Query with union

Question

Hi,
I have three table:
Candidates: (candidate_id primary key)
Purhcase_orders: (Po_number primary key)
Bids: (bid_number primary key)
Just wanted to retrieve candidate information like candidate_id, first_name etc, along with count of purchase orders issued against this candidate and count of bid_number.
For that I have written below query:
query - 1:
SELECT CANDIDATE_ID,
INITCAP (first_name) FIRST_NAME,
+(SELECT COUNT (DISTINCT bid_number)+
FROM bids
WHERE BIDS.CANDIDATE_ID = cand.candidate_id)
BID_COUNT,
+(SELECT COUNT (DISTINCT po_number)+
FROM purchase_orders po
WHERE PO.CONTRACTEE_ID = cand.candidate_id)
PO_COUNT,
+'Y' match_flag,+
DECODE ( (SELECT candidate_id
FROM v_ineligible_candidates vic
WHERE VIC.CANDIDATE_ID = cand.candidate_id),
NULL, 'N',
+'Y')+
INELIGIBLE_FLAG
FROM candidates cand
Based on business requirement, need to create another set of query (query -2) same like above. In that we have some decode statements to calculate match_flag, then need to perform UNION operation between query -1 and query -2.
have around 130563 number of rows in candidates table.
Individually both queries are taking 500 msec.
But when I have used union, it didnt give result for atleast 60 sec. Then I cancelled execution.
Execution plan for the final query with UNION: (final cost)
Cost: 2,873  Bytes: 9,130,170  Cardinality: 260,862                
Please help me in this issue.
Why its taking that much time in union.
Please correct my mistake and suggest me in this query.
UNION ALL is running but not UNION.
Thanks in advance

Answer 1

Please post complete Explain Plans.
To get detailed information of Explain Plan use
explain plan for
your_query;
select *
  from table(dbms_xplan.display(null, null, 'ALL'));Post it alongwith below details between {noformat}{noformat} (exactly as specified) tags to preserve formatting and ease of understanding.
On your requirement,
Using UNIONALL you will get two set of rows, with UNION you might get a single row if your MATCH_FLAG are same for both queries. Use of either, has to meet your requirements than to meet your performance. If it is Business Acceptable, then look for performance and tuning.
If you could do the folllowing, it might be helpful for you.
1. Post the Sample table structure (Table information of Candidates should suffice) in a Create Table/With Sub-Query clause.
2. Sample Data in Insert Into/With Sub-Query format.
3. Expected Outcome based on Sample data
4. The Explain Plan of your Queries as requested above.
Reason of asking these details are, there might be a way to avoid Hitting the table Twice and avoid use of UNION/UNIONALL clauses.