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select * from table where rownum=1

Posted on February 9, 2015 by Sayan Malakshinov Posted in adaptive serial direct path reads, CBO, oracle, troubleshooting 1 Comment

I never thought I would have to optimize so simple query as

select col1, col2, col4, col7 from table where rownum=1

(even though I read recently “SELECT * FROM TABLE” Runs Out Of TEMP Space)
But a few days ago frequent executions of this query caused big problems on the one of our databases(11.2.0.3) because of adaptive serial direct path reads.

I don’t know why, but I felt intuitively that full table scan with “First K rows” optimization (“_optimizer_rownum_pred_based_fkr“=true) should turn off adaptive serial direct path reads. It seems quite logical to me.

PS. Unfortunately I had a little time, so I didn’t investigate what process and why it was doing that, I just created profile with “index full scan” access, and it completely solved the problem.

cbo direct path reads
« INDEX FULL SCAN (MIN/MAX) with two identical MIN()
Easy quiz: rownum < NaN »
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