db2查询花费太多时间

Question

I have a task to fetch the data from table for some of the employees for specific years, but the query is taking approx. 50 mins to fetch 50 000 emp records.

Table has approx. 6 billion (6*10^9) data

Query :

select a, b
from t1
where t1.year in (2012,2013) and
      t1.name in (select name from name_tab fetch first 50000 rows only)

Partitioned table: t1
partitioned col: t1.year

Index col: t1.name

I have checked the Access plan and surprised to see that neither partition nor index is getting used.

Answer 1

First, try this query:

select a, b
from t1
where t1.year = 2012 and
      t1.name in (select name from name_tab fetch first 50000 rows only)

Does it recognize the partition? If it does, try writing the query as:

select a, b
from t1
where t1.year = 2012 and
      t1.name in (select name from name_tab fetch first 50000 rows only)
union all
select a, b
from t1
where t1.year = 2013 and
      t1.name in (select name from name_tab fetch first 50000 rows only)

You might want to put an order by in the subquery, so the names are guaranteed to be the same.

Then, put an index on name_tab(name) .