déchiffrage et optimissation

bonjour quelqu'un peut m'aider à déchiffrer ce code svp

select NATIONAL_NAME,
f.invoice_number num_facture,f.invoice_date date_facture,F.JOB_TYPE,F.INVOICE_TYPE,f.vrn IMMAT,F.VIN num_serie,F.MAKE marque, f.model, F.MILEAGE Kilométrage,
F.SCHEDULED_START_DATE Date_de_la_prestation,(select max(answer) from V_DW_COLLECTS c1 where f.job_id=c1.job_code and c1.COLLECT_NAME like ('AL12%'))Nom_conducteur,
(select max(answer) from V_DW_COLLECTS c2 where f.job_id=c2.job_code and c2.COLLECT_NAME like ('ID03%')) Date_sinistre,
(select max(answer) from V_DW_COLLECTS c3 where f.job_id=c3.job_code and c3.COLLECT_NAME like ('ID11%')) Circonstances_sinistre,
decode(substr(AR.ITEM_CATEGORY_CODE,1,2), '11','PB','12','GL','13','LU','14','REP','28','OPT','29','BEG','AUTRES') Nature_intervention,
AR.EUROCODE, AR.DESCRIPTION,
decode(substr(AR.ITEM_CATEGORY_CODE,1,2), '11',(F.IF_11_QUANTITY),'12',(F.IF_12_QUANTITY),'13',(F.IF_13_QUANTITY),'14',(F.IF_14_QUANTITY),'28',(F.IF_28_QUANTITY),'29',(F.IF_29_QUANTITY)) QTY,
F.AMOUNT_BO_ORIGINAL Montant_TTC,F.GROSSSALESVALUE_BO Montant_HT,F.TAX_BO_ORIGINAL1 Montant_TVA, b.pole_code Region,b.branch_code,b.name centre, B.POSTAL_CODE
from fr_invoices f, fr_items ar, fr_branchs b, fr_customers c
where F.INVENTORY_ITEM_ID1=AR.INVENTORY_ITEM_ID
and F.SALESREP_ID=B.SALESREP_ID
and f.invoice_type='INV'
and f.job_revision_number=0
and C.ACCOUNT_NUMBER=F.ACCOUNT2
AND c.site_use_code (+) = 'BILL_TO'
and c.site_use_status (+) = 'A'
--and C.SITE_BCF='002'
and c.address1 like ('WU%')
--and invoice_number='16743458'
and f.invoice_date >=to_date ('01012016','DDMMYYYY')
and f.invoice_date <to_date ('01012017','DDMMYYYY')