sql - Full table scan or index scan -


there large oracle table a , relatively large global temp table tb used following query:

update  set     a.field0 = ( select sum(tb.field0)                        tb                       tb.field1 = a.field1 ,                             tb.field2 = a.field2 ,                             tb.field3 = 'value' )   exist ( select 1                    tb                   tb.field1 = a.field1 ,                         tb.field2 = a.field2 ,                         tb.field3 = 'value' );

basically exist condition used check if record ever existed. query, if there index on a.field0 , a.field1, no index on tb @ all, going full table scan or index scan? furthermore, where condition necessary , performance impact it?

since there no indexes on tb select sub-queries in sql go full table scan on tb. if tb table large creating indexes on tb.field1,tb.field2,tb.field3 should give significant performance improvement.


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