Question

I was given a SQL query, saying that I have to optimize this query.

I came accross explain plan. So, in SQL developer, I ran explain plan for ,

It divided the query into different parts and showed the cost for each of them.

How do I go about optimizing the query? What do I look for? Elements with high costs?

I am a bit new to DB, so if you need more information, please ask me, and I will try to get it.

I am trying to understand the process rather than just posting the query itself and getting the answer.

The query in question:

SELECT cr.client_app_id,
  cr.personal_flg,
  r.requestor_type_id 
FROM credit_request cr,
  requestor r,
  evaluator e 
WHERE cr.evaluator_id = 96 AND
  cr.request_id = r.request_id AND
  cr.evaluator_id = e.evaluator_id AND
  cr.request_id != 143462 AND
  ((r.soc_sec_num_txt = 'xxxxxxxxx' AND         
  r.soc_sec_num_txt IS NOT NULL) OR
  (lower(r.first_name_txt) = 'test' AND
  lower(r.last_name_txt) = 'newprogram' AND
  to_char(r.birth_dt, 'MM/DD/YYYY') = '01/02/1960' AND
  r.last_name_txt IS NOT NULL AND
  r.first_name_txt IS NOT NULL AND
  r.birth_dt IS NOT NULL))

On running explain plan, I am trying to upload the screenshot.

OPERATION    OBJECT_NAME     OPTIONS     COST 
 SELECT STATEMENT                        15 
 NESTED LOOPS              
 NESTED LOOPS                            15 
 HASH JOIN                               12 
 Access Predicates 
 CR.EVALUATOR_ID=E.EVALUATOR_ID 
 INDEX  EVALUATOR_PK     UNIQUE SCAN     0 
 Access Predicates 
 E.EVALUATOR_ID=96 
 TABLE ACCESS  CREDIT_REQUEST    BY INDEX ROWID      11 
 INDEX  CRDRQ_DONE_EVAL_TASK_REQ_NDX     SKIP SCAN   10 
 Access Predicates 
 CR.EVALUATOR_ID=96 
 Filter Predicates 
 AND 
 CR.EVALUATOR_ID=96 
 CR.REQUEST_ID<>143462 
 INDEX  REQUESTOR_PK     RANGE SCAN      1 
 Access Predicates 
 CR.REQUEST_ID=R.REQUEST_ID 
 Filter Predicates 
 R.REQUEST_ID<>143462 
 TABLE ACCESS  REQUESTOR     BY INDEX ROWID      3 
 Filter Predicates 
 OR 
 R.SOC_SEC_NUM_TXT='XXXXXXXX' 
 AND 
 R.BIRTH_DT IS NOT NULL 
 R.LAST_NAME_TXT IS NOT NULL 
 R.FIRST_NAME_TXT IS NOT NULL 
 LOWER(R.FIRST_NAME_TXT)='test' 
 LOWER(R.LAST_NAME_TXT)='newprogram' 
 TO_CHAR(INTERNAL_FUNCTION(R.BIRTH_DT),'MM/DD/YYYY')='01/02/1960' 

Answer 1

After refactoring the query comes indexes, so following on from @eric’s post:

credit_request:
You’re joining this onto requestor on request_id, which I hope is unique. In your where clause you then have a condition on evaluator_id and select client_app_id and personal_flg in the query. So, you probably need a unique index, on credit_request of (request_id, evaulator_id, client_app_id, personal_flg.

By putting the columns you’re selecting into the index you avoid the by index rowid, which means that you have selected your values from the index then re-entered the table to pick up more information. If this information is already in the index then there’s no need.

You’re joining it onto evaluator on evaluator_id, which is included in the first index.

requestor:
This is being joined onto on request_id and your where clause include soc_sec_num_text, lower(first_name_txt), lower(last_name_txt) and birth_dt. So, you need a unique if possible, index on (request_id, soc_sec_num_text) because of the or this is further complicated because you should really have an index on as many of the conditions as possible. You’re also selecting requestor_type_iud.

In this case to avoid a functional index, with many columns, I’d index on (request_id, soc_sec_num_text, birth_dt ) if you have the space, time and inclination then adding lower(first_name_txt)... etc to this may improve the speed depending on how selective the column is. This means that if there are far more values in for instance, first_name_txt than birth_dt you’d be better of putting this in front of birth_dt in the index so your query has less to scan if it’s a non-unique index.

You notice that I haven’t added the selected column into this index as you’re already going to have to go into the table so you gain nothing by adding it.

evaluator:
This is only being joined on evaluator_id so you need a unique, if possible, index on this column.