Creating Indexes for Group By Fields?

sql oracle plsql indexing
onejigtwojig picture onejigtwojig · Sep 18, 2009 · Viewed 42.4k times · Source

Do you need to create an index for fields of group by fields in an Oracle database?

For example: 

select * 
from some_table
where field_one is not null and field_two = ?
group by field_three, field_four, field_five

I was testing the indexes I created for the above and the only relevant index for this query is an index created for field_two. Other single-field or composite indexes created on any of the other fields will not be used for the above query. Does this sound correct?

Answer

Eric Petroelje picture Eric Petroelje · Sep 18, 2009

It could be correct, but that would depend on how much data you have. Typically I would create an index for the columns I was using in a GROUP BY, but in your case the optimizer may have decided that after using the field_two index that there wouldn't be enough data returned to justify using the other index for the GROUP BY.

Related questions

SQL error "ORA-01722: invalid number"

A very easy one for someone, The following insert is giving me the ORA-01722: invalid number why? INSERT INTO CUSTOMER VALUES (1,'MALADY','Claire','27 Smith St Caulfield','0419 853 694'); INSERT INTO CUSTOMER VALUES (2,'GIBSON','Jake','27 Smith St Caulfield','0415 713 598…

Read More
Is there a combination of "LIKE" and "IN" in SQL?

In SQL I (sadly) often have to use "LIKE" conditions due to databases that violate nearly every rule of normalization. I can't change that right now. But that's irrelevant to the question. Further, I often use conditions like WHERE something …

Read More
ORA-06502: PL/SQL: numeric or value error: character string buffer too small

I tried the following code different ways, like by taking out the while or the if, but when I put both together (if and while), I always get the error at the end... undefine numero set serveroutput on accept numero …

Read More