and k.segment2 between nvl(:From_Cost_Center,k.segment2)
and nvl(:To_Cost_Center,k.segment2)
and substr(a.invoice_num,1,9) = l.invoice_num
and substr(c.invoice_num,1,9) = mgr_apprv_lst.user_key
and substr(c.invoice_num,1,9) = pbl_apprv_lst.user_key