Question

I have 2 tables table1 and table2 both having large amounts of data, Table1 has 5 million and Table2 has 80,000 records. I am running an update,

Update Table1 a
Set 
a.id1=(SELECT DISTINCT p.col21 
         FROM Table2 p 
         WHERE p.col21 = SUBSTR(a.id, 2, LENGTH(a.id));

The substr and distinct in the query are making it slow.

1. How can this query be re-written to speed up the process and
2. What columns do I need to index

Answer 1

May be a merge

 merge into Table1 a
  using Table2 p 
  on (p.col21 = SUBSTR(a.id, 2, LENGTH(a.id))
  When matched then
     update set a.id1 = p.col21;

and a Function Based Index on a.id.