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POAn oracle performance issue on COUNT()
 

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  1. POAn oracle performance issue on COUNT()
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    <p>I'm using Oracle 11g, the main table has about 10m records. Here is my query:</p> <pre><code>SELECT COUNT (*) FROM CONTACT c INNER JOIN STATUS S ON C.STATUS = S.STATUS WHERE C.USER = 1 AND S.REQUIRE = 1 AND ROWNUM = 1; </code></pre> <p>The Cost is 3736, but when I changed it to this form:</p> <pre><code>SELECT COUNT (*) FROM (SELECT 1 FROM CONTACT c INNER JOIN STATUS S ON C.STATUS = S.STATUS WHERE C.USER = 1 AND S.REQUIRE = 1 AND ROWNUM = 1); </code></pre> <p>The Cost became 5! What's the difference between these 2 queries?</p> <p>Here are the explain plan for both query:</p> <p>The first query:</p> <pre><code>---------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 10 | 3736 (1)| 00:00:45 | | 1 | SORT AGGREGATE | | 1 | 10 | | | |* 2 | COUNT STOPKEY | | | | | | | 3 | NESTED LOOPS | | 4627 | 46270 | 3736 (1)| 00:00:45 | | 4 | TABLE ACCESS BY INDEX ROWID| CONTACT | 6610 | 33050 | 3736 (1)| 00:00:45 | |* 5 | INDEX RANGE SCAN | IX_CONTACT_USR | 6610 | | 20 (0)| 00:00:01 | |* 6 | INDEX RANGE SCAN | IX_CONTACT_STATUS | 1 | 5 | 0 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter(ROWNUM=1) 5 - access("C"."USER"=1) 6 - access("C"."STATUS"="S"."STATUS" AND "S"."REQUIRE"=1) </code></pre> <p>The second query:</p> <pre><code>----------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | 5 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | | 2 | VIEW | | 1 | | 5 (0)| 00:00:01 | |* 3 | COUNT STOPKEY | | | | | | | 4 | NESTED LOOPS | | 2 | 20 | 5 (0)| 00:00:01 | | 5 | TABLE ACCESS BY INDEX ROWID| CONTACT | 3 | 15 | 5 (0)| 00:00:01 | |* 6 | INDEX RANGE SCAN | IX_CONTACT_USR | 6610 | | 3 (0)| 00:00:01 | |* 7 | INDEX RANGE SCAN | IX_CONTACT_STATUS | 1 | 5 | 0 (0)| 00:00:01 | ----------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - filter(ROWNUM=1) 6 - access("C"."USER"=1) 7 - access("C"."STATUS"="S"."STATUS" AND "S"."REQUIRE"=1) </code></pre> <p>I executed 2 queries, the first one sometimes cost 45s+ (e.g. first run or change the user id), otherwise it will cost &lt;1s. I totally don't know why it's such different, maybe db cache?</p> <p>When I executed the second query, I can always get result in 1s. So I think the second one is better, but I don't the reason why it improves a lot.</p>
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      1. POAn oracle performance issue on COUNT()
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    1. COComment on explain plan provided: As I supposed, your two querries will not be different. Only difference is line 2 - and that won't affect performance... (notice also that explain plan is not always what you will get in runtime - that's why I proposed to trace it.
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    2. CONice question and definitely going to get different answers! I'm glad you put in the plans not many do and it's frustrating, plus 1 just for that!
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    3. COcan you post explain plan for both the queries it will help us better understand the table statistics and compare the two queries. For getting explain plan of query follow this link: http://www.oraclebin.com/2012/12/explain-plan-how-to-get-execution-plan.html
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      1. USSushant Butta
 

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