Friday, November 24, 2006

Bad SQL design

Important point

If the statement is designed poorly, nothing much can be done by optimizer or indexes

Few known thumb rules

–Avoid Cartesian joins

–Use UNION ALL instead of UNION – if possible

–Use EXIST clause instead of IN - (Wherever appropiate)

–Use order by when you really require it – Its very costly

–When joining 2 views that themselves select from other views, check that the 2 views that you are using do not join the same tables!

–Avoid NOT in or NOT = on indexed columns. They prevent the optimizer from using indexes. Use where amount > 0 instead of where amount != 0

- Avoid writing where is not null. nulls can prevent the optimizer from using an index

- Avoid calculations on indexed columns. Write WHERE amount > 26000/3 instead of WHERE approved_amt/3 > 26000

- The query below will return any record where bmm_code = cORE, Core, CORE, COre, etc.

select appl_appl_id where upper(bmm_code) LIKE 'CORE%'

But this query can be very inefficient as it results in a full table scan. It cannot make use of the index on bmm_code.

Instead, write it like this:

select appl_appl_id from nci_appl_elements_t where (bmm_code like 'C%' or bmm_code like 'c%') and upper(bmm_code) LIKE 'CORE%'

This results in Index Range Scan.

You can also make this more efficient by using 2 characters instead of just one:

where ((bmm_code like 'CO%' or bmm_code like 'Co%' or bmm_code like 'cO%' or bmm_code like 'co%') and upper(bmm_code) LIKE 'CORE%')

Inviting Experts

Friends, feel free to correct me. I will appreciate if you can add your comments also.

1 comment:

Anonymous said...

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I have one Question......

How to create index..means in what sequence columns should be come in index......

Hoe to avoid sort at the time of group by clause?